diff --git a/README.md b/README.md
index 44d0fe6..8a4308d 100644
--- a/README.md
+++ b/README.md
@@ -10,6 +10,12 @@
 汇聚成 <strong>一个 API Key、一个入口</strong>，自动发现模型、智能路由、成本最优。
 </p>
 
+<p align="center">
+  <a href="https://linux.do/t/topic/1671489" alt="LINUX DO">
+    <img src="https://shorturl.at/ggSqS" alt="LINUX DO">
+  </a>
+</p>
+
 <p align="center">
 <a href="https://github.com/cita-777/metapi/releases">
   <img alt="GitHub Release" src="https://img.shields.io/github/v/release/cita-777/metapi?label=Release&logo=github&style=flat">
@@ -17,6 +23,9 @@
 --><a href="https://github.com/cita-777/metapi/stargazers">
   <img alt="GitHub Stars" src="https://img.shields.io/github/stars/cita-777/metapi?style=flat&logo=github&label=Stars">
 </a><!--
+--><a href="https://deepwiki.com/cita-777/metapi">
+  <img alt="Ask DeepWiki" src="https://deepwiki.com/badge.svg">
+</a><!--
 --><a href="https://hub.docker.com/r/1467078763/metapi">
   <img alt="Docker Pulls" src="https://img.shields.io/docker/pulls/1467078763/metapi?style=flat&logo=docker&label=Docker%20Pulls">
 </a><!--
diff --git a/README_EN.md b/README_EN.md
index 67f8c71..34c7834 100644
--- a/README_EN.md
+++ b/README_EN.md
@@ -10,6 +10,12 @@ Bring together all your New API / One API / OneHub / DoneHub / Veloera / AnyRout
 into <strong>one API Key, one endpoint</strong>, with automatic model discovery, smart routing, and cost optimization.
 </p>
 
+<p align="center">
+  <a href="https://linux.do/t/topic/1671489" alt="LINUX DO">
+    <img src="https://shorturl.at/ggSqS" alt="LINUX DO">
+  </a>
+</p>
+
 <p align="center">
 <a href="https://github.com/cita-777/metapi/releases">
   <img alt="GitHub Release" src="https://img.shields.io/github/v/release/cita-777/metapi?label=Release&logo=github&style=flat">
@@ -17,6 +23,9 @@ into <strong>one API Key, one endpoint</strong>, with automatic model discovery,
 --><a href="https://github.com/cita-777/metapi/stargazers">
   <img alt="GitHub Stars" src="https://img.shields.io/github/stars/cita-777/metapi?style=flat&logo=github&label=Stars">
 </a><!--
+--><a href="https://deepwiki.com/cita-777/metapi">
+  <img alt="Ask DeepWiki" src="https://deepwiki.com/badge.svg">
+</a><!--
 --><a href="https://hub.docker.com/r/1467078763/metapi">
   <img alt="Docker Pulls" src="https://img.shields.io/docker/pulls/1467078763/metapi?style=flat&logo=docker&label=Docker%20Pulls">
 </a><!--
diff --git a/src/server/proxy-core/surfaces/openAiResponsesSurface.ts b/src/server/proxy-core/surfaces/openAiResponsesSurface.ts
index a8f7ba9..f072a12 100644
--- a/src/server/proxy-core/surfaces/openAiResponsesSurface.ts
+++ b/src/server/proxy-core/surfaces/openAiResponsesSurface.ts
@@ -56,6 +56,12 @@ function isRecord(value: unknown): value is Record<string, unknown> {
   return !!value && typeof value === 'object';
 }
 
+function isResponsesWebsocketTransportRequest(headers: Record<string, unknown>): boolean {
+  return Object.entries(headers)
+    .some(([rawKey, rawValue]) => rawKey.trim().toLowerCase() === 'x-metapi-responses-websocket-transport'
+      && String(rawValue).trim() === '1');
+}
+
 function normalizeIncludeList(value: unknown): string[] {
   if (typeof value === 'string') {
     const trimmed = value.trim();
@@ -485,12 +491,10 @@ export async function handleOpenAiResponsesSurfaceRequest(
           const writeLines = (lines: string[]) => {
             for (const line of lines) reply.raw.write(line);
           };
+          const websocketTransportRequest = isResponsesWebsocketTransportRequest(request.headers as Record<string, unknown>);
           const streamSession = openAiResponsesTransformer.proxyStream.createSession({
             modelName,
             successfulUpstreamPath,
-            strictTerminalEvents: Object.entries(request.headers as Record<string, unknown>)
-              .some(([rawKey, rawValue]) => rawKey.trim().toLowerCase() === 'x-metapi-responses-websocket-transport'
-                && String(rawValue).trim() === '1'),
             getUsage: () => parsedUsage,
             onParsedPayload: (payload) => {
               if (payload && typeof payload === 'object') {
@@ -608,7 +612,65 @@ export async function handleOpenAiResponsesSurfaceRequest(
 
           startSseResponse();
 
-          const upstreamReader = upstream.body?.getReader();
+          let replayReader: ReturnType<typeof createSingleChunkStreamReader> | null = null;
+          if (websocketTransportRequest) {
+            const rawText = await upstream.text();
+            if (looksLikeResponsesSseText(rawText)) {
+              try {
+                const collectedPayload = collectResponsesFinalPayloadFromSseText(rawText, modelName).payload;
+                parsedUsage = mergeProxyUsage(parsedUsage, parseProxyUsage(collectedPayload));
+                const createdPayload = {
+                  ...collectedPayload,
+                  status: 'in_progress',
+                  output: [],
+                  output_text: '',
+                };
+                const terminalEventType = String(collectedPayload.status || '').trim().toLowerCase() === 'incomplete'
+                  ? 'response.incomplete'
+                  : 'response.completed';
+                writeLines([
+                  `event: response.created\ndata: ${JSON.stringify({ type: 'response.created', response: createdPayload })}\n\n`,
+                  `event: ${terminalEventType}\ndata: ${JSON.stringify({ type: terminalEventType, response: collectedPayload })}\n\n`,
+                  'data: [DONE]\n\n',
+                ]);
+                reply.raw.end();
+                const latency = Date.now() - startTime;
+                await finalizeStreamSuccess(parsedUsage, latency);
+                return;
+              } catch {
+                // Fall through to the generic stream session for response.failed/error terminals.
+              }
+
+              const streamResult = await streamSession.run(
+                createSingleChunkStreamReader(rawText),
+                reply.raw,
+              );
+              const latency = Date.now() - startTime;
+              if (streamResult.status === 'failed') {
+                await failureToolkit.recordStreamFailure({
+                  selected,
+                  requestedModel,
+                  modelName,
+                  errorMessage: streamResult.errorMessage,
+                  latencyMs: latency,
+                  retryCount,
+                  promptTokens: parsedUsage.promptTokens,
+                  completionTokens: parsedUsage.completionTokens,
+                  totalTokens: parsedUsage.totalTokens,
+                  upstreamPath: successfulUpstreamPath,
+                  runtimeFailureStatus: 502,
+                });
+                return;
+              }
+
+              await finalizeStreamSuccess(parsedUsage, latency);
+              return;
+            }
+
+            replayReader = createSingleChunkStreamReader(rawText);
+          }
+
+          const upstreamReader = replayReader ?? upstream.body?.getReader();
           const baseReader = String(selected.site.platform || '').trim().toLowerCase() === 'gemini-cli' && upstreamReader
             ? createGeminiCliStreamReader(upstreamReader)
             : upstreamReader;
diff --git a/src/server/routes/proxy/chat.stream.test.ts b/src/server/routes/proxy/chat.stream.test.ts
index a3811cb..6997139 100644
--- a/src/server/routes/proxy/chat.stream.test.ts
+++ b/src/server/routes/proxy/chat.stream.test.ts
@@ -1154,6 +1154,112 @@ describe('chat proxy stream behavior', () => {
     expect(response.body).toContain('[DONE]');
   });
 
+  it('replays downgraded chat-completions SSE for websocket transport without requiring native responses terminals', async () => {
+    fetchModelPricingCatalogMock.mockResolvedValue({
+      models: [
+        {
+          modelName: 'upstream-gpt',
+          supportedEndpointTypes: ['/v1/chat/completions'],
+        },
+      ],
+      groupRatio: {},
+    });
+
+    const encoder = new TextEncoder();
+    const upstreamBody = new ReadableStream<Uint8Array>({
+      start(controller) {
+        controller.enqueue(encoder.encode('data: {"id":"chatcmpl-r1-ws","model":"upstream-gpt","choices":[{"delta":{"role":"assistant"},"finish_reason":null}]}\n\n'));
+        controller.enqueue(encoder.encode('data: {"id":"chatcmpl-r1-ws","model":"upstream-gpt","choices":[{"delta":{"content":"hello from fallback"},"finish_reason":null}]}\n\n'));
+        controller.enqueue(encoder.encode('data: {"id":"chatcmpl-r1-ws","model":"upstream-gpt","choices":[{"delta":{},"finish_reason":"stop"}]}\n\n'));
+        controller.close();
+      },
+    });
+
+    fetchMock.mockResolvedValue(new Response(upstreamBody, {
+      status: 200,
+      headers: { 'content-type': 'text/event-stream; charset=utf-8' },
+    }));
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/v1/responses',
+      headers: {
+        'x-metapi-responses-websocket-transport': '1',
+      },
+      payload: {
+        model: 'gpt-5.2',
+        input: 'hello',
+        stream: true,
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+    expect(response.headers['content-type']).toContain('text/event-stream');
+    expect(response.body).toContain('response.output_item.added');
+    expect(response.body).toContain('response.output_text.delta');
+    expect(response.body).toContain('response.completed');
+    expect(response.body).not.toContain('response.failed');
+    expect(response.body).toContain('[DONE]');
+  });
+
+  it('initializes reasoning items before emitting reasoning summary deltas on /v1/responses streams', async () => {
+    fetchModelPricingCatalogMock.mockResolvedValue({
+      models: [
+        {
+          modelName: 'upstream-gpt',
+          supportedEndpointTypes: ['/v1/chat/completions'],
+        },
+      ],
+      groupRatio: {},
+    });
+
+    const encoder = new TextEncoder();
+    const upstreamBody = new ReadableStream<Uint8Array>({
+      start(controller) {
+        controller.enqueue(encoder.encode('data: {"id":"chatcmpl-r1-reasoning","model":"upstream-gpt","choices":[{"delta":{"role":"assistant"},"finish_reason":null}]}\n\n'));
+        controller.enqueue(encoder.encode('data: {"id":"chatcmpl-r1-reasoning","model":"upstream-gpt","choices":[{"delta":{"reasoning_content":"plan first"},"finish_reason":null}]}\n\n'));
+        controller.enqueue(encoder.encode('data: {"id":"chatcmpl-r1-reasoning","model":"upstream-gpt","choices":[{"delta":{},"finish_reason":"stop"}]}\n\n'));
+        controller.enqueue(encoder.encode('data: [DONE]\n\n'));
+        controller.close();
+      },
+    });
+
+    fetchMock.mockResolvedValue(new Response(upstreamBody, {
+      status: 200,
+      headers: { 'content-type': 'text/event-stream; charset=utf-8' },
+    }));
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/v1/responses',
+      payload: {
+        model: 'gpt-5.2',
+        input: 'hello',
+        stream: true,
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+    expect(response.headers['content-type']).toContain('text/event-stream');
+    expect(response.body).toContain('event: response.output_item.added');
+    expect(response.body).toContain('event: response.reasoning_summary_part.added');
+    expect(response.body).toContain('event: response.reasoning_summary_text.delta');
+    const eventBlocks = response.body.split('\n\n').filter((block) => block.trim().length > 0);
+    const reasoningItemAddedIndex = eventBlocks.findIndex(
+      (block) => block.includes('event: response.output_item.added') && block.includes('"type":"reasoning"'),
+    );
+    const reasoningSummaryPartAddedIndex = eventBlocks.findIndex(
+      (block) => block.includes('event: response.reasoning_summary_part.added'),
+    );
+    const reasoningSummaryTextDeltaIndex = eventBlocks.findIndex(
+      (block) => block.includes('event: response.reasoning_summary_text.delta'),
+    );
+
+    expect(reasoningItemAddedIndex).toBeGreaterThanOrEqual(0);
+    expect(reasoningItemAddedIndex).toBeLessThan(reasoningSummaryPartAddedIndex);
+    expect(reasoningSummaryPartAddedIndex).toBeLessThan(reasoningSummaryTextDeltaIndex);
+  });
+
   it('converts chat tool_calls SSE to Responses function_call events on /v1/responses', async () => {
     fetchModelPricingCatalogMock.mockResolvedValue({
       models: [
diff --git a/src/server/routes/proxy/responses.compact-upstream.test.ts b/src/server/routes/proxy/responses.compact-upstream.test.ts
index edec7c6..ce93ae2 100644
--- a/src/server/routes/proxy/responses.compact-upstream.test.ts
+++ b/src/server/routes/proxy/responses.compact-upstream.test.ts
@@ -279,7 +279,8 @@ describe('responses proxy compact upstream routing', () => {
     });
 
     expect(response.statusCode).toBe(200);
-    expect(response.json()).toEqual({
+    const body = response.json();
+    expect(body).toMatchObject({
       id: 'cmp_sse_123',
       object: 'response.compaction',
       input_tokens: 12,
@@ -292,6 +293,12 @@ describe('responses proxy compact upstream routing', () => {
           encrypted_content: 'enc-from-sse',
         },
       ],
+      usage: {
+        input_tokens: 12,
+        output_tokens: 3,
+        total_tokens: 15,
+      },
     });
+    expect(body.created_at).toEqual(expect.any(Number));
   });
 });
diff --git a/src/server/routes/proxy/responses.websocket.test.ts b/src/server/routes/proxy/responses.websocket.test.ts
index 376ea5c..615cfa8 100644
--- a/src/server/routes/proxy/responses.websocket.test.ts
+++ b/src/server/routes/proxy/responses.websocket.test.ts
@@ -534,7 +534,40 @@ describe('responses websocket transport', () => {
 
     const socket = createClientSocket(baseUrl);
     await waitForSocketOpen(socket);
-    const messagesPromise = waitForSocketMessages(socket, 1);
+    const messagesPromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.completed',
+    );
+
+    socket.send(JSON.stringify({
+      type: 'response.create',
+      model: 'gpt-5.4',
+      input: [],
+    }));
+
+    const message = await messagesPromise;
+    socket.close();
+
+    expect(fetchMock).toHaveBeenCalledTimes(1);
+    expect(message?.type).toBe('response.completed');
+    expect(message?.response?.id).toBe('resp_http_fallback');
+  });
+
+  it('treats response.incomplete as a terminal HTTP fallback payload without appending websocket error', async () => {
+    const selectedChannel = createSelectedChannel({
+      siteUrl: rejectedUpgradeSiteUrl,
+    });
+    selectChannelMock.mockReturnValue(selectedChannel);
+    previewSelectedChannelMock.mockResolvedValue(selectedChannel);
+    fetchMock.mockResolvedValueOnce(createSseResponse([
+      'event: response.incomplete\n',
+      'data: {"type":"response.incomplete","response":{"id":"resp_http_incomplete","model":"gpt-5.4","status":"incomplete","output":[{"id":"msg_http_incomplete","type":"message","role":"assistant","status":"incomplete","content":[{"type":"output_text","text":"partial"}]}],"output_text":"partial","incomplete_details":{"reason":"max_output_tokens"},"usage":{"input_tokens":1,"output_tokens":1,"total_tokens":2}}}\n\n',
+      'data: [DONE]\n\n',
+    ]));
+
+    const socket = createClientSocket(baseUrl);
+    await waitForSocketOpen(socket);
+    const messagesPromise = waitForSocketMessages(socket, 2);
 
     socket.send(JSON.stringify({
       type: 'response.create',
@@ -546,8 +579,13 @@ describe('responses websocket transport', () => {
     socket.close();
 
     expect(fetchMock).toHaveBeenCalledTimes(1);
-    expect(messages.map((message) => message.type)).toEqual(['response.completed']);
-    expect(messages[0]?.response?.id).toBe('resp_http_fallback');
+    expect(messages.map((message) => message?.type)).toEqual([
+      'response.created',
+      'response.incomplete',
+    ]);
+    expect(messages.some((message) => message?.type === 'error')).toBe(false);
+    const terminalMessage = messages[1];
+    expect(terminalMessage?.response?.incomplete_details?.reason).toBe('max_output_tokens');
   });
 
   it('falls back to the HTTP responses executor when the upstream codex websocket upgrade returns 401', async () => {
@@ -572,7 +610,10 @@ describe('responses websocket transport', () => {
 
     const socket = createClientSocket(baseUrl);
     await waitForSocketOpen(socket);
-    const messagesPromise = waitForSocketMessages(socket, 1);
+    const messagesPromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.completed',
+    );
 
     socket.send(JSON.stringify({
       type: 'response.create',
@@ -580,12 +621,58 @@ describe('responses websocket transport', () => {
       input: [],
     }));
 
-    const messages = await messagesPromise;
+    const message = await messagesPromise;
     socket.close();
 
     expect(fetchMock).toHaveBeenCalledTimes(1);
-    expect(messages.map((message) => message.type)).toEqual(['response.completed']);
-    expect(messages[0]?.response?.id).toBe('resp_http_fallback_401');
+    expect(message?.type).toBe('response.completed');
+    expect(message?.response?.id).toBe('resp_http_fallback_401');
+  });
+
+  it('treats response.incomplete as a terminal HTTP fallback event instead of appending a websocket error', async () => {
+    rejectedUpgradeStatus = 426;
+    rejectedUpgradeStatusText = 'Upgrade Required';
+    rejectedUpgradeBody = JSON.stringify({
+      error: {
+        message: 'upgrade required',
+        type: 'invalid_request_error',
+      },
+    });
+    const selectedChannel = createSelectedChannel({
+      siteUrl: rejectedUpgradeSiteUrl,
+    });
+    selectChannelMock.mockReturnValue(selectedChannel);
+    previewSelectedChannelMock.mockResolvedValue(selectedChannel);
+    fetchMock.mockResolvedValueOnce(createSseResponse([
+      'event: response.incomplete\n',
+      'data: {"type":"response.incomplete","response":{"id":"resp_http_incomplete","model":"gpt-5.4","status":"incomplete","output":[{"id":"msg_incomplete","type":"message","role":"assistant","status":"incomplete","content":[{"type":"output_text","text":"partial"}]}],"output_text":"partial","incomplete_details":{"reason":"max_output_tokens"},"usage":{"input_tokens":1,"output_tokens":1,"total_tokens":2}}}\n\n',
+      'data: [DONE]\n\n',
+    ]));
+
+    const socket = createClientSocket(baseUrl);
+    await waitForSocketOpen(socket);
+    const messagePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.incomplete',
+    );
+
+    socket.send(JSON.stringify({
+      type: 'response.create',
+      model: 'gpt-5.4',
+      input: [],
+    }));
+
+    const message = await messagePromise;
+    await expect(
+      waitForSocketMessageMatching(
+        socket,
+        (nextMessage) => nextMessage?.type === 'error',
+        150,
+      ),
+    ).rejects.toThrow('Timed out waiting for matching websocket message');
+    socket.close();
+
+    expect(message?.response?.id).toBe('resp_http_incomplete');
   });
 
   it('preserves previous_response_id when websocket upgrade fallback uses HTTP on incremental-capable upstreams', async () => {
@@ -617,7 +704,10 @@ describe('responses websocket transport', () => {
 
     const socket = createClientSocket(baseUrl);
     await waitForSocketOpen(socket);
-    const firstResponsePromise = waitForSocketMessages(socket, 1);
+    const firstResponsePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.completed',
+    );
 
     socket.send(JSON.stringify({
       type: 'response.create',
@@ -626,8 +716,11 @@ describe('responses websocket transport', () => {
       input: [],
     }));
 
-    const firstMessages = await firstResponsePromise;
-    const secondResponsePromise = waitForSocketMessages(socket, 1);
+    const firstMessage = await firstResponsePromise;
+    const secondResponsePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.completed' && message?.response?.id === 'resp_http_fallback_2',
+    );
     socket.send(JSON.stringify({
       type: 'response.create',
       model: 'gpt-5.4',
@@ -645,7 +738,7 @@ describe('responses websocket transport', () => {
     await secondResponsePromise;
     socket.close();
 
-    expect(firstMessages[0]?.type).toBe('response.completed');
+    expect(firstMessage?.type).toBe('response.completed');
     expect(fetchMock).toHaveBeenCalledTimes(2);
     const [, secondOptions] = fetchMock.mock.calls[1] as [string, RequestInit];
     const secondBody = JSON.parse(String(secondOptions.body));
@@ -660,6 +753,181 @@ describe('responses websocket transport', () => {
     ]);
   });
 
+  it('carries forward incomplete-turn output into the next fallback websocket request input', async () => {
+    rejectedUpgradeStatus = 426;
+    rejectedUpgradeStatusText = 'Upgrade Required';
+    rejectedUpgradeBody = JSON.stringify({
+      error: {
+        message: 'upgrade required',
+        type: 'invalid_request_error',
+      },
+    });
+
+    const selectedChannel = createSelectedChannel({
+      siteUrl: rejectedUpgradeSiteUrl,
+    });
+    selectChannelMock.mockReturnValue(selectedChannel);
+    previewSelectedChannelMock.mockResolvedValue(selectedChannel);
+    fetchMock
+      .mockResolvedValueOnce(createSseResponse([
+        'event: response.incomplete\n',
+        'data: {"type":"response.incomplete","response":{"id":"resp_http_incomplete_1","model":"gpt-5.4","status":"incomplete","output":[{"id":"msg_http_incomplete_1","type":"message","role":"assistant","status":"incomplete","content":[{"type":"output_text","text":"carry me"}]}],"output_text":"carry me","incomplete_details":{"reason":"max_output_tokens"},"usage":{"input_tokens":3,"output_tokens":1,"total_tokens":4}}}\n\n',
+        'data: [DONE]\n\n',
+      ]))
+      .mockResolvedValueOnce(createSseResponse([
+        'event: response.completed\n',
+        'data: {"type":"response.completed","response":{"id":"resp_http_incomplete_2","model":"gpt-5.4","status":"completed","output":[],"usage":{"input_tokens":5,"output_tokens":1,"total_tokens":6}}}\n\n',
+        'data: [DONE]\n\n',
+      ]));
+
+    const socket = createClientSocket(baseUrl);
+    await waitForSocketOpen(socket);
+    const firstResponsePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.incomplete',
+    );
+
+    socket.send(JSON.stringify({
+      type: 'response.create',
+      model: 'gpt-5.4',
+      input: [],
+    }));
+
+    const firstMessage = await firstResponsePromise;
+    const secondResponsePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.completed',
+    );
+    socket.send(JSON.stringify({
+      type: 'response.create',
+      model: 'gpt-5.4',
+      input: [
+        {
+          type: 'message',
+          role: 'user',
+          content: [{ type: 'input_text', text: 'continue' }],
+        },
+      ],
+    }));
+
+    await secondResponsePromise;
+    socket.close();
+
+    expect(firstMessage?.type).toBe('response.incomplete');
+    expect(fetchMock).toHaveBeenCalledTimes(2);
+    const [, secondOptions] = fetchMock.mock.calls[1] as [string, RequestInit];
+    const secondBody = JSON.parse(String(secondOptions.body));
+    expect(secondBody.input).toHaveLength(2);
+    expect(secondBody.input[0]).toMatchObject({
+      type: 'message',
+      role: 'assistant',
+      status: 'incomplete',
+      content: [{ type: 'output_text', text: 'carry me' }],
+    });
+    expect(secondBody.input[1]).toEqual({
+      type: 'message',
+      role: 'user',
+      content: [{ type: 'input_text', text: 'continue' }],
+    });
+  });
+
+  it('carries forward terminal output from incomplete HTTP fallback turns on non-incremental upstreams', async () => {
+    rejectedUpgradeStatus = 426;
+    rejectedUpgradeStatusText = 'Upgrade Required';
+    rejectedUpgradeBody = JSON.stringify({
+      error: {
+        message: 'upgrade required',
+        type: 'invalid_request_error',
+      },
+    });
+
+    const selectedChannel = createSelectedChannel({
+      sitePlatform: 'openai',
+      siteUrl: rejectedUpgradeSiteUrl,
+      actualModel: 'gpt-4.1',
+    });
+    selectChannelMock.mockReturnValue(selectedChannel);
+    previewSelectedChannelMock.mockResolvedValue(selectedChannel);
+    fetchMock
+      .mockResolvedValueOnce(createSseResponse([
+        'event: response.incomplete\n',
+        'data: {"type":"response.incomplete","response":{"id":"resp_http_incomplete_1","model":"gpt-4.1","status":"incomplete","output":[{"id":"msg_1","type":"message","role":"assistant","status":"incomplete","content":[{"type":"output_text","text":"partial tool call"}]}],"output_text":"partial tool call","incomplete_details":{"reason":"max_output_tokens"},"usage":{"input_tokens":3,"output_tokens":1,"total_tokens":4}}}\n\n',
+        'data: [DONE]\n\n',
+      ]))
+      .mockResolvedValueOnce(createSseResponse([
+        'event: response.completed\n',
+        'data: {"type":"response.completed","response":{"id":"resp_http_complete_2","model":"gpt-4.1","status":"completed","output":[{"id":"msg_2","type":"message","role":"assistant","status":"completed","content":[{"type":"output_text","text":"done"}]}],"usage":{"input_tokens":5,"output_tokens":1,"total_tokens":6}}}\n\n',
+        'data: [DONE]\n\n',
+      ]));
+
+    const socket = createClientSocket(baseUrl);
+    await waitForSocketOpen(socket);
+
+    const firstResponsePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.incomplete',
+    );
+    socket.send(JSON.stringify({
+      type: 'response.create',
+      model: 'gpt-4.1',
+      instructions: 'be helpful',
+      input: [
+        {
+          id: 'msg_user_1',
+          type: 'message',
+          role: 'user',
+          content: [{ type: 'input_text', text: 'call the tool' }],
+        },
+      ],
+    }));
+    await firstResponsePromise;
+
+    const secondResponsePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.completed',
+    );
+    socket.send(JSON.stringify({
+      type: 'response.create',
+      previous_response_id: 'resp_http_incomplete_1',
+      input: [
+        {
+          id: 'tool_out_1',
+          type: 'function_call_output',
+          call_id: 'call_1',
+          output: 'tool result',
+        },
+      ],
+    }));
+    await secondResponsePromise;
+    socket.close();
+
+    expect(fetchMock).toHaveBeenCalledTimes(2);
+    const [, secondOptions] = fetchMock.mock.calls[1] as [string, RequestInit];
+    const secondBody = JSON.parse(String(secondOptions.body));
+    expect(secondBody.previous_response_id).toBeUndefined();
+    expect(secondBody.model).toBe('gpt-4.1');
+    expect(secondBody.instructions).toBe('be helpful');
+    expect(secondBody.input).toHaveLength(3);
+    expect(secondBody.input[0]).toEqual({
+      id: 'msg_user_1',
+      type: 'message',
+      role: 'user',
+      content: [{ type: 'input_text', text: 'call the tool' }],
+    });
+    expect(secondBody.input[1]).toMatchObject({
+      type: 'message',
+      role: 'assistant',
+      status: 'incomplete',
+      content: [{ type: 'output_text', text: 'partial tool call' }],
+    });
+    expect(secondBody.input[2]).toEqual({
+      id: 'tool_out_1',
+      type: 'function_call_output',
+      call_id: 'call_1',
+      output: 'tool result',
+    });
+  });
+
   it('preserves query parameter auth when websocket transport falls back to the HTTP responses route', async () => {
     rejectedUpgradeStatus = 401;
     rejectedUpgradeStatusText = 'Unauthorized';
@@ -770,7 +1038,10 @@ describe('responses websocket transport', () => {
     const socket = createClientSocket(baseUrl);
     await waitForSocketOpen(socket);
 
-    const firstResponsePromise = waitForSocketMessages(socket, 1);
+    const firstResponsePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.completed',
+    );
     socket.send(JSON.stringify({
       type: 'response.create',
       model: 'gpt-4.1',
@@ -786,7 +1057,10 @@ describe('responses websocket transport', () => {
     }));
     await firstResponsePromise;
 
-    const secondResponsePromise = waitForSocketMessages(socket, 1);
+    const secondResponsePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.completed',
+    );
     socket.send(JSON.stringify({
       type: 'response.create',
       previous_response_id: 'resp_ws_1',
@@ -812,33 +1086,116 @@ describe('responses websocket transport', () => {
     expect(secondBody.previous_response_id).toBeUndefined();
     expect(secondBody.model).toBe('gpt-4.1');
     expect(secondBody.instructions).toBe('be helpful');
-    expect(secondBody.input).toEqual([
-      {
-        id: 'msg_user_1',
-        type: 'message',
-        role: 'user',
-        content: [{ type: 'input_text', text: 'call the tool' }],
-      },
-      {
-        id: 'fc_1',
-        type: 'function_call',
-        call_id: 'call_1',
-        status: 'completed',
-      },
-      {
-        id: 'msg_1',
-        type: 'message',
-        role: 'assistant',
-        status: 'completed',
-        content: [{ type: 'output_text', text: 'call tool' }],
-      },
-      {
-        id: 'tool_out_1',
-        type: 'function_call_output',
-        call_id: 'call_1',
-        output: 'tool result',
-      },
-    ]);
+    expect(secondBody.input).toHaveLength(4);
+    expect(secondBody.input[0]).toEqual({
+      id: 'msg_user_1',
+      type: 'message',
+      role: 'user',
+      content: [{ type: 'input_text', text: 'call the tool' }],
+    });
+    expect(secondBody.input[1]).toMatchObject({
+      id: 'fc_1',
+      type: 'function_call',
+      call_id: 'call_1',
+    });
+    expect(secondBody.input[2]).toEqual({
+      id: 'msg_1',
+      type: 'message',
+      role: 'assistant',
+      status: 'completed',
+      content: [{ type: 'output_text', text: 'call tool' }],
+    });
+    expect(secondBody.input[3]).toEqual({
+      id: 'tool_out_1',
+      type: 'function_call_output',
+      call_id: 'call_1',
+      output: 'tool result',
+    });
+  });
+
+  it('keeps streamed output items for follow-up turns when the terminal HTTP fallback payload has an empty output array', async () => {
+    const selectedChannel = createSelectedChannel({
+      sitePlatform: 'openai',
+      actualModel: 'gpt-4.1',
+    });
+    selectChannelMock.mockReturnValue(selectedChannel);
+    previewSelectedChannelMock.mockResolvedValue(selectedChannel);
+    fetchMock
+      .mockResolvedValueOnce(createSseResponse([
+        'event: response.output_item.done\n',
+        'data: {"type":"response.output_item.done","output_index":0,"item":{"id":"fc_1","type":"function_call","call_id":"call_1"}}\n\n',
+        'event: response.output_item.done\n',
+        'data: {"type":"response.output_item.done","output_index":1,"item":{"id":"msg_1","type":"message","role":"assistant","status":"completed","content":[{"type":"output_text","text":"call tool"}]}}\n\n',
+        'event: response.completed\n',
+        'data: {"type":"response.completed","response":{"id":"resp_ws_empty_output","model":"gpt-4.1","status":"completed","output":[],"usage":{"input_tokens":3,"output_tokens":1,"total_tokens":4}}}\n\n',
+        'data: [DONE]\n\n',
+      ]))
+      .mockResolvedValueOnce(createSseResponse([
+        'event: response.completed\n',
+        'data: {"type":"response.completed","response":{"id":"resp_ws_followup","model":"gpt-4.1","status":"completed","output":[{"id":"msg_2","type":"message","role":"assistant","status":"completed","content":[{"type":"output_text","text":"done"}]}],"usage":{"input_tokens":5,"output_tokens":1,"total_tokens":6}}}\n\n',
+        'data: [DONE]\n\n',
+      ]));
+
+    const socket = createClientSocket(baseUrl);
+    await waitForSocketOpen(socket);
+
+    const firstResponsePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.completed',
+    );
+    socket.send(JSON.stringify({
+      type: 'response.create',
+      model: 'gpt-4.1',
+      instructions: 'be helpful',
+      input: [
+        {
+          id: 'msg_user_1',
+          type: 'message',
+          role: 'user',
+          content: [{ type: 'input_text', text: 'call the tool' }],
+        },
+      ],
+    }));
+    await firstResponsePromise;
+
+    const secondResponsePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.completed',
+    );
+    socket.send(JSON.stringify({
+      type: 'response.create',
+      model: 'gpt-4.1',
+      instructions: 'be helpful',
+      previous_response_id: 'resp_ws_empty_output',
+      input: [
+        {
+          id: 'tool_out_1',
+          type: 'function_call_output',
+          call_id: 'call_1',
+          output: 'tool result',
+        },
+      ],
+    }));
+    await secondResponsePromise;
+    socket.close();
+
+    expect(fetchMock).toHaveBeenCalledTimes(2);
+    const [, secondOptions] = fetchMock.mock.calls[1] as [string, RequestInit];
+    const secondBody = JSON.parse(String(secondOptions.body));
+
+    expect(secondBody.input).toHaveLength(4);
+    expect(secondBody.input[1]).toMatchObject({
+      id: 'fc_1',
+      type: 'function_call',
+      call_id: 'call_1',
+    });
+    expect(secondBody.input[2]).toEqual({
+      id: 'msg_1',
+      type: 'message',
+      role: 'assistant',
+      status: 'completed',
+      content: [{ type: 'output_text', text: 'call tool' }],
+    });
   });
 
   it('serializes websocket messages per connection so follow-up turns wait for the previous HTTP fallback to finish', async () => {
@@ -924,33 +1281,31 @@ describe('responses websocket transport', () => {
     });
     const [, secondOptions] = fetchMock.mock.calls[1] as [string, RequestInit];
     const secondBody = JSON.parse(String(secondOptions.body));
-    expect(secondBody.input).toEqual([
-      {
-        id: 'msg_user_1',
-        type: 'message',
-        role: 'user',
-        content: [{ type: 'input_text', text: 'call the tool' }],
-      },
-      {
-        id: 'fc_1',
-        type: 'function_call',
-        call_id: 'call_1',
-        status: 'completed',
-      },
-      {
-        id: 'msg_1',
-        type: 'message',
-        role: 'assistant',
-        status: 'completed',
-        content: [{ type: 'output_text', text: 'call tool' }],
-      },
-      {
-        id: 'tool_out_1',
-        type: 'function_call_output',
-        call_id: 'call_1',
-        output: 'tool result',
-      },
-    ]);
+    expect(secondBody.input).toHaveLength(4);
+    expect(secondBody.input[0]).toEqual({
+      id: 'msg_user_1',
+      type: 'message',
+      role: 'user',
+      content: [{ type: 'input_text', text: 'call the tool' }],
+    });
+    expect(secondBody.input[1]).toMatchObject({
+      id: 'fc_1',
+      type: 'function_call',
+      call_id: 'call_1',
+    });
+    expect(secondBody.input[2]).toEqual({
+      id: 'msg_1',
+      type: 'message',
+      role: 'assistant',
+      status: 'completed',
+      content: [{ type: 'output_text', text: 'call tool' }],
+    });
+    expect(secondBody.input[3]).toEqual({
+      id: 'tool_out_1',
+      type: 'function_call_output',
+      call_id: 'call_1',
+      output: 'tool result',
+    });
   });
 
   it('preserves incremental response.create payloads with previous_response_id for websocket-capable upstreams', async () => {
@@ -1008,7 +1363,10 @@ describe('responses websocket transport', () => {
     const socket = createClientSocket(baseUrl);
     await waitForSocketOpen(socket);
 
-    const firstResponsePromise = waitForSocketMessages(socket, 1);
+    const firstResponsePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.completed',
+    );
     socket.send(JSON.stringify({
       type: 'response.create',
       model: 'gpt-5.4',
@@ -1100,7 +1458,10 @@ describe('responses websocket transport', () => {
 
     const socket = createClientSocket(baseUrl);
     await waitForSocketOpen(socket);
-    const firstResponsePromise = waitForSocketMessages(socket, 1);
+    const firstResponsePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.completed' && message?.response?.id === 'resp_http_1',
+    );
 
     socket.send(JSON.stringify({
       type: 'response.create',
@@ -1109,8 +1470,11 @@ describe('responses websocket transport', () => {
       input: [],
     }));
 
-    const firstMessages = await firstResponsePromise;
-    const secondResponsePromise = waitForSocketMessages(socket, 1);
+    const firstMessage = await firstResponsePromise;
+    const secondResponsePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.completed' && message?.response?.id === 'resp_http_2',
+    );
     socket.send(JSON.stringify({
       type: 'response.create',
       model: 'gpt-5.4',
@@ -1128,13 +1492,20 @@ describe('responses websocket transport', () => {
     await secondResponsePromise;
     socket.close();
 
-    expect(firstMessages[0]?.type).toBe('response.completed');
+    expect(firstMessage?.type).toBe('response.completed');
     expect(upstreamConnectionCount).toBe(0);
     expect(fetchMock).toHaveBeenCalledTimes(2);
     const [, secondOptions] = fetchMock.mock.calls[1] as [string, RequestInit];
     const secondBody = JSON.parse(String(secondOptions.body));
     expect(secondBody.previous_response_id).toBeUndefined();
     expect(secondBody.input).toEqual([
+      {
+        id: 'msg_1',
+        type: 'message',
+        role: 'assistant',
+        status: 'completed',
+        content: [{ type: 'output_text', text: 'first' }],
+      },
       {
         id: 'tool_out_1',
         type: 'function_call_output',
@@ -1282,4 +1653,238 @@ describe('responses websocket transport', () => {
     ]);
     expect(messages[2]?.error?.message).toContain('stream closed before response.completed');
   });
+
+  it('does not append websocket error after an upstream response.incomplete terminal event', async () => {
+    const selectedChannel = createSelectedChannel({
+      siteUrl: upstreamSiteUrl,
+    });
+    selectChannelMock.mockReturnValue(selectedChannel);
+    previewSelectedChannelMock.mockResolvedValue(selectedChannel);
+    upstreamMessageHandler = (socket) => {
+      socket.send(JSON.stringify({
+        type: 'response.incomplete',
+        response: {
+          id: 'resp_ws_incomplete',
+          model: 'gpt-5.4',
+          status: 'incomplete',
+          output: [{
+            id: 'msg_ws_incomplete',
+            type: 'message',
+            role: 'assistant',
+            status: 'incomplete',
+            content: [{ type: 'output_text', text: 'partial' }],
+          }],
+          incomplete_details: {
+            reason: 'max_output_tokens',
+          },
+        },
+      }));
+      socket.close();
+    };
+
+    const socket = createClientSocket(baseUrl);
+    await waitForSocketOpen(socket);
+    const incompletePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.incomplete',
+    );
+
+    socket.send(JSON.stringify({
+      type: 'response.create',
+      model: 'gpt-5.4',
+      input: [],
+    }));
+
+    const message = await incompletePromise;
+    socket.close();
+
+    expect(message?.type).toBe('response.incomplete');
+    expect(message?.response?.incomplete_details?.reason).toBe('max_output_tokens');
+  });
+
+  it('does not append websocket error after an upstream response.failed terminal event with output', async () => {
+    const selectedChannel = createSelectedChannel({
+      siteUrl: upstreamSiteUrl,
+    });
+    selectChannelMock.mockReturnValue(selectedChannel);
+    previewSelectedChannelMock.mockResolvedValue(selectedChannel);
+    upstreamMessageHandler = (socket) => {
+      socket.send(JSON.stringify({
+        type: 'response.failed',
+        response: {
+          id: 'resp_ws_failed',
+          model: 'gpt-5.4',
+          status: 'failed',
+          output: [{
+            id: 'msg_ws_failed',
+            type: 'message',
+            role: 'assistant',
+            status: 'failed',
+            content: [{ type: 'output_text', text: 'partial before failure' }],
+          }],
+          error: {
+            message: 'tool crashed',
+            type: 'server_error',
+          },
+        },
+      }));
+      socket.close();
+    };
+
+    const socket = createClientSocket(baseUrl);
+    await waitForSocketOpen(socket);
+    const failedPromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.failed',
+    );
+
+    socket.send(JSON.stringify({
+      type: 'response.create',
+      model: 'gpt-5.4',
+      input: [],
+    }));
+
+    const message = await failedPromise;
+    socket.close();
+
+    expect(message?.type).toBe('response.failed');
+    expect(message?.response?.error?.message).toBe('tool crashed');
+    expect(message?.response?.output?.[0]?.content?.[0]?.text).toBe('partial before failure');
+  });
+
+  it('carries forward output from response.incomplete terminal payloads on non-incremental websocket turns', async () => {
+    const selectedChannel = createSelectedChannel({
+      sitePlatform: 'openai',
+      actualModel: 'gpt-4.1',
+    });
+    selectChannelMock.mockReturnValue(selectedChannel);
+    previewSelectedChannelMock.mockResolvedValue(selectedChannel);
+    fetchMock
+      .mockResolvedValueOnce(createSseResponse([
+        'event: response.incomplete\n',
+        'data: {"type":"response.incomplete","response":{"id":"resp_incomplete_followup","model":"gpt-5.4","status":"incomplete","output":[{"id":"msg_incomplete_followup","type":"message","role":"assistant","status":"incomplete","content":[{"type":"output_text","text":"partial"}]}],"output_text":"partial","incomplete_details":{"reason":"max_output_tokens"},"usage":{"input_tokens":1,"output_tokens":1,"total_tokens":2}}}\n\n',
+        'data: [DONE]\n\n',
+      ]))
+      .mockResolvedValueOnce(createSseResponse([
+        'event: response.completed\n',
+        'data: {"type":"response.completed","response":{"id":"resp_followup_done","model":"gpt-5.4","status":"completed","output":[],"usage":{"input_tokens":2,"output_tokens":1,"total_tokens":3}}}\n\n',
+        'data: [DONE]\n\n',
+      ]));
+
+    const socket = createClientSocket(baseUrl);
+    await waitForSocketOpen(socket);
+    const firstMessagesPromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.incomplete',
+    );
+
+    socket.send(JSON.stringify({
+      type: 'response.create',
+      model: 'gpt-5.4',
+      input: [],
+    }));
+
+    const firstMessage = await firstMessagesPromise;
+    const secondMessagesPromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.completed' && message?.response?.id === 'resp_followup_done',
+    );
+    socket.send(JSON.stringify({
+      type: 'response.create',
+      model: 'gpt-5.4',
+      previous_response_id: 'resp_incomplete_followup',
+      input: [],
+    }));
+
+    await secondMessagesPromise;
+    socket.close();
+
+    expect(firstMessage?.response?.output).toEqual([
+      {
+        id: 'msg_incomplete_followup',
+        type: 'message',
+        role: 'assistant',
+        status: 'incomplete',
+        content: [{ type: 'output_text', text: 'partial' }],
+      },
+    ]);
+    expect(fetchMock).toHaveBeenCalledTimes(2);
+    const [, secondOptions] = fetchMock.mock.calls[1] as [string, RequestInit];
+    const secondBody = JSON.parse(String(secondOptions.body));
+    expect(secondBody.input).toHaveLength(1);
+    expect(secondBody.input[0]).toMatchObject({
+      type: 'message',
+      role: 'assistant',
+      status: 'incomplete',
+      content: [{ type: 'output_text', text: 'partial' }],
+    });
+  });
+
+  it('carries forward output from response.failed terminal payloads on non-incremental websocket turns', async () => {
+    const selectedChannel = createSelectedChannel({
+      sitePlatform: 'openai',
+      actualModel: 'gpt-4.1',
+    });
+    selectChannelMock.mockReturnValue(selectedChannel);
+    previewSelectedChannelMock.mockResolvedValue(selectedChannel);
+    fetchMock
+      .mockResolvedValueOnce(createSseResponse([
+        'event: response.failed\n',
+        'data: {"type":"response.failed","response":{"id":"resp_failed_followup","model":"gpt-4.1","status":"failed","output":[{"id":"msg_failed_followup","type":"message","role":"assistant","status":"failed","content":[{"type":"output_text","text":"partial failure"}]}],"error":{"message":"tool crashed","type":"server_error"},"usage":{"input_tokens":1,"output_tokens":1,"total_tokens":2}}}\n\n',
+        'data: [DONE]\n\n',
+      ]))
+      .mockResolvedValueOnce(createSseResponse([
+        'event: response.completed\n',
+        'data: {"type":"response.completed","response":{"id":"resp_failed_followup_done","model":"gpt-4.1","status":"completed","output":[],"usage":{"input_tokens":2,"output_tokens":1,"total_tokens":3}}}\n\n',
+        'data: [DONE]\n\n',
+      ]));
+
+    const socket = createClientSocket(baseUrl);
+    await waitForSocketOpen(socket);
+    const firstMessagePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.failed',
+    );
+
+    socket.send(JSON.stringify({
+      type: 'response.create',
+      model: 'gpt-4.1',
+      input: [],
+    }));
+
+    const firstMessage = await firstMessagePromise;
+    const secondMessagePromise = waitForSocketMessageMatching(
+      socket,
+      (message) => message?.type === 'response.completed' && message?.response?.id === 'resp_failed_followup_done',
+    );
+    socket.send(JSON.stringify({
+      type: 'response.create',
+      model: 'gpt-4.1',
+      previous_response_id: 'resp_failed_followup',
+      input: [],
+    }));
+
+    await secondMessagePromise;
+    socket.close();
+
+    expect(firstMessage?.response?.output).toEqual([
+      {
+        id: 'msg_failed_followup',
+        type: 'message',
+        role: 'assistant',
+        status: 'failed',
+        content: [{ type: 'output_text', text: 'partial failure' }],
+      },
+    ]);
+    expect(fetchMock).toHaveBeenCalledTimes(2);
+    const [, secondOptions] = fetchMock.mock.calls[1] as [string, RequestInit];
+    const secondBody = JSON.parse(String(secondOptions.body));
+    expect(secondBody.input).toHaveLength(1);
+    expect(secondBody.input[0]).toMatchObject({
+      type: 'message',
+      role: 'assistant',
+      content: [{ type: 'output_text', text: 'partial failure' }],
+    });
+    expect(['failed', 'incomplete']).toContain(secondBody.input[0]?.status);
+  });
 });
diff --git a/src/server/routes/proxy/responsesSseFinal.test.ts b/src/server/routes/proxy/responsesSseFinal.test.ts
index 64c96e5..eaba0a6 100644
--- a/src/server/routes/proxy/responsesSseFinal.test.ts
+++ b/src/server/routes/proxy/responsesSseFinal.test.ts
@@ -72,4 +72,161 @@ describe('collectResponsesFinalPayloadFromSse', () => {
         },
       });
   });
+
+  it('returns response.incomplete payloads instead of treating them as upstream failures', async () => {
+    const upstream = {
+      async text() {
+        return [
+          'event: response.created',
+          'data: {"type":"response.created","response":{"id":"resp_incomplete_terminal","model":"gpt-5.4","created_at":1706000000,"status":"in_progress","output":[]}}',
+          '',
+          'event: response.incomplete',
+          'data: {"type":"response.incomplete","response":{"id":"resp_incomplete_terminal","model":"gpt-5.4","status":"incomplete","output":[{"id":"msg_incomplete_1","type":"message","role":"assistant","status":"incomplete","content":[{"type":"output_text","text":"partial answer"}]}],"output_text":"partial answer","incomplete_details":{"reason":"max_output_tokens"},"usage":{"input_tokens":3,"output_tokens":1,"total_tokens":4}}}',
+          '',
+          'data: [DONE]',
+          '',
+        ].join('\n');
+      },
+    };
+
+    await expect(collectResponsesFinalPayloadFromSse(upstream, 'gpt-5.4'))
+      .resolves
+      .toMatchObject({
+        payload: {
+          id: 'resp_incomplete_terminal',
+          status: 'incomplete',
+          output_text: 'partial answer',
+          incomplete_details: {
+            reason: 'max_output_tokens',
+          },
+        },
+      });
+  });
+
+  it('returns response.incomplete terminal payloads instead of throwing', async () => {
+    const upstream = {
+      async text() {
+        return [
+          'event: response.created',
+          'data: {"type":"response.created","response":{"id":"resp_incomplete_1","model":"gpt-5.4","created_at":1706000000,"status":"in_progress","output":[]}}',
+          '',
+          'event: response.output_text.delta',
+          'data: {"type":"response.output_text.delta","output_index":0,"item_id":"msg_incomplete_1","delta":"partial"}',
+          '',
+          'event: response.incomplete',
+          'data: {"type":"response.incomplete","response":{"id":"resp_incomplete_1","model":"gpt-5.4","status":"incomplete","output":[{"id":"msg_incomplete_1","type":"message","role":"assistant","status":"incomplete","content":[{"type":"output_text","text":"partial"}]}],"incomplete_details":{"reason":"max_output_tokens"},"usage":{"input_tokens":3,"output_tokens":1,"total_tokens":4}}}',
+          '',
+          'data: [DONE]',
+          '',
+        ].join('\n');
+      },
+    };
+
+    await expect(collectResponsesFinalPayloadFromSse(upstream, 'gpt-5.4'))
+      .resolves
+      .toMatchObject({
+        payload: {
+          id: 'resp_incomplete_1',
+          status: 'incomplete',
+          incomplete_details: {
+            reason: 'max_output_tokens',
+          },
+          output_text: 'partial',
+        },
+      });
+  });
+
+  it('treats response.incomplete as terminal even when only response.output carries the visible text', async () => {
+    const upstream = {
+      async text() {
+        return [
+          'event: response.created',
+          'data: {"type":"response.created","response":{"id":"resp_incomplete_output_only","model":"gpt-5.4","created_at":1706000000,"status":"in_progress","output":[]}}',
+          '',
+          'event: response.incomplete',
+          'data: {"type":"response.incomplete","response":{"id":"resp_incomplete_output_only","model":"gpt-5.4","status":"incomplete","output":[{"id":"msg_incomplete_output_only","type":"message","role":"assistant","status":"incomplete","content":[{"type":"output_text","text":"partial from output"}]}],"incomplete_details":{"reason":"max_output_tokens"},"usage":{"input_tokens":3,"output_tokens":1,"total_tokens":4}}}',
+          '',
+          'data: [DONE]',
+          '',
+        ].join('\n');
+      },
+    };
+
+    await expect(collectResponsesFinalPayloadFromSse(upstream, 'gpt-5.4'))
+      .resolves
+      .toMatchObject({
+        payload: {
+          id: 'resp_incomplete_output_only',
+          status: 'incomplete',
+          output: [
+            {
+              id: 'msg_incomplete_output_only',
+              type: 'message',
+              role: 'assistant',
+              status: 'incomplete',
+              content: [
+                {
+                  type: 'output_text',
+                  text: 'partial from output',
+                },
+              ],
+            },
+          ],
+          output_text: 'partial from output',
+          incomplete_details: {
+            reason: 'max_output_tokens',
+          },
+        },
+      });
+  });
+
+  it('preserves incomplete item status when response.incomplete needs aggregate output repair', async () => {
+    const upstream = {
+      async text() {
+        return [
+          'event: response.created',
+          'data: {"type":"response.created","response":{"id":"resp_incomplete_repair","model":"gpt-5.4","created_at":1706000000,"status":"in_progress","output":[]}}',
+          '',
+          'event: response.output_item.added',
+          'data: {"type":"response.output_item.added","output_index":0,"item":{"id":"msg_incomplete_repair","type":"message","role":"assistant","status":"in_progress","content":[]}}',
+          '',
+          'event: response.output_text.delta',
+          'data: {"type":"response.output_text.delta","output_index":0,"item_id":"msg_incomplete_repair","delta":"partial repair"}',
+          '',
+          'event: response.incomplete',
+          'data: {"type":"response.incomplete","response":{"id":"resp_incomplete_repair","model":"gpt-5.4","status":"incomplete","output":[],"incomplete_details":{"reason":"max_output_tokens"},"usage":{"input_tokens":3,"output_tokens":1,"total_tokens":4}}}',
+          '',
+          'data: [DONE]',
+          '',
+        ].join('\n');
+      },
+    };
+
+    await expect(collectResponsesFinalPayloadFromSse(upstream, 'gpt-5.4'))
+      .resolves
+      .toMatchObject({
+        payload: {
+          id: 'resp_incomplete_repair',
+          status: 'incomplete',
+          output: [
+            {
+              id: 'msg_incomplete_repair',
+              type: 'message',
+              role: 'assistant',
+              status: 'incomplete',
+              content: [
+                {
+                  type: 'output_text',
+                  text: 'partial repair',
+                },
+              ],
+            },
+          ],
+          output_text: 'partial repair',
+          incomplete_details: {
+            reason: 'max_output_tokens',
+          },
+        },
+      });
+  });
 });
diff --git a/src/server/routes/proxy/responsesSseFinal.ts b/src/server/routes/proxy/responsesSseFinal.ts
index 3fee5ac..4863384 100644
--- a/src/server/routes/proxy/responsesSseFinal.ts
+++ b/src/server/routes/proxy/responsesSseFinal.ts
@@ -1,10 +1,16 @@
 import { openAiResponsesTransformer } from '../../transformers/openai/responses/index.js';
 import { mergeProxyUsage, parseProxyUsage } from '../../services/proxyUsageParser.js';
 
+type ResponsesTerminalStatus = 'completed' | 'incomplete';
+
 function isRecord(value: unknown): value is Record<string, unknown> {
   return !!value && typeof value === 'object';
 }
 
+function asTrimmedString(value: unknown): string {
+  return typeof value === 'string' ? value.trim() : '';
+}
+
 function parseResponsesSsePayload(data: string): Record<string, unknown> | null {
   try {
     const parsed = JSON.parse(data);
@@ -72,6 +78,27 @@ function hasMeaningfulFinalResponsesPayload(payload: Record<string, unknown>): b
   return hasMeaningfulResponsesOutput(payload.output);
 }
 
+function collectResponsesOutputText(payload: Record<string, unknown>): string {
+  const output = Array.isArray(payload.output) ? payload.output : [];
+  const parts: string[] = [];
+
+  for (const item of output) {
+    if (!isRecord(item)) continue;
+    if (asTrimmedString(item.type).toLowerCase() !== 'message') continue;
+    const content = Array.isArray(item.content) ? item.content : [];
+    for (const part of content) {
+      if (!isRecord(part)) continue;
+      const partType = asTrimmedString(part.type).toLowerCase();
+      const text = typeof part.text === 'string' ? part.text : '';
+      if ((partType === 'output_text' || partType === 'text') && text) {
+        parts.push(text);
+      }
+    }
+  }
+
+  return parts.join('');
+}
+
 function rememberStreamResponseEnvelope(
   streamContext: ReturnType<typeof openAiResponsesTransformer.createStreamContext>,
   payload: Record<string, unknown>,
@@ -95,29 +122,132 @@ function rememberStreamResponseEnvelope(
   }
 }
 
-function materializeCompletedPayloadFromAggregate(
+function ensureResponseId(rawId: string): string {
+  const trimmed = rawId.trim() || `resp_${Date.now()}`;
+  return trimmed.startsWith('resp_') ? trimmed : `resp_${trimmed}`;
+}
+
+function buildUsagePayload(usage: ReturnType<typeof parseProxyUsage>): Record<string, unknown> {
+  const payload: Record<string, unknown> = {
+    input_tokens: usage.promptTokens,
+    output_tokens: usage.completionTokens,
+    total_tokens: usage.totalTokens,
+  };
+  const inputDetails: Record<string, unknown> = {};
+  if ((usage.cacheReadTokens || 0) > 0) inputDetails.cached_tokens = usage.cacheReadTokens;
+  if ((usage.cacheCreationTokens || 0) > 0) inputDetails.cache_creation_tokens = usage.cacheCreationTokens;
+  if (Object.keys(inputDetails).length > 0) payload.input_tokens_details = inputDetails;
+  return payload;
+}
+
+function cloneAggregateOutputItem(
+  item: unknown,
+  terminalStatus: ResponsesTerminalStatus,
+): Record<string, unknown> | null {
+  if (!isRecord(item)) return null;
+  const next = structuredClone(item);
+  const currentStatus = asTrimmedString(next.status).toLowerCase();
+  next.status = currentStatus && currentStatus !== 'in_progress'
+    ? currentStatus
+    : terminalStatus;
+  return next;
+}
+
+function materializeTerminalPayloadFromAggregate(
   aggregateState: ReturnType<typeof openAiResponsesTransformer.aggregator.createState>,
   streamContext: ReturnType<typeof openAiResponsesTransformer.createStreamContext>,
   usage: ReturnType<typeof parseProxyUsage>,
-): Record<string, unknown> | null {
-  const lines = openAiResponsesTransformer.aggregator.complete(
+  terminalStatus: ResponsesTerminalStatus,
+): Record<string, unknown> {
+  const output = aggregateState.outputItems
+    .map((item) => cloneAggregateOutputItem(item, terminalStatus))
+    .filter((item): item is Record<string, unknown> => !!item);
+  const usagePayload = buildUsagePayload(usage);
+  const usageWithExtras = Object.keys(aggregateState.usageExtras).length > 0
+    ? { ...usagePayload, ...structuredClone(aggregateState.usageExtras) }
+    : usagePayload;
+
+  return {
+    id: ensureResponseId(
+      asTrimmedString(aggregateState.responseId)
+      || asTrimmedString(streamContext.id)
+      || asTrimmedString(aggregateState.modelName),
+    ),
+    object: 'response',
+    created_at: (
+      typeof aggregateState.createdAt === 'number' && Number.isFinite(aggregateState.createdAt)
+        ? aggregateState.createdAt
+        : streamContext.created
+    ) || Math.floor(Date.now() / 1000),
+    status: terminalStatus,
+    model: asTrimmedString(streamContext.model) || asTrimmedString(aggregateState.modelName),
+    output,
+    output_text: collectResponsesOutputText({ output }),
+    usage: usageWithExtras,
+  };
+}
+
+function enrichTerminalPayload(
+  payload: Record<string, unknown>,
+  aggregateState: ReturnType<typeof openAiResponsesTransformer.aggregator.createState>,
+  streamContext: ReturnType<typeof openAiResponsesTransformer.createStreamContext>,
+  usage: ReturnType<typeof parseProxyUsage>,
+  terminalStatus: ResponsesTerminalStatus,
+): Record<string, unknown> {
+  const next = structuredClone(payload);
+  const materialized = materializeTerminalPayloadFromAggregate(
     aggregateState,
     streamContext,
     usage,
+    terminalStatus,
   );
-  const { events } = openAiResponsesTransformer.pullSseEvents(lines.join(''));
-  for (const event of events) {
-    if (event.data === '[DONE]') continue;
-    const payload = parseResponsesSsePayload(event.data);
-    if (!payload) continue;
-    if (event.event === 'response.completed' && isRecord(payload.response)) {
-      return payload.response;
-    }
-    if (payload.type === 'response.completed' && hasCompleteFinalResponsesPayload(payload)) {
-      return payload;
+
+  if (!hasMeaningfulResponsesOutput(next.output) && materialized && hasMeaningfulResponsesOutput(materialized.output)) {
+    next.output = materialized.output;
+  }
+
+  const currentOutputText = typeof next.output_text === 'string' ? next.output_text : '';
+  if (!currentOutputText) {
+    const derivedOutputText = collectResponsesOutputText(next)
+      || (materialized && typeof materialized.output_text === 'string' ? materialized.output_text : '');
+    if (derivedOutputText) {
+      next.output_text = derivedOutputText;
     }
   }
-  return null;
+
+  if (materialized && next.usage === undefined && materialized.usage !== undefined) {
+    next.usage = materialized.usage;
+  }
+
+  return next;
+}
+
+function mergeMissingResponsesTerminalFields(
+  payload: Record<string, unknown>,
+  fallbackPayload: Record<string, unknown> | null,
+): Record<string, unknown> {
+  if (!fallbackPayload) return payload;
+  const merged = { ...payload };
+  if (merged.output === undefined && fallbackPayload.output !== undefined) {
+    merged.output = fallbackPayload.output;
+  }
+  if (
+    (typeof merged.output_text !== 'string' || merged.output_text.length === 0)
+    && typeof fallbackPayload.output_text === 'string'
+    && fallbackPayload.output_text.length > 0
+  ) {
+    merged.output_text = fallbackPayload.output_text;
+  }
+  if (merged.object === undefined && fallbackPayload.object !== undefined) {
+    merged.object = fallbackPayload.object;
+  }
+  if (merged.created_at === undefined && fallbackPayload.created_at !== undefined) {
+    merged.created_at = fallbackPayload.created_at;
+  }
+  if (merged.usage === undefined && fallbackPayload.usage !== undefined) {
+    merged.usage = fallbackPayload.usage;
+  }
+  return merged;
 }
 
 export function looksLikeResponsesSseText(rawText: string): boolean {
@@ -169,18 +299,20 @@ export function collectResponsesFinalPayloadFromSseText(
     promptTokensIncludeCache: null as boolean | null,
   };
   let completedPayload: Record<string, unknown> | null = null;
+  let terminalStatus: ResponsesTerminalStatus = 'completed';
 
   const captureCompletedPayloadFromEvent = (
     eventType: string,
     payload: Record<string, unknown>,
   ) => {
     if (completedPayload) return;
-    if (eventType === 'response.failed' || eventType === 'response.incomplete' || eventType === 'error') {
+    if (eventType === 'response.failed' || eventType === 'error') {
       throw new Error(getResponsesFailureMessage(payload));
     }
-    if (eventType !== 'response.completed') {
+    if (eventType !== 'response.completed' && eventType !== 'response.incomplete') {
       return;
     }
+    terminalStatus = eventType === 'response.incomplete' ? 'incomplete' : 'completed';
     if (isRecord(payload.response) && hasCompleteFinalResponsesPayload(payload.response)) {
       completedPayload = payload.response;
       return;
@@ -236,11 +368,36 @@ export function collectResponsesFinalPayloadFromSseText(
     && !hasMeaningfulFinalResponsesPayload(completedPayload)
     && hasMeaningfulResponsesOutput(aggregateState.outputItems)
   ) {
-    completedPayload = materializeCompletedPayloadFromAggregate(aggregateState, streamContext, usage);
+    completedPayload = mergeMissingResponsesTerminalFields(
+      completedPayload,
+      materializeTerminalPayloadFromAggregate(
+        aggregateState,
+        streamContext,
+        usage,
+        terminalStatus,
+      ),
+    );
+  }
+
+  if (completedPayload) {
+    completedPayload = mergeMissingResponsesTerminalFields(
+      completedPayload,
+      materializeTerminalPayloadFromAggregate(
+        aggregateState,
+        streamContext,
+        usage,
+        terminalStatus,
+      ),
+    );
   }
 
   if (!completedPayload) {
-    const materialized = materializeCompletedPayloadFromAggregate(aggregateState, streamContext, usage);
+    const materialized = materializeTerminalPayloadFromAggregate(
+      aggregateState,
+      streamContext,
+      usage,
+      terminalStatus,
+    );
     if (materialized) {
       completedPayload = materialized;
     }
@@ -248,12 +405,18 @@ export function collectResponsesFinalPayloadFromSseText(
 
   if (completedPayload) {
     return {
-      payload: completedPayload,
+      payload: enrichTerminalPayload(
+        completedPayload,
+        aggregateState,
+        streamContext,
+        usage,
+        terminalStatus,
+      ),
       rawText,
     };
   }
 
-  throw new Error('stream disconnected before response.completed');
+  throw new Error('stream disconnected before terminal responses event');
 }
 
 export async function collectResponsesFinalPayloadFromSse(
diff --git a/src/server/routes/proxy/responsesWebsocket.ts b/src/server/routes/proxy/responsesWebsocket.ts
index f068c86..019645d 100644
--- a/src/server/routes/proxy/responsesWebsocket.ts
+++ b/src/server/routes/proxy/responsesWebsocket.ts
@@ -312,6 +312,11 @@ function synthesizePrewarmResponsePayloads(request: Record<string, unknown>) {
 function collectResponsesOutput(payloads: unknown[]): unknown[] {
   const outputByIndex = new Map<number, unknown>();
   let completedOutput: unknown[] | null = null;
+  const fallbackStatusForType = (type: string): string => {
+    if (type === 'response.completed') return 'completed';
+    if (type === 'response.failed') return 'failed';
+    return 'incomplete';
+  };
 
   for (const payload of payloads) {
     if (!isRecord(payload)) continue;
@@ -322,8 +327,54 @@ function collectResponsesOutput(payloads: unknown[]): unknown[] {
       outputByIndex.set(Number(payload.output_index), cloneJsonObject(payload.item));
       continue;
     }
-    if (type === 'response.completed' && isRecord(payload.response) && Array.isArray(payload.response.output)) {
-      completedOutput = cloneJsonObject(payload.response.output);
+    if (
+      (type === 'response.completed' || type === 'response.incomplete' || type === 'response.failed')
+      && isRecord(payload.response)
+      && Array.isArray(payload.response.output)
+    ) {
+      const terminalOutput = cloneJsonObject(payload.response.output);
+      if (terminalOutput.length > 0 || outputByIndex.size === 0) {
+        completedOutput = terminalOutput;
+      }
+      continue;
+    }
+    if (
+      (type === 'response.completed' || type === 'response.incomplete' || type === 'response.failed')
+      && isRecord(payload.response)
+      && typeof payload.response.output_text === 'string'
+      && payload.response.output_text.trim()
+    ) {
+      completedOutput = [{
+        id: `msg_${asTrimmedString(payload.response.id) || type}`,
+        type: 'message',
+        role: 'assistant',
+        status: asTrimmedString(payload.response.status) || fallbackStatusForType(type),
+        content: [{
+          type: 'output_text',
+          text: payload.response.output_text,
+        }],
+      }];
+      continue;
+    }
+    if (Array.isArray(payload.output)) {
+      const terminalOutput = cloneJsonObject(payload.output);
+      if (terminalOutput.length > 0 || outputByIndex.size === 0) {
+        completedOutput = terminalOutput;
+      }
+      continue;
+    }
+    if (typeof payload.output_text === 'string' && payload.output_text.trim()) {
+      const fallbackStatus = asTrimmedString(payload.status) || fallbackStatusForType(type || 'response.completed');
+      completedOutput = [{
+        id: `msg_${type || 'response'}`,
+        type: 'message',
+        role: 'assistant',
+        status: fallbackStatus,
+        content: [{
+          type: 'output_text',
+          text: payload.output_text,
+        }],
+      }];
     }
   }
 
@@ -381,10 +432,9 @@ async function forwardResponsesRequestViaHttp(input: {
   if (!contentType.includes('text/event-stream')) {
     try {
       const payload = JSON.parse(response.body);
+      const output = collectResponsesOutput([payload]);
       input.socket.send(JSON.stringify(payload));
-      return isRecord(payload?.response) && Array.isArray(payload.response.output)
-        ? cloneJsonObject(payload.response.output)
-        : [];
+      return output;
     } catch {
       writeResponsesWebsocketError(input.socket, 502, 'Unexpected non-JSON websocket proxy response');
       return null;
@@ -400,7 +450,7 @@ async function forwardResponsesRequestViaHttp(input: {
       const payload = JSON.parse(event.data);
       forwardedPayloads.push(payload);
       const type = isRecord(payload) ? asTrimmedString(payload.type) : '';
-      if (type === 'response.completed' || type === 'response.failed') {
+      if (type === 'response.completed' || type === 'response.failed' || type === 'response.incomplete') {
         sawTerminalPayload = true;
       }
       input.socket.send(JSON.stringify(payload));
@@ -597,7 +647,7 @@ async function handleResponsesWebsocketConnection(
                 ? error
                 : new CodexWebsocketRuntimeError('upstream websocket request failed');
               if (runtimeError.status && runtimeError.events.length === 0) {
-                const fallbackOutput = await forwardResponsesRequestViaHttp({
+                const forwarded = await forwardResponsesRequestViaHttp({
                   app,
                   socket,
                   request,
@@ -605,8 +655,8 @@ async function handleResponsesWebsocketConnection(
                   preserveIncrementalMode: supportsIncrementalInput,
                   authToken: authContext.token,
                 });
-                if (fallbackOutput) {
-                  lastResponseOutput = fallbackOutput;
+                if (forwarded) {
+                  lastResponseOutput = forwarded;
                 }
                 return;
               }
@@ -614,17 +664,24 @@ async function handleResponsesWebsocketConnection(
               for (const payload of runtimeError.events) {
                 socket.send(JSON.stringify(payload));
               }
-              writeResponsesWebsocketError(
-                socket,
-                runtimeError.status || 408,
-                runtimeError.message,
-                runtimeError.payload,
-              );
+              const emittedTerminalResponsesEvent = runtimeError.events.some((payload) => {
+                if (!isRecord(payload)) return false;
+                const type = asTrimmedString(payload.type);
+                return type === 'response.completed' || type === 'response.failed' || type === 'response.incomplete';
+              });
+              if (!emittedTerminalResponsesEvent) {
+                writeResponsesWebsocketError(
+                  socket,
+                  runtimeError.status || 408,
+                  runtimeError.message,
+                  runtimeError.payload,
+                );
+              }
             }
             return;
           }
 
-          const forwardedOutput = await forwardResponsesRequestViaHttp({
+          const forwarded = await forwardResponsesRequestViaHttp({
             app,
             socket,
             request,
@@ -632,8 +689,8 @@ async function handleResponsesWebsocketConnection(
             preserveIncrementalMode: supportsIncrementalInput,
             authToken: authContext.token,
           });
-          if (forwardedOutput) {
-            lastResponseOutput = forwardedOutput;
+          if (forwarded) {
+            lastResponseOutput = forwarded;
           }
         } catch {
           writeResponsesWebsocketError(socket, 500, 'internal websocket proxy error');
diff --git a/src/server/transformers/openai/responses/aggregator.test.ts b/src/server/transformers/openai/responses/aggregator.test.ts
index 17d0765..6347040 100644
--- a/src/server/transformers/openai/responses/aggregator.test.ts
+++ b/src/server/transformers/openai/responses/aggregator.test.ts
@@ -264,6 +264,246 @@ describe('serializeConvertedResponsesEvents', () => {
     ]);
   });
 
+  it('emits response.output_item.added before synthetic reasoning summary events for delta-only reasoning streams', () => {
+    const state = createOpenAiResponsesAggregateState('gpt-5');
+    const streamContext = createStreamTransformContext('gpt-5');
+    const usage = {
+      promptTokens: 1,
+      completionTokens: 2,
+      totalTokens: 3,
+    };
+
+    const lines = serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        reasoningDelta: 'Think ',
+      } as any,
+    });
+
+    const events = parseSseEvents(lines);
+
+    expect(events.map((entry) => entry.event)).toEqual([
+      'response.output_item.added',
+      'response.reasoning_summary_part.added',
+      'response.reasoning_summary_text.delta',
+    ]);
+    expect(events[0]?.payload).toMatchObject({
+      type: 'response.output_item.added',
+      output_index: 0,
+      item: {
+        id: 'rs_0',
+        type: 'reasoning',
+        status: 'in_progress',
+        summary: [],
+      },
+    });
+    expect(events[1]?.payload).toMatchObject({
+      type: 'response.reasoning_summary_part.added',
+      item_id: 'rs_0',
+      output_index: 0,
+      summary_index: 0,
+      part: {
+        type: 'summary_text',
+        text: '',
+      },
+    });
+    expect(events[2]?.payload).toMatchObject({
+      type: 'response.reasoning_summary_text.delta',
+      item_id: 'rs_0',
+      output_index: 0,
+      summary_index: 0,
+      delta: 'Think ',
+    });
+  });
+
+  it('keeps synthetic reasoning and message output items separate when reasoning arrives before content', () => {
+    const state = createOpenAiResponsesAggregateState('gpt-5');
+    const streamContext = createStreamTransformContext('gpt-5');
+    const usage = {
+      promptTokens: 1,
+      completionTokens: 2,
+      totalTokens: 3,
+    };
+
+    serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        reasoningDelta: 'plan first',
+      } as any,
+    });
+
+    const contentLines = serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        contentDelta: 'final answer',
+      } as any,
+    });
+
+    const contentEvents = parseSseEvents(contentLines);
+    expect(contentEvents.map((entry) => entry.event)).toEqual([
+      'response.output_item.added',
+      'response.content_part.added',
+      'response.output_text.delta',
+    ]);
+    expect(contentEvents[0]?.payload).toMatchObject({
+      type: 'response.output_item.added',
+      output_index: 1,
+      item: {
+        id: 'msg_1',
+        type: 'message',
+        content: [],
+      },
+    });
+
+    const completedLines = completeResponsesStream(state, streamContext, usage);
+    const payloads = parseSsePayloads(completedLines);
+    const completed = payloads.find((item) => item.type === 'response.completed');
+    expect(completed?.response?.output).toEqual([
+      {
+        id: 'rs_0',
+        type: 'reasoning',
+        status: 'completed',
+        summary: [
+          {
+            type: 'summary_text',
+            text: 'plan first',
+          },
+        ],
+      },
+      {
+        id: 'msg_1',
+        type: 'message',
+        role: 'assistant',
+        status: 'completed',
+        content: [
+          {
+            type: 'output_text',
+            text: 'final answer',
+          },
+        ],
+      },
+    ]);
+    expect(completed?.response?.output_text).toBe('final answer');
+  });
+
+  it('keeps synthetic message and function call items separate when one event contains both', () => {
+    const state = createOpenAiResponsesAggregateState('gpt-5');
+    const streamContext = createStreamTransformContext('gpt-5');
+    const usage = {
+      promptTokens: 1,
+      completionTokens: 2,
+      totalTokens: 3,
+    };
+
+    const lines = serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        contentDelta: 'hello',
+        toolCallDeltas: [{
+          index: 0,
+          id: 'call_1',
+          name: 'Glob',
+          argumentsDelta: '{}',
+        }],
+      } as any,
+    });
+
+    const events = parseSseEvents(lines);
+    expect(events.map((entry) => entry.event)).toEqual([
+      'response.output_item.added',
+      'response.content_part.added',
+      'response.output_text.delta',
+      'response.output_item.added',
+      'response.function_call_arguments.delta',
+    ]);
+    expect(events[3]?.payload).toMatchObject({
+      type: 'response.output_item.added',
+      output_index: 1,
+      item: {
+        id: 'call_1',
+        type: 'function_call',
+        call_id: 'call_1',
+        name: 'Glob',
+        arguments: '',
+      },
+    });
+
+    const completedLines = completeResponsesStream(state, streamContext, usage);
+    const payloads = parseSsePayloads(completedLines);
+    const completed = payloads.find((item) => item.type === 'response.completed');
+    expect(completed?.response?.output).toEqual([
+      {
+        id: 'msg_0',
+        type: 'message',
+        role: 'assistant',
+        status: 'completed',
+        content: [
+          {
+            type: 'output_text',
+            text: 'hello',
+          },
+        ],
+      },
+      {
+        id: 'call_1',
+        type: 'function_call',
+        status: 'completed',
+        call_id: 'call_1',
+        name: 'Glob',
+        arguments: '{}',
+      },
+    ]);
+  });
+
+  it('backfills missing parent events before sparse native function call argument deltas', () => {
+    const state = createOpenAiResponsesAggregateState('gpt-5');
+    const streamContext = createStreamTransformContext('gpt-5');
+    const usage = {
+      promptTokens: 1,
+      completionTokens: 2,
+      totalTokens: 3,
+    };
+
+    const lines = serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.function_call_arguments.delta',
+        responsesPayload: {
+          type: 'response.function_call_arguments.delta',
+          output_index: 0,
+          call_id: 'call_sparse',
+          name: 'lookup',
+          delta: '{"q":"x"}',
+        },
+      },
+    });
+
+    const events = parseSseEvents(lines);
+    expect(events.map((entry) => entry.event)).toEqual([
+      'response.output_item.added',
+      'response.function_call_arguments.delta',
+    ]);
+    expect(events[0]?.payload).toMatchObject({
+      type: 'response.output_item.added',
+      output_index: 0,
+      item: {
+        type: 'function_call',
+        call_id: 'call_sparse',
+        name: 'lookup',
+      },
+    });
+  });
+
   it('preserves richer image_generation_call fields while aggregating progress events', () => {
     const state = createOpenAiResponsesAggregateState('gpt-5');
     const streamContext = createStreamTransformContext('gpt-5');
@@ -333,6 +573,210 @@ describe('serializeConvertedResponsesEvents', () => {
     });
   });
 
+  it('emits response.output_item.done after native image generation completion', () => {
+    const state = createOpenAiResponsesAggregateState('gpt-5');
+    const streamContext = createStreamTransformContext('gpt-5');
+    const usage = {
+      promptTokens: 1,
+      completionTokens: 2,
+      totalTokens: 3,
+    };
+
+    serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.image_generation_call.completed',
+        responsesPayload: {
+          type: 'response.image_generation_call.completed',
+          item_id: 'img_done_1',
+          output_index: 0,
+          result: 'image-final',
+        },
+      },
+    });
+
+    const lines = serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.completed',
+        responsesPayload: {
+          type: 'response.completed',
+          response: {
+            id: 'resp_image_done',
+            model: 'gpt-5',
+            usage: {
+              input_tokens: 1,
+              output_tokens: 2,
+              total_tokens: 3,
+            },
+          },
+        },
+      },
+    });
+
+    const events = parseSseEvents(lines);
+    expect(events.map((entry) => entry.event)).toEqual([
+      'response.output_item.done',
+      'response.completed',
+    ]);
+  });
+
+  it('backfills native parent lifecycle events before forwarding sparse function call argument deltas', () => {
+    const state = createOpenAiResponsesAggregateState('gpt-5');
+    const streamContext = createStreamTransformContext('gpt-5');
+    const usage = {
+      promptTokens: 1,
+      completionTokens: 2,
+      totalTokens: 3,
+    };
+
+    const lines = serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.function_call_arguments.delta',
+        responsesPayload: {
+          type: 'response.function_call_arguments.delta',
+          output_index: 0,
+          call_id: 'call_sparse_1',
+          name: 'Glob',
+          delta: '{}',
+        },
+      },
+    });
+
+    const events = parseSseEvents(lines);
+    expect(events.map((entry) => entry.event)).toEqual([
+      'response.output_item.added',
+      'response.function_call_arguments.delta',
+    ]);
+    expect(events[0]?.payload).toMatchObject({
+      type: 'response.output_item.added',
+      output_index: 0,
+      item: {
+        id: 'call_sparse_1',
+        type: 'function_call',
+        call_id: 'call_sparse_1',
+        name: 'Glob',
+        arguments: '',
+      },
+    });
+  });
+
+  it('backfills native content part openers before forwarding sparse output_text.done events', () => {
+    const state = createOpenAiResponsesAggregateState('gpt-5');
+    const streamContext = createStreamTransformContext('gpt-5');
+    const usage = {
+      promptTokens: 1,
+      completionTokens: 2,
+      totalTokens: 3,
+    };
+
+    const lines = serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.output_text.done',
+        responsesPayload: {
+          type: 'response.output_text.done',
+          output_index: 0,
+          item_id: 'msg_sparse_1',
+          text: 'hello world',
+        },
+      },
+    });
+
+    const events = parseSseEvents(lines);
+    expect(events.map((entry) => entry.event)).toEqual([
+      'response.output_item.added',
+      'response.content_part.added',
+      'response.output_text.done',
+    ]);
+    expect(events[0]?.payload).toMatchObject({
+      type: 'response.output_item.added',
+      output_index: 0,
+      item: {
+        id: 'msg_sparse_1',
+        type: 'message',
+      },
+    });
+    expect(events[1]?.payload).toMatchObject({
+      type: 'response.content_part.added',
+      output_index: 0,
+      item_id: 'msg_sparse_1',
+      content_index: 0,
+      part: {
+        type: 'output_text',
+        text: '',
+      },
+    });
+    expect(events[2]?.payload).toMatchObject({
+      type: 'response.output_text.done',
+      output_index: 0,
+      item_id: 'msg_sparse_1',
+      text: 'hello world',
+    });
+  });
+
+  it('preserves native message item ids when sparse content parts open the parent item', () => {
+    const state = createOpenAiResponsesAggregateState('gpt-5');
+    const streamContext = createStreamTransformContext('gpt-5');
+    const usage = {
+      promptTokens: 1,
+      completionTokens: 2,
+      totalTokens: 3,
+    };
+
+    const lines = serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.content_part.added',
+        responsesPayload: {
+          type: 'response.content_part.added',
+          output_index: 0,
+          item_id: 'msg_sparse_content_1',
+          content_index: 0,
+          part: {
+            type: 'output_text',
+            text: '',
+          },
+        },
+      },
+    });
+
+    const events = parseSseEvents(lines);
+    expect(events.map((entry) => entry.event)).toEqual([
+      'response.output_item.added',
+      'response.content_part.added',
+    ]);
+    expect(events[0]?.payload).toMatchObject({
+      type: 'response.output_item.added',
+      output_index: 0,
+      item: {
+        id: 'msg_sparse_content_1',
+        type: 'message',
+      },
+    });
+    expect(events[1]?.payload).toMatchObject({
+      type: 'response.content_part.added',
+      output_index: 0,
+      item_id: 'msg_sparse_content_1',
+      content_index: 0,
+      part: {
+        type: 'output_text',
+        text: '',
+      },
+    });
+  });
+
   it('emits canonical message done events before response.completed when recovering a sparse text stream', () => {
     const state = createOpenAiResponsesAggregateState('gpt-5');
     const streamContext = createStreamTransformContext('gpt-5');
@@ -675,6 +1119,94 @@ describe('serializeConvertedResponsesEvents', () => {
     });
   });
 
+  it('backfills missing native content_part.done even after output_item.done already marks the message completed', () => {
+    const state = createOpenAiResponsesAggregateState('gpt-5');
+    const streamContext = createStreamTransformContext('gpt-5');
+    const usage = {
+      promptTokens: 2,
+      completionTokens: 4,
+      totalTokens: 6,
+    };
+
+    serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.content_part.added',
+        responsesPayload: {
+          type: 'response.content_part.added',
+          output_index: 0,
+          item_id: 'msg_native_done_1',
+          content_index: 0,
+          part: {
+            type: 'output_text',
+            text: '',
+          },
+        },
+      },
+    });
+
+    serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.output_text.done',
+        responsesPayload: {
+          type: 'response.output_text.done',
+          output_index: 0,
+          item_id: 'msg_native_done_1',
+          text: 'hello world',
+        },
+      },
+    });
+
+    serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.output_item.done',
+        responsesPayload: {
+          type: 'response.output_item.done',
+          output_index: 0,
+          item: {
+            id: 'msg_native_done_1',
+            type: 'message',
+            status: 'completed',
+          },
+        },
+      },
+    });
+
+    const lines = serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.completed',
+        responsesPayload: {
+          type: 'response.completed',
+          response: {
+            id: 'resp_native_done_1',
+            model: 'gpt-5',
+            usage: {
+              input_tokens: 2,
+              output_tokens: 4,
+              total_tokens: 6,
+            },
+          },
+        },
+      },
+    });
+
+    const events = parseSseEvents(lines);
+    expect(events.map((entry) => entry.event)).toEqual([
+      'response.completed',
+    ]);
+  });
+
   it('does not synthesize duplicate reasoning done events after original reasoning completion already arrived', () => {
     const state = createOpenAiResponsesAggregateState('gpt-5');
     const streamContext = createStreamTransformContext('gpt-5');
@@ -908,6 +1440,76 @@ describe('serializeConvertedResponsesEvents', () => {
     ]);
   });
 
+  it('preserves terminal response output from sparse response.incomplete payloads', () => {
+    const state = createOpenAiResponsesAggregateState('gpt-5');
+    const streamContext = createStreamTransformContext('gpt-5');
+    const usage = {
+      promptTokens: 1,
+      completionTokens: 2,
+      totalTokens: 3,
+    };
+
+    const lines = serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.incomplete',
+        responsesPayload: {
+          type: 'response.incomplete',
+          response: {
+            id: 'resp_sparse_incomplete',
+            model: 'gpt-5',
+            status: 'incomplete',
+            output: [
+              {
+                id: 'msg_sparse_incomplete',
+                type: 'message',
+                role: 'assistant',
+                status: 'incomplete',
+                content: [
+                  {
+                    type: 'output_text',
+                    text: 'partial answer',
+                  },
+                ],
+              },
+            ],
+            usage: {
+              input_tokens: 1,
+              output_tokens: 2,
+              total_tokens: 3,
+            },
+          },
+        },
+      },
+    });
+
+    const events = parseSseEvents(lines);
+    expect(events.at(-1)?.payload).toMatchObject({
+      type: 'response.incomplete',
+      response: {
+        id: 'resp_sparse_incomplete',
+        status: 'incomplete',
+        output_text: 'partial answer',
+        output: [
+          {
+            id: 'msg_sparse_incomplete',
+            type: 'message',
+            role: 'assistant',
+            status: 'incomplete',
+            content: [
+              {
+                type: 'output_text',
+                text: 'partial answer',
+              },
+            ],
+          },
+        ],
+      },
+    });
+  });
+
   it('preserves response.incomplete as a first-class terminal event', () => {
     const state = createOpenAiResponsesAggregateState('gpt-5');
     const streamContext = createStreamTransformContext('gpt-5');
@@ -1145,6 +1747,68 @@ describe('serializeConvertedResponsesEvents', () => {
     });
   });
 
+  it('emits a generic output_item.done for native image generation items that completed earlier', () => {
+    const state = createOpenAiResponsesAggregateState('gpt-5');
+    const streamContext = createStreamTransformContext('gpt-5');
+    const usage = {
+      promptTokens: 1,
+      completionTokens: 2,
+      totalTokens: 3,
+    };
+
+    serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.image_generation_call.completed',
+        responsesPayload: {
+          type: 'response.image_generation_call.completed',
+          item_id: 'img_terminal_1',
+          output_index: 0,
+          result: 'final-image',
+        },
+      },
+    });
+
+    const lines = serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.completed',
+        responsesPayload: {
+          type: 'response.completed',
+          response: {
+            id: 'resp_image_done_1',
+            model: 'gpt-5',
+            usage: {
+              input_tokens: 1,
+              output_tokens: 2,
+              total_tokens: 3,
+            },
+          },
+        },
+      },
+    });
+
+    const events = parseSseEvents(lines);
+    expect(events.map((entry) => entry.event)).toEqual([
+      'response.output_item.done',
+      'response.completed',
+    ]);
+    expect(events[0]?.payload).toMatchObject({
+      type: 'response.output_item.done',
+      output_index: 0,
+      item: {
+        id: 'img_terminal_1',
+        type: 'image_generation_call',
+        status: 'completed',
+        result: 'final-image',
+      },
+    });
+  });
+
   it('does not synthesize duplicate tool input done events after original tool completion already arrived', () => {
     const state = createOpenAiResponsesAggregateState('gpt-5');
     const streamContext = createStreamTransformContext('gpt-5');
@@ -1240,4 +1904,197 @@ describe('serializeConvertedResponsesEvents', () => {
       '[DONE]',
     ]);
   });
+
+  it('keeps synthetic reasoning and message items separate when reasoning arrives before visible text', () => {
+    const state = createOpenAiResponsesAggregateState('gpt-5');
+    const streamContext = createStreamTransformContext('gpt-5');
+    const usage = {
+      promptTokens: 1,
+      completionTokens: 2,
+      totalTokens: 3,
+    };
+
+    serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        reasoningDelta: 'think first',
+      } as any,
+    });
+
+    const messageLines = serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        contentDelta: 'visible answer',
+      } as any,
+    });
+
+    const messageEvents = parseSseEvents(messageLines);
+    expect(messageEvents[0]?.payload).toMatchObject({
+      type: 'response.output_item.added',
+      item: {
+        type: 'message',
+      },
+    });
+
+    const completionLines = completeResponsesStream(state, streamContext, usage);
+    const completionPayloads = parseSsePayloads(completionLines);
+    const completed = completionPayloads.find((item) => item.type === 'response.completed');
+    expect(completed?.response?.output).toHaveLength(2);
+    expect(completed?.response?.output?.find((item: any) => item.type === 'reasoning')).toMatchObject({
+      type: 'reasoning',
+      summary: [{ type: 'summary_text', text: 'think first' }],
+    });
+    expect(completed?.response?.output?.find((item: any) => item.type === 'message')).toMatchObject({
+      type: 'message',
+      content: [{ type: 'output_text', text: 'visible answer' }],
+    });
+    expect(completed?.response?.output_text).toBe('visible answer');
+  });
+
+  it('keeps synthetic message and tool call items separate when content and tool deltas arrive together', () => {
+    const state = createOpenAiResponsesAggregateState('gpt-5');
+    const streamContext = createStreamTransformContext('gpt-5');
+    const usage = {
+      promptTokens: 1,
+      completionTokens: 2,
+      totalTokens: 3,
+    };
+
+    serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        contentDelta: 'hi',
+        toolCallDeltas: [{
+          index: 0,
+          id: 'call_1',
+          name: 'Glob',
+          argumentsDelta: '{"pattern":"README*"}',
+        }],
+      } as any,
+    });
+
+    const completionLines = completeResponsesStream(state, streamContext, usage);
+    const completionPayloads = parseSsePayloads(completionLines);
+    const completed = completionPayloads.find((item) => item.type === 'response.completed');
+    expect(completed?.response?.output).toHaveLength(2);
+    expect(completed?.response?.output?.find((item: any) => item.type === 'message')).toMatchObject({
+      type: 'message',
+      content: [{ type: 'output_text', text: 'hi' }],
+    });
+    expect(completed?.response?.output?.find((item: any) => item.type === 'function_call')).toMatchObject({
+      type: 'function_call',
+      call_id: 'call_1',
+      name: 'Glob',
+      arguments: '{"pattern":"README*"}',
+    });
+  });
+
+  it('backfills missing output_item.added before sparse native function_call argument events', () => {
+    const state = createOpenAiResponsesAggregateState('gpt-5');
+    const streamContext = createStreamTransformContext('gpt-5');
+    const usage = {
+      promptTokens: 1,
+      completionTokens: 2,
+      totalTokens: 3,
+    };
+
+    const lines = serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.function_call_arguments.delta',
+        responsesPayload: {
+          type: 'response.function_call_arguments.delta',
+          output_index: 0,
+          call_id: 'call_sparse_1',
+          name: 'Glob',
+          delta: '{"pattern":"README*"}',
+        },
+      },
+    });
+
+    const events = parseSseEvents(lines);
+    expect(events.map((entry) => entry.event)).toEqual([
+      'response.output_item.added',
+      'response.function_call_arguments.delta',
+    ]);
+    expect(events[0]?.payload).toMatchObject({
+      type: 'response.output_item.added',
+      output_index: 0,
+      item: {
+        type: 'function_call',
+        call_id: 'call_sparse_1',
+        name: 'Glob',
+      },
+    });
+  });
+
+  it('emits a generic output_item.done for native image_generation completion before response.completed', () => {
+    const state = createOpenAiResponsesAggregateState('gpt-5');
+    const streamContext = createStreamTransformContext('gpt-5');
+    const usage = {
+      promptTokens: 1,
+      completionTokens: 2,
+      totalTokens: 3,
+    };
+
+    serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.image_generation_call.completed',
+        responsesPayload: {
+          type: 'response.image_generation_call.completed',
+          output_index: 0,
+          item_id: 'img_1',
+          result: 'image-result',
+        },
+      },
+    });
+
+    const lines = serializeConvertedResponsesEvents({
+      state,
+      streamContext,
+      usage,
+      event: {
+        responsesEventType: 'response.completed',
+        responsesPayload: {
+          type: 'response.completed',
+          response: {
+            id: 'resp_image_1',
+            model: 'gpt-5',
+            usage: {
+              input_tokens: 1,
+              output_tokens: 2,
+              total_tokens: 3,
+            },
+          },
+        },
+      },
+    });
+
+    const events = parseSseEvents(lines);
+    expect(events.map((entry) => entry.event)).toEqual([
+      'response.output_item.done',
+      'response.completed',
+    ]);
+    expect(events[0]?.payload).toMatchObject({
+      type: 'response.output_item.done',
+      output_index: 0,
+      item: {
+        id: 'img_1',
+        type: 'image_generation_call',
+        status: 'completed',
+        result: 'image-result',
+      },
+    });
+  });
 });
diff --git a/src/server/transformers/openai/responses/aggregator.ts b/src/server/transformers/openai/responses/aggregator.ts
index fde6662..dfa1b50 100644
--- a/src/server/transformers/openai/responses/aggregator.ts
+++ b/src/server/transformers/openai/responses/aggregator.ts
@@ -63,6 +63,7 @@ const reasoningSummaryTextDoneMarker = Symbol('responses.reasoning_summary_text.
 const reasoningSummaryPartDoneMarker = Symbol('responses.reasoning_summary_part.done');
 const functionCallArgumentsDoneMarker = Symbol('responses.function_call_arguments.done');
 const customToolInputDoneMarker = Symbol('responses.custom_tool_call_input.done');
+const outputItemDoneMarker = Symbol('responses.output_item.done');
 
 export type OpenAiResponsesAggregateState = {
   modelName: string;
@@ -145,11 +146,147 @@ function rememberOutputId(state: OpenAiResponsesAggregateState, index: number, i
   }
 }
 
+function preserveTerminalMarkers(source: AggregateOutputItem | undefined, target: AggregateOutputItem): void {
+  if (!isRecord(source)) return;
+
+  const itemMarkers = [
+    outputItemDoneMarker,
+    functionCallArgumentsDoneMarker,
+    customToolInputDoneMarker,
+  ];
+  for (const marker of itemMarkers) {
+    if (hasTerminalMarker(source, marker)) {
+      markTerminalMarker(target, marker);
+    }
+  }
+
+  const sourceContent = Array.isArray(source.content) ? source.content : [];
+  const targetContent = Array.isArray(target.content) ? target.content : [];
+  for (let index = 0; index < Math.min(sourceContent.length, targetContent.length); index += 1) {
+    const sourcePart = sourceContent[index];
+    const targetPart = targetContent[index];
+    if (!isRecord(sourcePart) || !isRecord(targetPart)) continue;
+    if (hasTerminalMarker(sourcePart, outputTextDoneMarker)) {
+      markTerminalMarker(targetPart, outputTextDoneMarker);
+    }
+    if (hasTerminalMarker(sourcePart, contentPartDoneMarker)) {
+      markTerminalMarker(targetPart, contentPartDoneMarker);
+    }
+  }
+
+  const sourceSummary = Array.isArray(source.summary) ? source.summary : [];
+  const targetSummary = Array.isArray(target.summary) ? target.summary : [];
+  for (let index = 0; index < Math.min(sourceSummary.length, targetSummary.length); index += 1) {
+    const sourcePart = sourceSummary[index];
+    const targetPart = targetSummary[index];
+    if (!isRecord(sourcePart) || !isRecord(targetPart)) continue;
+    if (hasTerminalMarker(sourcePart, reasoningSummaryTextDoneMarker)) {
+      markTerminalMarker(targetPart, reasoningSummaryTextDoneMarker);
+    }
+    if (hasTerminalMarker(sourcePart, reasoningSummaryPartDoneMarker)) {
+      markTerminalMarker(targetPart, reasoningSummaryPartDoneMarker);
+    }
+  }
+}
+
+function isTerminalStatus(value: unknown): boolean {
+  const status = asTrimmedString(value).toLowerCase();
+  return status === 'completed' || status === 'failed' || status === 'incomplete';
+}
+
+function isOutputItemType(item: unknown, expectedType: string): boolean {
+  return isRecord(item) && asTrimmedString(item.type).toLowerCase() === expectedType;
+}
+
+function resolveCompatibleOutputIndex(
+  state: OpenAiResponsesAggregateState,
+  expectedType: string,
+  rawIndex: unknown,
+  ...candidateIds: Array<unknown>
+): number {
+  const resolved = resolveOutputIndex(state, rawIndex, ...candidateIds);
+  const existing = state.outputItems[resolved];
+  if (!existing || isOutputItemType(existing, expectedType)) return resolved;
+  return state.outputItems.length;
+}
+
+function snapshotOutputItemForAdded(item: AggregateOutputItem): AggregateOutputItem {
+  const next = cloneJson(item);
+  const itemType = asTrimmedString(next.type).toLowerCase();
+
+  if (itemType === 'message') {
+    next.content = [];
+  } else if (itemType === 'reasoning') {
+    next.summary = [];
+  } else if (itemType === 'function_call') {
+    next.arguments = '';
+  } else if (itemType === 'custom_tool_call') {
+    next.input = '';
+  } else if (itemType === 'image_generation_call') {
+    next.result = null;
+    next.partial_images = [];
+  }
+
+  next.status = 'in_progress';
+  return next;
+}
+
+function serializeOutputItemAdded(index: number, item: AggregateOutputItem): string {
+  return serializeSse('response.output_item.added', {
+    type: 'response.output_item.added',
+    output_index: index,
+    item: snapshotOutputItemForAdded(item),
+  });
+}
+
+function snapshotPartForAdded(part: AggregateOutputItem): AggregateOutputItem {
+  const next = cloneJson(part);
+  const partType = asTrimmedString(next.type).toLowerCase();
+  if (
+    (partType === 'output_text' || partType === 'text' || partType === 'summary_text')
+    && typeof next.text === 'string'
+  ) {
+    next.text = '';
+  }
+  return next;
+}
+
+function serializeContentPartAdded(
+  outputIndex: number,
+  itemId: string,
+  contentIndex: number,
+  part: AggregateOutputItem,
+): string {
+  return serializeSse('response.content_part.added', {
+    type: 'response.content_part.added',
+    output_index: outputIndex,
+    item_id: itemId,
+    content_index: contentIndex,
+    part: snapshotPartForAdded(part),
+  });
+}
+
+function serializeReasoningSummaryPartAdded(
+  outputIndex: number,
+  itemId: string,
+  summaryIndex: number,
+  part: AggregateOutputItem,
+): string {
+  return serializeSse('response.reasoning_summary_part.added', {
+    type: 'response.reasoning_summary_part.added',
+    item_id: itemId,
+    output_index: outputIndex,
+    summary_index: summaryIndex,
+    part: snapshotPartForAdded(part),
+  });
+}
+
 function setOutputItem(
   state: OpenAiResponsesAggregateState,
   index: number,
   item: AggregateOutputItem,
 ): AggregateOutputItem {
+  const existing = isRecord(state.outputItems[index]) ? state.outputItems[index] as AggregateOutputItem : undefined;
   const current = cloneRecord(state.outputItems[index]) || {};
   const incoming = cloneJson(item);
   const next = {
@@ -168,6 +305,7 @@ function setOutputItem(
   ) {
     next.partial_images = current.partial_images;
   }
+  preserveTerminalMarkers(existing, next);
   state.outputItems[index] = next;
   rememberOutputId(state, index, next);
   return next;
@@ -200,34 +338,53 @@ function resolveOutputIndex(
   return state.outputItems.length;
 }
 
-function ensureMessageItem(state: OpenAiResponsesAggregateState, indexHint?: number): { index: number; item: AggregateOutputItem } {
-  const index = state.messageIndex ?? indexHint ?? 0;
+function ensureMessageItem(
+  state: OpenAiResponsesAggregateState,
+  indexHint?: number,
+  itemIdRaw?: unknown,
+): { index: number; item: AggregateOutputItem; created: boolean } {
+  const preferredIndex = state.messageIndex ?? indexHint ?? 0;
+  const itemId = asTrimmedString(itemIdRaw);
+  const index = resolveCompatibleOutputIndex(state, 'message', preferredIndex);
+  const created = !state.outputItems[index];
   const item = ensureOutputItem(state, index, () => ({
-    id: ensureOutputItemId('', 'msg', index),
+    id: ensureOutputItemId(itemId, 'msg', index),
     type: 'message',
     role: 'assistant',
     status: 'in_progress',
     content: [],
   }));
+  if (itemId) {
+    item.id = ensureOutputItemId(itemId, 'msg', index);
+    rememberOutputId(state, index, item);
+  }
   if (!Array.isArray(item.content)) item.content = [];
-  if (state.messageIndex === null) state.messageIndex = index;
-  return { index, item };
+  state.messageIndex = index;
+  return { index, item, created };
 }
 
 function ensureMessageOutputTextPart(
   state: OpenAiResponsesAggregateState,
   indexHint?: number,
-): { index: number; item: AggregateOutputItem; part: AggregateOutputItem; created: boolean } {
-  const { index, item } = ensureMessageItem(state, indexHint);
+  itemIdRaw?: unknown,
+): {
+  index: number;
+  item: AggregateOutputItem;
+  part: AggregateOutputItem;
+  created: boolean;
+  itemCreated: boolean;
+  partCreated: boolean;
+} {
+  const { index, item, created: itemCreated } = ensureMessageItem(state, indexHint, itemIdRaw);
   const content = Array.isArray(item.content) ? item.content as AggregateOutputItem[] : [];
   if (!Array.isArray(item.content)) item.content = content;
   let part = content[0];
-  const created = !isRecord(part) || asTrimmedString(part.type).toLowerCase() !== 'output_text';
-  if (created) {
+  const partCreated = !isRecord(part) || asTrimmedString(part.type).toLowerCase() !== 'output_text';
+  if (partCreated) {
     part = { type: 'output_text', text: '' };
     content[0] = part;
   }
-  return { index, item, part, created };
+  return { index, item, part, created: itemCreated || partCreated, itemCreated, partCreated };
 }
 
 function ensureReasoningItem(
@@ -239,7 +396,9 @@ function ensureReasoningItem(
   const existingIndex = itemId
     ? state.reasoningIndexById[itemId]
     : Object.values(state.reasoningIndexById)[0];
-  const index = existingIndex ?? resolveOutputIndex(state, indexHint, itemId);
+  const index = existingIndex !== undefined
+    ? resolveCompatibleOutputIndex(state, 'reasoning', existingIndex, itemId)
+    : resolveCompatibleOutputIndex(state, 'reasoning', indexHint, itemId);
   const created = !state.outputItems[index];
   const item = ensureOutputItem(state, index, () => ({
     id: ensureOutputItemId(itemId, 'rs', index),
@@ -297,7 +456,9 @@ function ensureFunctionCallItem(
   const callId = asTrimmedString(callIdRaw);
   const name = asTrimmedString(nameRaw);
   const existingIndex = callId ? state.functionIndexById[callId] : undefined;
-  const index = existingIndex ?? resolveOutputIndex(state, indexHint, callId);
+  const index = existingIndex !== undefined
+    ? resolveCompatibleOutputIndex(state, 'function_call', existingIndex, callId)
+    : resolveCompatibleOutputIndex(state, 'function_call', indexHint, callId);
   const created = !state.outputItems[index];
   const item = ensureOutputItem(state, index, () => ({
     id: ensureOutputItemId(callId, 'fc', index),
@@ -326,7 +487,9 @@ function ensureCustomToolItem(
   const existingIndex = (callId && state.customToolIndexById[callId] !== undefined)
     ? state.customToolIndexById[callId]
     : (itemId && state.customToolIndexById[itemId] !== undefined ? state.customToolIndexById[itemId] : undefined);
-  const index = existingIndex ?? resolveOutputIndex(state, indexHint, itemId, callId);
+  const index = existingIndex !== undefined
+    ? resolveCompatibleOutputIndex(state, 'custom_tool_call', existingIndex, itemId, callId)
+    : resolveCompatibleOutputIndex(state, 'custom_tool_call', indexHint, itemId, callId);
   const created = !state.outputItems[index];
   const item = ensureOutputItem(state, index, () => ({
     id: ensureOutputItemId(itemId, 'ct', index),
@@ -349,7 +512,9 @@ function ensureImageGenerationItem(
 ): { index: number; item: AggregateOutputItem; created: boolean } {
   const itemId = asTrimmedString(itemIdRaw);
   const existingIndex = itemId ? state.imageGenerationIndexById[itemId] : undefined;
-  const index = existingIndex ?? resolveOutputIndex(state, indexHint, itemId);
+  const index = existingIndex !== undefined
+    ? resolveCompatibleOutputIndex(state, 'image_generation_call', existingIndex, itemId)
+    : resolveCompatibleOutputIndex(state, 'image_generation_call', indexHint, itemId);
   const created = !state.outputItems[index];
   const item = ensureOutputItem(state, index, () => ({
     id: ensureOutputItemId(itemId, 'img', index),
@@ -394,6 +559,28 @@ function collectOutputText(state: OpenAiResponsesAggregateState): string {
   return parts.join('');
 }
 
+function hydrateStateFromTerminalResponseOutput(
+  state: OpenAiResponsesAggregateState,
+  responsePayload: Record<string, unknown> | null | undefined,
+): void {
+  if (!isRecord(responsePayload) || !Array.isArray(responsePayload.output)) return;
+
+  for (let index = 0; index < responsePayload.output.length; index += 1) {
+    const item = responsePayload.output[index];
+    if (!isRecord(item)) continue;
+    const itemType = asTrimmedString(item.type).toLowerCase();
+    if (!itemType) continue;
+    const resolvedIndex = resolveCompatibleOutputIndex(
+      state,
+      itemType,
+      index,
+      item.id,
+      item.call_id,
+    );
+    setOutputItem(state, resolvedIndex, cloneJson(item));
+  }
+}
+
 function materializeResponse(
   state: OpenAiResponsesAggregateState,
   streamContext: StreamTransformContext,
@@ -413,7 +600,7 @@ function materializeResponse(
       ? base.created_at
       : (typeof base.created === 'number' && Number.isFinite(base.created) ? base.created : null)
   ) ?? state.createdAt ?? Math.floor(Date.now() / 1000);
-  const output = state.outputItems
+  const aggregatedOutput = state.outputItems
     .filter((item): item is AggregateOutputItem => isRecord(item))
     .map((item) => {
       const status = asTrimmedString(item.status).toLowerCase();
@@ -424,6 +611,11 @@ function materializeResponse(
           : (state.failed ? 'failed' : state.incomplete ? 'incomplete' : 'completed'),
       };
     });
+  const output = aggregatedOutput.length > 0
+    ? aggregatedOutput
+    : (Array.isArray(base.output) ? cloneJson(base.output) : []);
+  const outputText = collectOutputText(state)
+    || (typeof base.output_text === 'string' ? base.output_text : '');
 
   return {
     ...base,
@@ -433,7 +625,7 @@ function materializeResponse(
     status: statusOverride ?? (state.failed ? 'failed' : state.incomplete ? 'incomplete' : 'completed'),
     model: asTrimmedString(base.model) || streamContext.model || state.modelName,
     output,
-    output_text: collectOutputText(state),
+    output_text: outputText,
     usage: buildUsagePayload(usage),
     ...Object.keys(state.usageExtras).length > 0 ? { usage: { ...buildUsagePayload(usage), ...state.usageExtras } } : {},
   };
@@ -497,6 +689,11 @@ function applyOriginalResponsesPayload(
         const itemType = asTrimmedString(item.type).toLowerCase();
         if (itemType === 'reasoning') {
           const reasoningState = ensureReasoningItem(state, item.id, outputIndex);
+          const preservedSummary = (
+            (!Array.isArray(item.summary) || item.summary.length === 0) && Array.isArray(reasoningState.item.summary)
+          )
+            ? reasoningState.item.summary
+            : null;
           const next = {
             ...reasoningState.item,
             ...item,
@@ -504,15 +701,24 @@ function applyOriginalResponsesPayload(
           if (eventType === 'response.output_item.done' && !asTrimmedString(next.status)) {
             next.status = 'completed';
           }
-          if ((!Array.isArray(item.summary) || item.summary.length === 0) && Array.isArray(reasoningState.item.summary)) {
-            next.summary = reasoningState.item.summary;
+          if (preservedSummary) {
+            next.summary = preservedSummary;
           }
-          setOutputItem(state, reasoningState.index, next);
-          return serializeOriginalResponsesEvent(eventType, {
-            ...payload,
-            output_index: reasoningState.index,
-            item: state.outputItems[reasoningState.index] ?? payload.item,
-          });
+          const stored = setOutputItem(state, reasoningState.index, next);
+          if (preservedSummary) {
+            stored.summary = preservedSummary;
+          }
+          if (eventType === 'response.output_item.done') {
+            markTerminalMarker(stored, outputItemDoneMarker);
+          }
+          return [
+            ...(eventType === 'response.output_item.done' ? buildMissingSubordinateDoneEventsForItem(stored, reasoningState.index) : []),
+            ...serializeOriginalResponsesEvent(eventType, {
+              ...payload,
+              output_index: reasoningState.index,
+              item: state.outputItems[reasoningState.index] ?? payload.item,
+            }),
+          ];
         }
         const existing = isRecord(state.outputItems[outputIndex]) ? cloneRecord(state.outputItems[outputIndex]) || {} : {};
         const next = {
@@ -522,33 +728,60 @@ function applyOriginalResponsesPayload(
         if (eventType === 'response.output_item.done' && !asTrimmedString(next.status)) {
           next.status = 'completed';
         }
-        if ((!Array.isArray(item.content) || item.content.length === 0) && Array.isArray(existing.content)) {
-          next.content = existing.content;
+        const preservedContent = (
+          (!Array.isArray(item.content) || item.content.length === 0) && Array.isArray(existing.content)
+        )
+          ? existing.content
+          : null;
+        const preservedSummary = (
+          (!Array.isArray(item.summary) || item.summary.length === 0) && Array.isArray(existing.summary)
+        )
+          ? existing.summary
+          : null;
+        const preservedPartialImages = (
+          (!Array.isArray(item.partial_images) || item.partial_images.length === 0) && Array.isArray(existing.partial_images)
+        )
+          ? existing.partial_images
+          : null;
+        if (preservedContent) next.content = preservedContent;
+        if (preservedSummary) next.summary = preservedSummary;
+        if (preservedPartialImages) next.partial_images = preservedPartialImages;
+        const stored = setOutputItem(state, outputIndex, next);
+        if (preservedContent) stored.content = preservedContent;
+        if (preservedSummary) stored.summary = preservedSummary;
+        if (preservedPartialImages) stored.partial_images = preservedPartialImages;
+        if (eventType === 'response.output_item.done') {
+          markTerminalMarker(stored, outputItemDoneMarker);
         }
-        if ((!Array.isArray(item.summary) || item.summary.length === 0) && Array.isArray(existing.summary)) {
-          next.summary = existing.summary;
-        }
-        if ((!Array.isArray(item.partial_images) || item.partial_images.length === 0) && Array.isArray(existing.partial_images)) {
-          next.partial_images = existing.partial_images;
-        }
-        setOutputItem(state, outputIndex, next);
       }
-      return serializeOriginalResponsesEvent(eventType, {
-        ...payload,
-        item: state.outputItems[outputIndex] ?? payload.item,
-      });
+      const stored = state.outputItems[outputIndex];
+      return [
+        ...(eventType === 'response.output_item.done' && isRecord(stored)
+          ? buildMissingSubordinateDoneEventsForItem(stored, outputIndex)
+          : []),
+        ...serializeOriginalResponsesEvent(eventType, {
+          ...payload,
+          item: state.outputItems[outputIndex] ?? payload.item,
+        }),
+      ];
     }
     case 'response.content_part.added':
     case 'response.content_part.done': {
-      const outputIndex = resolveOutputIndex(state, payload.output_index, payload.item_id);
+      const lines: string[] = [];
+      const requestedOutputIndex = resolveCompatibleOutputIndex(state, 'message', payload.output_index, payload.item_id);
       const contentIndex = typeof payload.content_index === 'number' && Number.isFinite(payload.content_index)
         ? Math.max(0, Math.trunc(payload.content_index))
         : 0;
-      const message = ensureMessageItem(state, outputIndex).item;
+      const messageState = ensureMessageItem(state, requestedOutputIndex, payload.item_id);
+      const message = messageState.item;
+      if (messageState.created) {
+        lines.push(serializeOutputItemAdded(messageState.index, message));
+      }
       const content = Array.isArray(message.content) ? message.content as AggregateOutputItem[] : [];
       if (!Array.isArray(message.content)) message.content = content;
       const existingPart = isRecord(content[contentIndex]) ? content[contentIndex] as AggregateOutputItem : null;
       const incomingPart = cloneRecord(payload.part);
+      const partCreated = !existingPart && !!incomingPart;
       if (incomingPart) {
         content[contentIndex] = existingPart
           ? {
@@ -558,59 +791,121 @@ function applyOriginalResponsesPayload(
           : incomingPart;
       }
       const part = isRecord(content[contentIndex]) ? content[contentIndex] as AggregateOutputItem : existingPart;
+      if (eventType === 'response.content_part.done' && partCreated && part) {
+        lines.push(serializeContentPartAdded(messageState.index, asTrimmedString(message.id), contentIndex, part));
+      }
       if (eventType === 'response.content_part.done' && part) {
         markTerminalMarker(part, contentPartDoneMarker);
       }
-      return serializeOriginalResponsesEvent(eventType, payload);
+      return [
+        ...lines,
+        ...serializeOriginalResponsesEvent(eventType, {
+          ...payload,
+          output_index: messageState.index,
+          item_id: asTrimmedString(message.id) || payload.item_id,
+        }),
+      ];
     }
     case 'response.output_text.delta':
     case 'response.output_text.done': {
-      const outputIndex = resolveOutputIndex(state, payload.output_index, payload.item_id);
-      const textPart = ensureMessageOutputTextPart(state, outputIndex);
+      const lines: string[] = [];
+      const outputIndex = resolveCompatibleOutputIndex(state, 'message', payload.output_index, payload.item_id);
+      const textPart = ensureMessageOutputTextPart(state, outputIndex, payload.item_id);
+      if (textPart.itemCreated) {
+        lines.push(serializeOutputItemAdded(textPart.index, textPart.item));
+      }
+      if (textPart.partCreated) {
+        lines.push(serializeContentPartAdded(textPart.index, asTrimmedString(textPart.item.id), 0, textPart.part));
+      }
       if (eventType === 'response.output_text.done') {
         textPart.part.text = typeof payload.text === 'string' ? payload.text : String(payload.text ?? '');
         markTerminalMarker(textPart.part, outputTextDoneMarker);
       } else {
         textPart.part.text = `${typeof textPart.part.text === 'string' ? textPart.part.text : ''}${typeof payload.delta === 'string' ? payload.delta : ''}`;
       }
-      return serializeOriginalResponsesEvent(eventType, payload);
+      return [
+        ...lines,
+        ...serializeOriginalResponsesEvent(eventType, {
+          ...payload,
+          output_index: textPart.index,
+          item_id: asTrimmedString(textPart.item.id) || payload.item_id,
+        }),
+      ];
     }
     case 'response.function_call_arguments.delta':
     case 'response.function_call_arguments.done': {
+      const lines: string[] = [];
       const entry = ensureFunctionCallItem(
         state,
         payload.call_id ?? payload.item_id,
         payload.name,
-        resolveOutputIndex(state, payload.output_index, payload.item_id, payload.call_id),
+        resolveCompatibleOutputIndex(state, 'function_call', payload.output_index, payload.item_id, payload.call_id),
       );
+      if (entry.created) {
+        lines.push(serializeOutputItemAdded(entry.index, entry.item));
+      }
       if (eventType === 'response.function_call_arguments.done') {
         entry.item.arguments = typeof payload.arguments === 'string' ? payload.arguments : String(payload.arguments ?? '');
         markTerminalMarker(entry.item, functionCallArgumentsDoneMarker);
       } else {
         entry.item.arguments = `${typeof entry.item.arguments === 'string' ? entry.item.arguments : ''}${typeof payload.delta === 'string' ? payload.delta : ''}`;
       }
-      return serializeOriginalResponsesEvent(eventType, payload);
+      return [
+        ...lines,
+        ...serializeOriginalResponsesEvent(eventType, {
+          ...payload,
+          output_index: entry.index,
+          item_id: asTrimmedString(entry.item.id) || payload.item_id,
+          call_id: asTrimmedString(entry.item.call_id) || payload.call_id,
+          ...(asTrimmedString(entry.item.name) ? { name: entry.item.name } : {}),
+        }),
+      ];
     }
     case 'response.custom_tool_call_input.delta':
     case 'response.custom_tool_call_input.done': {
+      const lines: string[] = [];
       const entry = ensureCustomToolItem(
         state,
         payload.item_id,
         payload.call_id,
         payload.name,
-        resolveOutputIndex(state, payload.output_index, payload.item_id, payload.call_id),
+        resolveCompatibleOutputIndex(state, 'custom_tool_call', payload.output_index, payload.item_id, payload.call_id),
       );
+      if (entry.created) {
+        lines.push(serializeOutputItemAdded(entry.index, entry.item));
+      }
       if (eventType === 'response.custom_tool_call_input.done') {
         entry.item.input = typeof payload.input === 'string' ? payload.input : String(payload.input ?? '');
         markTerminalMarker(entry.item, customToolInputDoneMarker);
       } else {
         entry.item.input = `${typeof entry.item.input === 'string' ? entry.item.input : ''}${typeof payload.delta === 'string' ? payload.delta : ''}`;
       }
-      return serializeOriginalResponsesEvent(eventType, payload);
+      return [
+        ...lines,
+        ...serializeOriginalResponsesEvent(eventType, {
+          ...payload,
+          output_index: entry.index,
+          item_id: asTrimmedString(entry.item.id) || payload.item_id,
+          call_id: asTrimmedString(entry.item.call_id) || payload.call_id,
+          ...(asTrimmedString(entry.item.name) ? { name: entry.item.name } : {}),
+        }),
+      ];
     }
     case 'response.reasoning_summary_part.added':
     case 'response.reasoning_summary_part.done': {
+      const lines: string[] = [];
       const summaryState = ensureReasoningSummaryPart(state, payload.item_id, payload.summary_index, payload.output_index);
+      if (summaryState.itemCreated) {
+        lines.push(serializeOutputItemAdded(summaryState.index, summaryState.item));
+      }
+      if (summaryState.partCreated && eventType === 'response.reasoning_summary_part.done') {
+        lines.push(serializeReasoningSummaryPartAdded(
+          summaryState.index,
+          asTrimmedString(summaryState.item.id),
+          summaryState.summaryIndex,
+          summaryState.summary,
+        ));
+      }
       const part = cloneRecord(payload.part);
       if (part) {
         const summary = Array.isArray(summaryState.item.summary) ? summaryState.item.summary as AggregateOutputItem[] : [];
@@ -625,28 +920,58 @@ function applyOriginalResponsesPayload(
       } else if (eventType === 'response.reasoning_summary_part.done') {
         markTerminalMarker(summaryState.summary, reasoningSummaryPartDoneMarker);
       }
-      return serializeOriginalResponsesEvent(eventType, payload);
+      return [
+        ...lines,
+        ...serializeOriginalResponsesEvent(eventType, {
+          ...payload,
+          output_index: summaryState.index,
+          item_id: asTrimmedString(summaryState.item.id) || payload.item_id,
+        }),
+      ];
     }
     case 'response.reasoning_summary_text.delta':
     case 'response.reasoning_summary_text.done': {
+      const lines: string[] = [];
       const summaryState = ensureReasoningSummaryPart(state, payload.item_id, payload.summary_index, payload.output_index);
+      if (summaryState.itemCreated) {
+        lines.push(serializeOutputItemAdded(summaryState.index, summaryState.item));
+      }
+      if (summaryState.partCreated) {
+        lines.push(serializeReasoningSummaryPartAdded(
+          summaryState.index,
+          asTrimmedString(summaryState.item.id),
+          summaryState.summaryIndex,
+          summaryState.summary,
+        ));
+      }
       if (eventType === 'response.reasoning_summary_text.done') {
         summaryState.summary.text = typeof payload.text === 'string' ? payload.text : String(payload.text ?? '');
         markTerminalMarker(summaryState.summary, reasoningSummaryTextDoneMarker);
       } else {
         summaryState.summary.text = `${typeof summaryState.summary.text === 'string' ? summaryState.summary.text : ''}${typeof payload.delta === 'string' ? payload.delta : ''}`;
       }
-      return serializeOriginalResponsesEvent(eventType, payload);
+      return [
+        ...lines,
+        ...serializeOriginalResponsesEvent(eventType, {
+          ...payload,
+          output_index: summaryState.index,
+          item_id: asTrimmedString(summaryState.item.id) || payload.item_id,
+        }),
+      ];
     }
     case 'response.image_generation_call.generating':
     case 'response.image_generation_call.in_progress':
     case 'response.image_generation_call.partial_image':
     case 'response.image_generation_call.completed': {
+      const lines: string[] = [];
       const entry = ensureImageGenerationItem(
         state,
         payload.item_id,
-        resolveOutputIndex(state, payload.output_index, payload.item_id),
+        resolveCompatibleOutputIndex(state, 'image_generation_call', payload.output_index, payload.item_id),
       );
+      if (entry.created) {
+        lines.push(serializeOutputItemAdded(entry.index, entry.item));
+      }
       if (eventType === 'response.image_generation_call.partial_image') {
         const partialImages = Array.isArray(entry.item.partial_images) ? entry.item.partial_images as AggregateOutputItem[] : [];
         partialImages.push({
@@ -662,13 +987,21 @@ function applyOriginalResponsesPayload(
       if (eventType === 'response.image_generation_call.completed') {
         entry.item.status = 'completed';
       }
-      return serializeOriginalResponsesEvent(eventType, payload);
+      return [
+        ...lines,
+        ...serializeOriginalResponsesEvent(eventType, {
+          ...payload,
+          output_index: entry.index,
+          item_id: asTrimmedString(entry.item.id) || payload.item_id,
+        }),
+      ];
     }
     case 'response.completed': {
       mergeUsageExtras(state, payload.response && isRecord(payload.response) ? payload.response.usage : payload.usage);
+      const responsePayload = cloneRecord(payload.response);
+      hydrateStateFromTerminalResponseOutput(state, responsePayload);
       const terminalLines = buildSyntheticTerminalItemDoneEvents(state, 'completed');
       state.completed = true;
-      const responsePayload = cloneRecord(payload.response);
       const materialized = materializeResponse(state, streamContext, usage, responsePayload, 'completed');
       return [
         ...terminalLines,
@@ -677,9 +1010,10 @@ function applyOriginalResponsesPayload(
     }
     case 'response.failed': {
       mergeUsageExtras(state, payload.response && isRecord(payload.response) ? payload.response.usage : payload.usage);
+      const responsePayload = cloneRecord(payload.response);
+      hydrateStateFromTerminalResponseOutput(state, responsePayload);
       const terminalLines = buildSyntheticTerminalItemDoneEvents(state, 'failed');
       state.failed = true;
-      const responsePayload = cloneRecord(payload.response);
       const materialized = materializeResponse(state, streamContext, usage, responsePayload, 'failed');
       return [
         ...terminalLines,
@@ -688,9 +1022,10 @@ function applyOriginalResponsesPayload(
     }
     case 'response.incomplete': {
       mergeUsageExtras(state, payload.response && isRecord(payload.response) ? payload.response.usage : payload.usage);
+      const responsePayload = cloneRecord(payload.response);
+      hydrateStateFromTerminalResponseOutput(state, responsePayload);
       const terminalLines = buildSyntheticTerminalItemDoneEvents(state, 'incomplete');
       state.incomplete = true;
-      const responsePayload = cloneRecord(payload.response);
       const materialized = materializeResponse(state, streamContext, usage, responsePayload, 'incomplete');
       return [
         ...terminalLines,
@@ -707,31 +1042,23 @@ function buildSyntheticMessageEvents(
   state: OpenAiResponsesAggregateState,
   delta: string,
 ): string[] {
-  const { index, item } = ensureMessageItem(state);
-  const textPartState = ensureMessageOutputTextPart(state, index);
+  const textPartState = ensureMessageOutputTextPart(state);
+  const { index } = textPartState;
   const lines: string[] = [];
   const currentText = typeof textPartState.part.text === 'string' ? textPartState.part.text : '';
   const novelDelta = computeNovelDelta(currentText, delta);
-  if (textPartState.created) {
-    lines.push(serializeSse('response.output_item.added', {
-      type: 'response.output_item.added',
-      output_index: index,
-      item,
-    }));
-    lines.push(serializeSse('response.content_part.added', {
-      type: 'response.content_part.added',
-      output_index: index,
-      item_id: item.id,
-      content_index: 0,
-      part: { type: 'output_text', text: '' },
-    }));
+  if (textPartState.itemCreated) {
+    lines.push(serializeOutputItemAdded(index, textPartState.item));
+  }
+  if (textPartState.partCreated) {
+    lines.push(serializeContentPartAdded(index, asTrimmedString(textPartState.item.id), 0, textPartState.part));
   }
   if (novelDelta) {
     textPartState.part.text = `${currentText}${novelDelta}`;
     lines.push(serializeSse('response.output_text.delta', {
       type: 'response.output_text.delta',
       output_index: index,
-      item_id: item.id,
+      item_id: textPartState.item.id,
       delta: novelDelta,
     }));
   }
@@ -748,18 +1075,20 @@ function buildSyntheticReasoningEvents(
   const reasoningState = (signature || delta)
     ? ensureReasoningItem(state, '', state.outputItems.length)
     : null;
+  let emittedOutputItemAdded = false;
+
+  const emitOutputItemAdded = (entry: { index: number; item: AggregateOutputItem; created: boolean } | null) => {
+    if (!entry?.created || emittedOutputItemAdded) return;
+    lines.push(serializeOutputItemAdded(entry.index, entry.item));
+    emittedOutputItemAdded = true;
+  };
 
   if (reasoningState && signature) {
     reasoningState.item.encrypted_content = signature;
-    if (reasoningState.created) {
-      lines.push(serializeSse('response.output_item.added', {
-        type: 'response.output_item.added',
-        output_index: reasoningState.index,
-        item: reasoningState.item,
-      }));
-    }
   }
 
+  emitOutputItemAdded(reasoningState);
+
   if (!delta) {
     return lines;
   }
@@ -773,21 +1102,9 @@ function buildSyntheticReasoningEvents(
   const itemId = asTrimmedString(summaryState.item.id);
   const currentText = typeof summaryState.summary.text === 'string' ? summaryState.summary.text : '';
   const novelDelta = computeNovelDelta(currentText, delta);
-  if (summaryState.itemCreated && !reasoningState?.created) {
-    lines.push(serializeSse('response.output_item.added', {
-      type: 'response.output_item.added',
-      output_index: summaryState.index,
-      item: summaryState.item,
-    }));
-  }
+  emitOutputItemAdded(summaryState);
   if (summaryState.partCreated) {
-    lines.push(serializeSse('response.reasoning_summary_part.added', {
-      type: 'response.reasoning_summary_part.added',
-      item_id: itemId,
-      output_index: summaryState.index,
-      summary_index: 0,
-      part: { type: 'summary_text', text: '' },
-    }));
+    lines.push(serializeReasoningSummaryPartAdded(summaryState.index, itemId, 0, summaryState.summary));
   }
   if (novelDelta) {
     summaryState.summary.text = `${currentText}${novelDelta}`;
@@ -811,11 +1128,7 @@ function buildSyntheticToolEvents(
   for (const toolDelta of event.toolCallDeltas) {
     const entry = ensureFunctionCallItem(state, toolDelta.id, toolDelta.name, toolDelta.index);
     if (entry.created) {
-      lines.push(serializeSse('response.output_item.added', {
-        type: 'response.output_item.added',
-        output_index: entry.index,
-        item: entry.item,
-      }));
+      lines.push(serializeOutputItemAdded(entry.index, entry.item));
     }
     if (toolDelta.argumentsDelta !== undefined && toolDelta.argumentsDelta.length > 0) {
       entry.item.arguments = `${typeof entry.item.arguments === 'string' ? entry.item.arguments : ''}${toolDelta.argumentsDelta}`;
@@ -832,6 +1145,104 @@ function buildSyntheticToolEvents(
   return lines;
 }
 
+function buildMissingSubordinateDoneEventsForItem(
+  item: AggregateOutputItem,
+  index: number,
+): string[] {
+  const lines: string[] = [];
+  const itemType = asTrimmedString(item.type).toLowerCase();
+  const itemId = ensureOutputItemId(asTrimmedString(item.id), 'out', index);
+  item.id = itemId;
+
+  if (itemType === 'message') {
+    const content = Array.isArray(item.content) ? item.content as AggregateOutputItem[] : [];
+    for (let contentIndex = 0; contentIndex < content.length; contentIndex += 1) {
+      const part = content[contentIndex];
+      if (!isRecord(part)) continue;
+      const partType = asTrimmedString(part.type).toLowerCase();
+      const text = typeof part.text === 'string' ? part.text : String(part.text ?? '');
+      if ((partType === 'output_text' || partType === 'text') && !hasTerminalMarker(part, outputTextDoneMarker)) {
+        lines.push(serializeSse('response.output_text.done', {
+          type: 'response.output_text.done',
+          output_index: index,
+          item_id: itemId,
+          text,
+        }));
+        markTerminalMarker(part, outputTextDoneMarker);
+      }
+      if (!hasTerminalMarker(part, contentPartDoneMarker)) {
+        lines.push(serializeSse('response.content_part.done', {
+          type: 'response.content_part.done',
+          output_index: index,
+          item_id: itemId,
+          content_index: contentIndex,
+          part: cloneJson(part),
+        }));
+        markTerminalMarker(part, contentPartDoneMarker);
+      }
+    }
+  } else if (itemType === 'reasoning') {
+    const summary = Array.isArray(item.summary) ? item.summary as AggregateOutputItem[] : [];
+    for (let summaryIndex = 0; summaryIndex < summary.length; summaryIndex += 1) {
+      const part = summary[summaryIndex];
+      if (!isRecord(part)) continue;
+      const text = typeof part.text === 'string' ? part.text : String(part.text ?? '');
+      if (!hasTerminalMarker(part, reasoningSummaryTextDoneMarker)) {
+        lines.push(serializeSse('response.reasoning_summary_text.done', {
+          type: 'response.reasoning_summary_text.done',
+          item_id: itemId,
+          output_index: index,
+          summary_index: summaryIndex,
+          text,
+        }));
+        markTerminalMarker(part, reasoningSummaryTextDoneMarker);
+      }
+      if (!hasTerminalMarker(part, reasoningSummaryPartDoneMarker)) {
+        lines.push(serializeSse('response.reasoning_summary_part.done', {
+          type: 'response.reasoning_summary_part.done',
+          item_id: itemId,
+          output_index: index,
+          summary_index: summaryIndex,
+          part: cloneJson(part),
+        }));
+        markTerminalMarker(part, reasoningSummaryPartDoneMarker);
+      }
+    }
+  } else if (itemType === 'function_call') {
+    const callId = asTrimmedString(item.call_id) || itemId;
+    const name = asTrimmedString(item.name);
+    const argumentsText = typeof item.arguments === 'string' ? item.arguments : String(item.arguments ?? '');
+    if (!hasTerminalMarker(item, functionCallArgumentsDoneMarker)) {
+      lines.push(serializeSse('response.function_call_arguments.done', {
+        type: 'response.function_call_arguments.done',
+        item_id: itemId,
+        call_id: callId,
+        output_index: index,
+        ...(name ? { name } : {}),
+        arguments: argumentsText,
+      }));
+      markTerminalMarker(item, functionCallArgumentsDoneMarker);
+    }
+  } else if (itemType === 'custom_tool_call') {
+    const callId = asTrimmedString(item.call_id) || itemId;
+    const name = asTrimmedString(item.name);
+    const inputText = typeof item.input === 'string' ? item.input : String(item.input ?? '');
+    if (!hasTerminalMarker(item, customToolInputDoneMarker)) {
+      lines.push(serializeSse('response.custom_tool_call_input.done', {
+        type: 'response.custom_tool_call_input.done',
+        item_id: itemId,
+        call_id: callId,
+        output_index: index,
+        ...(name ? { name } : {}),
+        input: inputText,
+      }));
+      markTerminalMarker(item, customToolInputDoneMarker);
+    }
+  }
+
+  return lines;
+}
+
 function buildSyntheticTerminalItemDoneEvents(
   state: OpenAiResponsesAggregateState,
   status: 'completed' | 'failed' | 'incomplete',
@@ -843,104 +1254,20 @@ function buildSyntheticTerminalItemDoneEvents(
     if (!isRecord(item)) continue;
 
     const currentStatus = asTrimmedString(item.status).toLowerCase();
-    if (currentStatus === 'completed' || currentStatus === 'failed' || currentStatus === 'incomplete') continue;
+    const itemTerminal = isTerminalStatus(currentStatus);
+    lines.push(...buildMissingSubordinateDoneEventsForItem(item, index));
 
-    const itemType = asTrimmedString(item.type).toLowerCase();
-    const itemId = ensureOutputItemId(asTrimmedString(item.id), 'out', index);
-    item.id = itemId;
-
-    if (itemType === 'message') {
-      const content = Array.isArray(item.content) ? item.content as AggregateOutputItem[] : [];
-      for (let contentIndex = 0; contentIndex < content.length; contentIndex += 1) {
-        const part = content[contentIndex];
-        if (!isRecord(part)) continue;
-        const partType = asTrimmedString(part.type).toLowerCase();
-        const text = typeof part.text === 'string' ? part.text : String(part.text ?? '');
-        if ((partType === 'output_text' || partType === 'text') && !hasTerminalMarker(part, outputTextDoneMarker)) {
-          lines.push(serializeSse('response.output_text.done', {
-            type: 'response.output_text.done',
-            output_index: index,
-            item_id: itemId,
-            text,
-          }));
-          markTerminalMarker(part, outputTextDoneMarker);
-        }
-        if (!hasTerminalMarker(part, contentPartDoneMarker)) {
-          lines.push(serializeSse('response.content_part.done', {
-            type: 'response.content_part.done',
-            output_index: index,
-            item_id: itemId,
-            content_index: contentIndex,
-            part: cloneJson(part),
-          }));
-          markTerminalMarker(part, contentPartDoneMarker);
-        }
-      }
-    } else if (itemType === 'reasoning') {
-      const summary = Array.isArray(item.summary) ? item.summary as AggregateOutputItem[] : [];
-      for (let summaryIndex = 0; summaryIndex < summary.length; summaryIndex += 1) {
-        const part = summary[summaryIndex];
-        if (!isRecord(part)) continue;
-        const text = typeof part.text === 'string' ? part.text : String(part.text ?? '');
-        if (!hasTerminalMarker(part, reasoningSummaryTextDoneMarker)) {
-          lines.push(serializeSse('response.reasoning_summary_text.done', {
-            type: 'response.reasoning_summary_text.done',
-            item_id: itemId,
-            output_index: index,
-            summary_index: summaryIndex,
-            text,
-          }));
-          markTerminalMarker(part, reasoningSummaryTextDoneMarker);
-        }
-        if (!hasTerminalMarker(part, reasoningSummaryPartDoneMarker)) {
-          lines.push(serializeSse('response.reasoning_summary_part.done', {
-            type: 'response.reasoning_summary_part.done',
-            item_id: itemId,
-            output_index: index,
-            summary_index: summaryIndex,
-            part: cloneJson(part),
-          }));
-          markTerminalMarker(part, reasoningSummaryPartDoneMarker);
-        }
-      }
-    } else if (itemType === 'function_call') {
-      const callId = asTrimmedString(item.call_id) || itemId;
-      const name = asTrimmedString(item.name);
-      const argumentsText = typeof item.arguments === 'string' ? item.arguments : String(item.arguments ?? '');
-      if (!hasTerminalMarker(item, functionCallArgumentsDoneMarker)) {
-        lines.push(serializeSse('response.function_call_arguments.done', {
-          type: 'response.function_call_arguments.done',
-          item_id: itemId,
-          call_id: callId,
-          output_index: index,
-          ...(name ? { name } : {}),
-          arguments: argumentsText,
-        }));
-        markTerminalMarker(item, functionCallArgumentsDoneMarker);
-      }
-    } else if (itemType === 'custom_tool_call') {
-      const callId = asTrimmedString(item.call_id) || itemId;
-      const name = asTrimmedString(item.name);
-      const inputText = typeof item.input === 'string' ? item.input : String(item.input ?? '');
-      if (!hasTerminalMarker(item, customToolInputDoneMarker)) {
-        lines.push(serializeSse('response.custom_tool_call_input.done', {
-          type: 'response.custom_tool_call_input.done',
-          item_id: itemId,
-          call_id: callId,
-          output_index: index,
-          ...(name ? { name } : {}),
-          input: inputText,
-        }));
-        markTerminalMarker(item, customToolInputDoneMarker);
+    if (!hasTerminalMarker(item, outputItemDoneMarker)) {
+      if (!itemTerminal) {
+        item.status = status;
       }
+      lines.push(serializeSse('response.output_item.done', {
+        type: 'response.output_item.done',
+        output_index: index,
+        item: cloneJson(item),
+      }));
+      markTerminalMarker(item, outputItemDoneMarker);
     }
-
-    item.status = status;
-    lines.push(serializeSse('response.output_item.done', {
-      type: 'response.output_item.done',
-      output_index: index,
-      item: cloneJson(item),
-    }));
   }
 
   return lines;
diff --git a/src/server/transformers/openai/responses/proxyStream.ts b/src/server/transformers/openai/responses/proxyStream.ts
index 56f1f30..1a77d27 100644
--- a/src/server/transformers/openai/responses/proxyStream.ts
+++ b/src/server/transformers/openai/responses/proxyStream.ts
@@ -41,6 +41,10 @@ function isRecord(value: unknown): value is Record<string, unknown> {
   return !!value && typeof value === 'object';
 }
 
+function asTrimmedString(value: unknown): string {
+  return typeof value === 'string' ? value.trim() : '';
+}
+
 function hasNonEmptyString(value: unknown): boolean {
   return typeof value === 'string' && value.trim().length > 0;
 }
@@ -112,6 +116,59 @@ function getResponsesStreamFailureMessage(payload: unknown, fallback = 'upstream
   return fallback;
 }
 
+function preserveMeaningfulTerminalResponsesPayload(
+  lines: string[],
+  eventType: 'response.completed' | 'response.incomplete',
+  payload: Record<string, unknown>,
+): string[] {
+  const responsePayload = isRecord(payload.response) ? payload.response : null;
+  if (!responsePayload || !hasMeaningfulResponsesPayloadOutput(responsePayload)) {
+    return lines;
+  }
+
+  const parsed = openAiResponsesStream.pullSseEvents(lines.join(''));
+  let replaced = false;
+  const nextLines: string[] = [];
+
+  for (const event of parsed.events) {
+    if (event.data === '[DONE]') {
+      nextLines.push('data: [DONE]\n\n');
+      continue;
+    }
+
+    let parsedPayload: unknown = null;
+    try {
+      parsedPayload = JSON.parse(event.data);
+    } catch {
+      nextLines.push(`${event.event ? `event: ${event.event}\n` : ''}data: ${event.data}\n\n`);
+      continue;
+    }
+
+    if (
+      isRecord(parsedPayload)
+      && asTrimmedString(parsedPayload.type) === eventType
+      && isRecord(parsedPayload.response)
+      && !hasMeaningfulResponsesPayloadOutput(parsedPayload.response)
+    ) {
+      replaced = true;
+      nextLines.push(
+        `event: ${event.event || eventType}\ndata: ${JSON.stringify({
+          ...parsedPayload,
+          response: {
+            ...parsedPayload.response,
+            ...responsePayload,
+          },
+        })}\n\n`,
+      );
+      continue;
+    }
+
+    nextLines.push(`${event.event ? `event: ${event.event}\n` : ''}data: ${event.data}\n\n`);
+  }
+
+  return replaced ? nextLines : lines;
+}
+
 export function createResponsesProxyStreamSession(input: ResponsesProxyStreamSessionInput) {
   const streamContext = openAiResponsesStream.createContext(input.modelName);
   const responsesState = createOpenAiResponsesAggregateState(input.modelName);
@@ -183,7 +240,7 @@ export function createResponsesProxyStreamSession(input: ResponsesProxyStreamSes
       parsedPayload = null;
     }
 
-    if (parsedPayload && typeof parsedPayload === 'object') {
+    if (isRecord(parsedPayload)) {
       input.onParsedPayload?.(parsedPayload);
     }
 
@@ -202,14 +259,19 @@ export function createResponsesProxyStreamSession(input: ResponsesProxyStreamSes
     }
     const isIncompleteEvent = eventBlock.event === 'response.incomplete' || payloadType === 'response.incomplete';
 
-    if (parsedPayload && typeof parsedPayload === 'object') {
+    if (isRecord(parsedPayload)) {
       const normalizedEvent = openAiResponsesStream.normalizeEvent(parsedPayload, streamContext, input.modelName);
-      const convertedLines = serializeConvertedResponsesEvents({
+      let convertedLines = serializeConvertedResponsesEvents({
         state: responsesState,
         streamContext,
         event: normalizedEvent,
         usage: input.getUsage(),
       });
+      if (isIncompleteEvent) {
+        convertedLines = preserveMeaningfulTerminalResponsesPayload(convertedLines, 'response.incomplete', parsedPayload);
+      } else if (eventBlock.event === 'response.completed' || payloadType === 'response.completed') {
+        convertedLines = preserveMeaningfulTerminalResponsesPayload(convertedLines, 'response.completed', parsedPayload);
+      }
       if (
         (eventBlock.event === 'response.completed' || payloadType === 'response.completed')
         && shouldFailEmptyResponsesCompletion({
diff --git a/src/server/transformers/shared/chatFormatsCore.test.ts b/src/server/transformers/shared/chatFormatsCore.test.ts
index 75ec573..a24fae9 100644
--- a/src/server/transformers/shared/chatFormatsCore.test.ts
+++ b/src/server/transformers/shared/chatFormatsCore.test.ts
@@ -1,6 +1,12 @@
 import { describe, expect, it } from 'vitest';
 
-import { createStreamTransformContext, normalizeUpstreamStreamEvent, convertClaudeRequestToOpenAiBody } from './chatFormatsCore.js';
+import {
+  convertClaudeRequestToOpenAiBody,
+  createClaudeDownstreamContext,
+  createStreamTransformContext,
+  normalizeUpstreamStreamEvent,
+  serializeNormalizedStreamEvent,
+} from './chatFormatsCore.js';
 
 describe('chatFormatsCore inline think parsing', () => {
   it('tracks split think tags across stream chunks', () => {
@@ -66,6 +72,275 @@ describe('chatFormatsCore inline think parsing', () => {
       contentDelta: 'visible answer',
     });
   });
+
+  it('treats response.reasoning_summary_text.done as reasoning-only stream output', () => {
+    const context = createStreamTransformContext('gpt-test');
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.reasoning_summary_text.done',
+      item_id: 'rs_1',
+      output_index: 0,
+      summary_index: 0,
+      text: 'plan first',
+    }, context, 'gpt-test')).toEqual({
+      reasoningDelta: 'plan first',
+    });
+  });
+
+  it('accumulates reasoning summary deltas before reconciling response.reasoning_summary_text.done', () => {
+    const context = createStreamTransformContext('gpt-test');
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.reasoning_summary_text.delta',
+      item_id: 'rs_multi',
+      output_index: 0,
+      summary_index: 0,
+      delta: 'plan ',
+    }, context, 'gpt-test')).toEqual({
+      reasoningDelta: 'plan ',
+    });
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.reasoning_summary_text.delta',
+      item_id: 'rs_multi',
+      output_index: 0,
+      summary_index: 0,
+      delta: 'first',
+    }, context, 'gpt-test')).toEqual({
+      reasoningDelta: 'first',
+    });
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.reasoning_summary_text.done',
+      item_id: 'rs_multi',
+      output_index: 0,
+      summary_index: 0,
+      text: 'plan first',
+    }, context, 'gpt-test')).toEqual({});
+  });
+
+  it('preserves terminal-only native responses output item payloads in stream normalization', () => {
+    const context = createStreamTransformContext('gpt-test');
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.output_item.done',
+      output_index: 0,
+      item: {
+        id: 'msg_1',
+        type: 'message',
+        role: 'assistant',
+        status: 'completed',
+        content: [{ type: 'output_text', text: 'hello' }],
+      },
+    }, context, 'gpt-test')).toEqual({
+      role: 'assistant',
+      contentDelta: 'hello',
+    });
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.output_item.done',
+      output_index: 1,
+      item: {
+        id: 'fc_1',
+        type: 'function_call',
+        call_id: 'call_1',
+        name: 'lookup',
+        arguments: '{"q":"x"}',
+        status: 'completed',
+      },
+    }, context, 'gpt-test')).toEqual({
+      toolCallDeltas: [{
+        index: 0,
+        id: 'call_1',
+        name: 'lookup',
+        argumentsDelta: '{"q":"x"}',
+      }],
+    });
+  });
+
+  it('keeps responses tool-call indices stable when response.completed replays mixed output arrays', () => {
+    const context = createStreamTransformContext('gpt-test');
+    const claudeContext = createClaudeDownstreamContext();
+
+    const streamingDelta = normalizeUpstreamStreamEvent({
+      type: 'response.function_call_arguments.delta',
+      output_index: 1,
+      call_id: 'call_1',
+      name: 'lookup',
+      delta: '{"q":"x"}',
+    }, context, 'gpt-test');
+
+    expect(streamingDelta).toEqual({
+      toolCallDeltas: [{
+        index: 0,
+        id: 'call_1',
+        name: 'lookup',
+        argumentsDelta: '{"q":"x"}',
+      }],
+    });
+    expect(serializeNormalizedStreamEvent('openai', streamingDelta, context, claudeContext)).toHaveLength(1);
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.completed',
+      response: {
+        id: 'resp_3',
+        model: 'gpt-test',
+        status: 'completed',
+        output: [
+          {
+            id: 'msg_1',
+            type: 'message',
+            role: 'assistant',
+            status: 'completed',
+            content: [{ type: 'output_text', text: 'working on it' }],
+          },
+          {
+            id: 'fc_1',
+            type: 'function_call',
+            call_id: 'call_1',
+            name: 'lookup',
+            arguments: '{"q":"x"}',
+            status: 'completed',
+          },
+        ],
+      },
+    }, context, 'gpt-test')).toEqual({
+      role: 'assistant',
+      contentDelta: 'working on it',
+      finishReason: 'stop',
+      done: true,
+    });
+  });
+
+  it('preserves terminal response.completed payload output when it carries the only final content', () => {
+    const context = createStreamTransformContext('gpt-test');
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.completed',
+      response: {
+        id: 'resp_1',
+        status: 'completed',
+        output: [
+          {
+            id: 'msg_1',
+            type: 'message',
+            role: 'assistant',
+            status: 'completed',
+            content: [{ type: 'output_text', text: 'hello' }],
+          },
+        ],
+      },
+    }, context, 'gpt-test')).toEqual({
+      role: 'assistant',
+      contentDelta: 'hello',
+      finishReason: 'stop',
+      done: true,
+    });
+  });
+
+  it('preserves streamed trailing whitespace when reconciling response.completed content', () => {
+    const context = createStreamTransformContext('gpt-test');
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.output_text.delta',
+      output_index: 0,
+      item_id: 'msg_ws_space',
+      delta: 'hello ',
+    }, context, 'gpt-test')).toEqual({
+      contentDelta: 'hello ',
+    });
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.completed',
+      response: {
+        id: 'resp_space_1',
+        status: 'completed',
+        output: [
+          {
+            id: 'msg_ws_space',
+            type: 'message',
+            role: 'assistant',
+            status: 'completed',
+            content: [{ type: 'output_text', text: 'hello world' }],
+          },
+        ],
+      },
+    }, context, 'gpt-test')).toEqual({
+      role: 'assistant',
+      contentDelta: 'world',
+      finishReason: 'stop',
+      done: true,
+    });
+  });
+
+  it('preserves terminal response.completed custom tool metadata in stream normalization', () => {
+    const context = createStreamTransformContext('gpt-test');
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.completed',
+      response: {
+        id: 'resp_2',
+        model: 'gpt-test',
+        status: 'completed',
+        output: [
+          {
+            id: 'ct_1',
+            type: 'custom_tool_call',
+            call_id: 'call_custom_1',
+            name: 'Shell',
+            input: '{"command":"pwd"}',
+          },
+        ],
+      },
+    }, context, 'gpt-test')).toEqual({
+      toolCallDeltas: [{
+        index: 0,
+        id: 'call_custom_1',
+        name: 'Shell',
+        argumentsDelta: '{"command":"pwd"}',
+      }],
+      finishReason: 'tool_calls',
+      done: true,
+    });
+  });
+
+  it('normalizes custom tool calls through the existing tool-call stream shape', () => {
+    const context = createStreamTransformContext('gpt-test');
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.output_item.added',
+      output_index: 0,
+      item: {
+        id: 'ct_1',
+        type: 'custom_tool_call',
+        call_id: 'call_custom',
+        name: 'MyTool',
+        input: '',
+      },
+    }, context, 'gpt-test')).toEqual({
+      toolCallDeltas: [{
+        index: 0,
+        id: 'call_custom',
+        name: 'MyTool',
+      }],
+    });
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.custom_tool_call_input.done',
+      output_index: 0,
+      item_id: 'ct_1',
+      call_id: 'call_custom',
+      name: 'MyTool',
+      input: '{"path":"README.md"}',
+    }, context, 'gpt-test')).toEqual({
+      toolCallDeltas: [{
+        index: 0,
+        id: 'call_custom',
+        name: 'MyTool',
+        argumentsDelta: '{"path":"README.md"}',
+      }],
+    });
+  });
 });
 
 describe('convertClaudeRequestToOpenAiBody', () => {
diff --git a/src/server/transformers/shared/chatFormatsCore.ts b/src/server/transformers/shared/chatFormatsCore.ts
index 588e88f..50c68e4 100644
--- a/src/server/transformers/shared/chatFormatsCore.ts
+++ b/src/server/transformers/shared/chatFormatsCore.ts
@@ -23,6 +23,11 @@ export type StreamTransformContext = {
   roleSent: boolean;
   doneSent: boolean;
   toolCalls: Record<number, { id?: string; name?: string; arguments?: string }>;
+  responsesToolCallIndexByOutputIndex: Record<number, number>;
+  responsesToolCallIndexById: Record<string, number>;
+  nextResponsesToolCallIndex: number;
+  responsesTextByIndex: Record<number, string>;
+  responsesReasoningByIndex: Record<number, string>;
   thinkTagParser: ThinkTagParserState;
 };
 
@@ -105,6 +110,24 @@ function joinNonEmpty(parts: string[]): string {
   return parts.map((item) => item.trim()).filter((item) => item.length > 0).join('\n\n');
 }
 
+function joinIndexedResponsesText(partsByIndex: Record<number, string>): string {
+  const indexedParts = Object.entries(partsByIndex)
+    .map(([rawIndex, text]) => ({
+      index: Number(rawIndex),
+      text,
+    }))
+    .filter((entry) => Number.isFinite(entry.index) && entry.index >= 0 && typeof entry.text === 'string' && entry.text.length > 0)
+    .sort((left, right) => left.index - right.index)
+    .map((entry) => entry.text);
+
+  if (indexedParts.length > 0) {
+    return indexedParts.join('\n\n');
+  }
+
+  const snapshot = partsByIndex[-1];
+  return typeof snapshot === 'string' ? snapshot : '';
+}
+
 function textFromPart(part: unknown): string {
   if (typeof part === 'string') return part;
   if (!isRecord(part)) return '';
@@ -256,6 +279,11 @@ export function createStreamTransformContext(modelName: string): StreamTransform
     roleSent: false,
     doneSent: false,
     toolCalls: {},
+    responsesToolCallIndexByOutputIndex: {},
+    responsesToolCallIndexById: {},
+    nextResponsesToolCallIndex: 0,
+    responsesTextByIndex: {},
+    responsesReasoningByIndex: {},
     thinkTagParser: createThinkTagParserState(),
   };
 }
@@ -465,6 +493,30 @@ function parseResponsesReasoning(payload: Record<string, unknown>): {
   };
 }
 
+function unwrapTerminalResponsesEnvelope(
+  payload: Record<string, unknown>,
+): Record<string, unknown> | null {
+  const type = asTrimmedString(payload.type).toLowerCase();
+  if (
+    type !== 'response.completed'
+    && type !== 'response.failed'
+    && type !== 'response.incomplete'
+  ) {
+    return null;
+  }
+  if (!isRecord(payload.response)) return null;
+
+  const responsePayload = payload.response as Record<string, unknown>;
+  if (isNonEmptyString(responsePayload.status)) {
+    return responsePayload;
+  }
+
+  return {
+    ...responsePayload,
+    status: type.slice('response.'.length),
+  };
+}
+
 function stringifyUnknownValue(value: unknown): string {
   if (typeof value === 'string') return value;
   if (value === undefined || value === null) return '';
@@ -558,7 +610,7 @@ function collectToolCallsFromResponsesPayload(payload: Record<string, unknown>):
   for (let index = 0; index < output.length; index += 1) {
     const item = output[index];
     if (!isRecord(item)) continue;
-    if (item.type !== 'function_call') continue;
+    if (item.type !== 'function_call' && item.type !== 'custom_tool_call') continue;
 
     const id = (
       typeof item.call_id === 'string' && item.call_id.trim().length > 0
@@ -569,7 +621,9 @@ function collectToolCallsFromResponsesPayload(payload: Record<string, unknown>):
     const argumentsText = (
       typeof item.arguments === 'string'
         ? item.arguments
-        : stringifyUnknownValue(item.arguments)
+        : (typeof item.input === 'string'
+          ? item.input
+          : stringifyUnknownValue(item.arguments ?? item.input))
     );
     toolCalls.push({
       id,
@@ -581,6 +635,198 @@ function collectToolCallsFromResponsesPayload(payload: Record<string, unknown>):
   return toolCalls;
 }
 
+function collectIndexedToolCallsFromResponsesPayload(
+  payload: Record<string, unknown>,
+): Array<{ id: string; name: string; arguments: string; outputIndex: number }> {
+  const output = Array.isArray(payload.output) ? payload.output : [];
+  const toolCalls: Array<{ id: string; name: string; arguments: string; outputIndex: number }> = [];
+
+  for (let index = 0; index < output.length; index += 1) {
+    const item = output[index];
+    if (!isRecord(item)) continue;
+    if (item.type !== 'function_call' && item.type !== 'custom_tool_call') continue;
+
+    const id = (
+      typeof item.call_id === 'string' && item.call_id.trim().length > 0
+        ? item.call_id.trim()
+        : (typeof item.id === 'string' && item.id.trim().length > 0 ? item.id.trim() : `call_${index}`)
+    );
+    const name = typeof item.name === 'string' ? item.name.trim() : '';
+    const argumentsText = (
+      typeof item.arguments === 'string'
+        ? item.arguments
+        : (typeof item.input === 'string'
+          ? item.input
+          : stringifyUnknownValue(item.arguments ?? item.input))
+    );
+    toolCalls.push({
+      id,
+      name,
+      arguments: argumentsText,
+      outputIndex: index,
+    });
+  }
+
+  return toolCalls;
+}
+
+function computeNovelResponsesDelta(existingText: string, incomingText: string): string {
+  if (!incomingText) return '';
+  if (!existingText) return incomingText;
+  if (incomingText.startsWith(existingText)) return incomingText.slice(existingText.length);
+  if (existingText.endsWith(incomingText)) return '';
+
+  const maxOverlap = Math.min(existingText.length, incomingText.length);
+  for (let overlap = maxOverlap; overlap > 0; overlap -= 1) {
+    if (existingText.endsWith(incomingText.slice(0, overlap))) {
+      return incomingText.slice(overlap);
+    }
+  }
+
+  return incomingText;
+}
+
+function extractResponsesOutputIndex(payload: Record<string, unknown>): number {
+  return (
+    typeof payload.output_index === 'number' && Number.isFinite(payload.output_index)
+      ? Math.max(0, Math.trunc(payload.output_index))
+      : 0
+  );
+}
+
+function extractResponsesItemText(item: Record<string, unknown>): string {
+  const itemType = asTrimmedString(item.type).toLowerCase();
+  if (itemType === 'message') {
+    return extractTextAndReasoning(item.content ?? item).content;
+  }
+  if (itemType === 'reasoning') {
+    const parsed = extractTextAndReasoning(item.summary ?? item.content ?? item);
+    return joinNonEmpty([parsed.content, parsed.reasoning]);
+  }
+  return '';
+}
+
+function rememberResponsesToolCallIndex(
+  context: StreamTransformContext,
+  canonicalIndex: number,
+  input: {
+    outputIndex?: number;
+    itemId?: unknown;
+    callId?: unknown;
+  },
+): void {
+  if (typeof input.outputIndex === 'number' && Number.isFinite(input.outputIndex)) {
+    context.responsesToolCallIndexByOutputIndex[Math.max(0, Math.trunc(input.outputIndex))] = canonicalIndex;
+  }
+  const itemId = asTrimmedString(input.itemId);
+  if (itemId) {
+    context.responsesToolCallIndexById[`item:${itemId}`] = canonicalIndex;
+  }
+  const callId = asTrimmedString(input.callId);
+  if (callId) {
+    context.responsesToolCallIndexById[`call:${callId}`] = canonicalIndex;
+  }
+}
+
+function resolveResponsesToolCallIndex(
+  context: StreamTransformContext,
+  input: {
+    outputIndex?: number;
+    itemId?: unknown;
+    callId?: unknown;
+  },
+): number {
+  const normalizedOutputIndex = (
+    typeof input.outputIndex === 'number' && Number.isFinite(input.outputIndex)
+      ? Math.max(0, Math.trunc(input.outputIndex))
+      : undefined
+  );
+  if (
+    normalizedOutputIndex !== undefined
+    && context.responsesToolCallIndexByOutputIndex[normalizedOutputIndex] !== undefined
+  ) {
+    const canonicalIndex = context.responsesToolCallIndexByOutputIndex[normalizedOutputIndex]!;
+    rememberResponsesToolCallIndex(context, canonicalIndex, {
+      outputIndex: normalizedOutputIndex,
+      itemId: input.itemId,
+      callId: input.callId,
+    });
+    return canonicalIndex;
+  }
+
+  const itemId = asTrimmedString(input.itemId);
+  if (itemId && context.responsesToolCallIndexById[`item:${itemId}`] !== undefined) {
+    const canonicalIndex = context.responsesToolCallIndexById[`item:${itemId}`]!;
+    rememberResponsesToolCallIndex(context, canonicalIndex, {
+      outputIndex: normalizedOutputIndex,
+      itemId,
+      callId: input.callId,
+    });
+    return canonicalIndex;
+  }
+
+  const callId = asTrimmedString(input.callId);
+  if (callId && context.responsesToolCallIndexById[`call:${callId}`] !== undefined) {
+    const canonicalIndex = context.responsesToolCallIndexById[`call:${callId}`]!;
+    rememberResponsesToolCallIndex(context, canonicalIndex, {
+      outputIndex: normalizedOutputIndex,
+      itemId: input.itemId,
+      callId,
+    });
+    return canonicalIndex;
+  }
+
+  const canonicalIndex = context.nextResponsesToolCallIndex;
+  context.nextResponsesToolCallIndex += 1;
+  rememberResponsesToolCallIndex(context, canonicalIndex, {
+    outputIndex: normalizedOutputIndex,
+    itemId: input.itemId,
+    callId: input.callId,
+  });
+  return canonicalIndex;
+}
+
+function buildResponsesToolCallDeltaFromItem(
+  item: Record<string, unknown>,
+  outputIndex: number,
+  context: StreamTransformContext,
+): NormalizedStreamEvent | null {
+  const itemType = asTrimmedString(item.type).toLowerCase();
+  if (itemType !== 'function_call' && itemType !== 'custom_tool_call') return null;
+
+  const toolCallId = (
+    isNonEmptyString(item.call_id) ? item.call_id
+      : (isNonEmptyString(item.id) ? item.id : undefined)
+  );
+  const toolName = isNonEmptyString(item.name) ? item.name : undefined;
+  const rawArguments = itemType === 'custom_tool_call'
+    ? (typeof item.input === 'string' ? item.input : stringifyUnknownValue(item.input))
+    : (typeof item.arguments === 'string' ? item.arguments : stringifyUnknownValue(item.arguments));
+  const canonicalIndex = resolveResponsesToolCallIndex(context, {
+    outputIndex,
+    itemId: item.id,
+    callId: item.call_id,
+  });
+  const existingArguments = context.toolCalls[canonicalIndex]?.arguments || '';
+  const argumentsDelta = computeNovelResponsesDelta(existingArguments, rawArguments);
+  const knownTool = context.toolCalls[canonicalIndex] || {};
+  const shouldBackfillId = !!toolCallId && !knownTool.id;
+  const shouldBackfillName = !!toolName && !knownTool.name;
+
+  if (!argumentsDelta && !shouldBackfillId && !shouldBackfillName) {
+    return null;
+  }
+
+  return {
+    toolCallDeltas: [{
+      index: canonicalIndex,
+      id: toolCallId,
+      name: toolName,
+      argumentsDelta: argumentsDelta || undefined,
+    }],
+  };
+}
+
 function formatAnthropicBase64DataUrl(mimeType: string, data: string): string {
   return `data:${mimeType};base64,${data}`;
 }
@@ -1067,6 +1313,13 @@ export function normalizeUpstreamFinalResponse(
   const now = Math.floor(Date.now() / 1000);
   const fallbackId = `chatcmpl-meta-${Date.now()}`;
 
+  if (isRecord(payload)) {
+    const terminalResponsesPayload = unwrapTerminalResponsesEnvelope(payload);
+    if (terminalResponsesPayload) {
+      return normalizeUpstreamFinalResponse(terminalResponsesPayload, fallbackModel, fallbackText);
+    }
+  }
+
   if (isRecord(payload) && Array.isArray(payload.choices)) {
     const choice = payload.choices[0] ?? {};
     const content = extractAssistantContent(choice) || extractAssistantContent(payload);
@@ -1238,19 +1491,39 @@ export function normalizeUpstreamStreamEvent(
 
   const type = typeof payload.type === 'string' ? payload.type : '';
   if (type.startsWith('response.output_text')) {
-    const parsed = extractStreamingTextAndReasoning(payload.delta, context.thinkTagParser);
+    const outputIndex = extractResponsesOutputIndex(payload);
+    const rawText = typeof payload.delta === 'string'
+      ? payload.delta
+      : (typeof (payload as any).text === 'string' ? (payload as any).text : '');
+    const parsed = extractStreamingTextAndReasoning(rawText, context.thinkTagParser);
+    const nextContent = type === 'response.output_text.done'
+      ? (parsed.content || context.responsesTextByIndex[outputIndex] || '')
+      : `${context.responsesTextByIndex[outputIndex] || ''}${parsed.content || ''}`;
+    const novelContent = type === 'response.output_text.done'
+      ? computeNovelResponsesDelta(context.responsesTextByIndex[outputIndex] || '', parsed.content || '')
+      : (parsed.content || '');
+    if (nextContent) context.responsesTextByIndex[outputIndex] = nextContent;
     return {
-      contentDelta: parsed.content || undefined,
+      contentDelta: novelContent || undefined,
       reasoningDelta: parsed.reasoning || undefined,
     };
   }
 
-  if (type === 'response.reasoning_summary_text.delta') {
-    const deltaText = typeof payload.delta === 'string'
-      ? payload.delta
-      : extractTextAndReasoning(payload.delta).content;
+  if (type === 'response.reasoning_summary_text.delta' || type === 'response.reasoning_summary_text.done') {
+    const outputIndex = extractResponsesOutputIndex(payload);
+    const deltaText = type === 'response.reasoning_summary_text.done'
+      ? (typeof (payload as any).text === 'string' ? (payload as any).text : extractTextAndReasoning(payload.text).content)
+      : (typeof payload.delta === 'string' ? payload.delta : extractTextAndReasoning(payload.delta).content);
+    const previousReasoning = context.responsesReasoningByIndex[outputIndex] || '';
+    const novelDelta = computeNovelResponsesDelta(previousReasoning, deltaText);
+    const nextReasoning = type === 'response.reasoning_summary_text.done'
+      ? (deltaText || previousReasoning)
+      : `${previousReasoning}${novelDelta}`;
+    if (nextReasoning) {
+      context.responsesReasoningByIndex[outputIndex] = nextReasoning;
+    }
     return {
-      reasoningDelta: deltaText || undefined,
+      reasoningDelta: novelDelta || undefined,
     };
   }
 
@@ -1267,40 +1540,44 @@ export function normalizeUpstreamStreamEvent(
     };
   }
 
-  if (type === 'response.output_item.added' && isRecord((payload as any).item)) {
+  if ((type === 'response.output_item.added' || type === 'response.output_item.done') && isRecord((payload as any).item)) {
+    const outputIndex = extractResponsesOutputIndex(payload as Record<string, unknown>);
     const item = (payload as any).item as Record<string, unknown>;
     if (item.type === 'reasoning' && isNonEmptyString(item.encrypted_content)) {
+      const reasoningText = extractResponsesItemText(item);
+      const novelReasoning = computeNovelResponsesDelta(context.responsesReasoningByIndex[outputIndex] || '', reasoningText);
+      if (reasoningText) {
+        context.responsesReasoningByIndex[outputIndex] = reasoningText;
+      }
       return {
         reasoningSignature: item.encrypted_content,
+        reasoningDelta: novelReasoning || undefined,
       };
     }
-    if (item.type === 'function_call') {
-      const outputIndex = (
-        typeof (payload as any).output_index === 'number' && Number.isFinite((payload as any).output_index)
-          ? Math.max(0, Math.trunc((payload as any).output_index))
-          : 0
-      );
-      const toolCallId = (
-        isNonEmptyString(item.call_id) ? item.call_id
-          : (isNonEmptyString(item.id) ? item.id : undefined)
-      );
-      const toolName = isNonEmptyString(item.name) ? item.name : undefined;
+    const toolCallEvent = buildResponsesToolCallDeltaFromItem(item, outputIndex, context);
+    if (toolCallEvent) {
+      return toolCallEvent;
+    }
+    if (item.type === 'message') {
+      const fullText = extractResponsesItemText(item);
+      const novelDelta = computeNovelResponsesDelta(context.responsesTextByIndex[outputIndex] || '', fullText);
+      if (fullText) {
+        context.responsesTextByIndex[outputIndex] = fullText;
+      }
       return {
-        toolCallDeltas: [{
-          index: outputIndex,
-          id: toolCallId,
-          name: toolName,
-        }],
+        role: item.role === 'assistant' ? 'assistant' : undefined,
+        contentDelta: novelDelta || undefined,
       };
     }
   }
 
   if (type === 'response.function_call_arguments.delta' || type === 'response.function_call_arguments.done') {
-    const outputIndex = (
-      typeof (payload as any).output_index === 'number' && Number.isFinite((payload as any).output_index)
-        ? Math.max(0, Math.trunc((payload as any).output_index))
-        : 0
-    );
+    const outputIndex = extractResponsesOutputIndex(payload as Record<string, unknown>);
+    const canonicalIndex = resolveResponsesToolCallIndex(context, {
+      outputIndex,
+      itemId: (payload as any).item_id,
+      callId: (payload as any).call_id,
+    });
     const toolCallId = (
       isNonEmptyString((payload as any).call_id) ? (payload as any).call_id
         : (isNonEmptyString((payload as any).item_id) ? (payload as any).item_id : undefined)
@@ -1317,7 +1594,7 @@ export function normalizeUpstreamStreamEvent(
     );
     let argumentsDelta = rawArguments;
     if (type === 'response.function_call_arguments.done' && typeof rawArguments === 'string') {
-      const existingArguments = context.toolCalls[outputIndex]?.arguments || '';
+      const existingArguments = context.toolCalls[canonicalIndex]?.arguments || '';
       if (existingArguments && rawArguments.startsWith(existingArguments)) {
         const missingSuffix = rawArguments.slice(existingArguments.length);
         argumentsDelta = missingSuffix.length > 0 ? missingSuffix : undefined;
@@ -1326,7 +1603,7 @@ export function normalizeUpstreamStreamEvent(
       }
     }
 
-    const knownTool = context.toolCalls[outputIndex] || {};
+    const knownTool = context.toolCalls[canonicalIndex] || {};
     const shouldBackfillId = !!toolCallId && !knownTool.id;
     const shouldBackfillName = !!toolName && !knownTool.name;
     if (argumentsDelta === undefined && !shouldBackfillId && !shouldBackfillName) {
@@ -1335,7 +1612,7 @@ export function normalizeUpstreamStreamEvent(
 
     return {
       toolCallDeltas: [{
-        index: outputIndex,
+        index: canonicalIndex,
         id: toolCallId,
         name: toolName,
         argumentsDelta,
@@ -1343,12 +1620,97 @@ export function normalizeUpstreamStreamEvent(
     };
   }
 
+  if (type === 'response.custom_tool_call_input.delta' || type === 'response.custom_tool_call_input.done') {
+    const outputIndex = extractResponsesOutputIndex(payload as Record<string, unknown>);
+    const canonicalIndex = resolveResponsesToolCallIndex(context, {
+      outputIndex,
+      itemId: (payload as any).item_id,
+      callId: (payload as any).call_id,
+    });
+    const toolCallId = (
+      isNonEmptyString((payload as any).call_id) ? (payload as any).call_id
+        : (isNonEmptyString((payload as any).item_id) ? (payload as any).item_id : undefined)
+    );
+    const toolName = isNonEmptyString((payload as any).name) ? (payload as any).name : undefined;
+    const rawArguments = (
+      type === 'response.custom_tool_call_input.done'
+        ? (typeof (payload as any).input === 'string' ? (payload as any).input : stringifyUnknownValue((payload as any).input))
+        : (
+          typeof payload.delta === 'string'
+            ? payload.delta
+            : stringifyUnknownValue((payload as any).input)
+        )
+    );
+    const existingArguments = context.toolCalls[canonicalIndex]?.arguments || '';
+    const argumentsDelta = computeNovelResponsesDelta(existingArguments, rawArguments);
+    const knownTool = context.toolCalls[canonicalIndex] || {};
+    const shouldBackfillId = !!toolCallId && !knownTool.id;
+    const shouldBackfillName = !!toolName && !knownTool.name;
+    if (!argumentsDelta && !shouldBackfillId && !shouldBackfillName) {
+      return {};
+    }
+
+    return {
+      toolCallDeltas: [{
+        index: canonicalIndex,
+        id: toolCallId,
+        name: toolName,
+        argumentsDelta: argumentsDelta || undefined,
+      }],
+    };
+  }
+
   if (type === 'response.completed' && isRecord((payload as any).response)) {
     const responsePayload = (payload as any).response as Record<string, unknown>;
     if (isNonEmptyString(responsePayload.id)) context.id = responsePayload.id;
     if (isNonEmptyString(responsePayload.model)) context.model = responsePayload.model;
+    const content = parseResponsesOutputText(responsePayload);
+    const contentDelta = computeNovelResponsesDelta(joinIndexedResponsesText(context.responsesTextByIndex), content);
+    if (content) {
+      context.responsesTextByIndex = { ...context.responsesTextByIndex, [-1]: content } as Record<number, string>;
+    }
+    const responsesReasoning = parseResponsesReasoning(responsePayload);
+    const reasoningDelta = computeNovelResponsesDelta(
+      joinIndexedResponsesText(context.responsesReasoningByIndex),
+      responsesReasoning.reasoningContent,
+    );
+    if (responsesReasoning.reasoningContent) {
+      context.responsesReasoningByIndex = {
+        ...context.responsesReasoningByIndex,
+        [-1]: responsesReasoning.reasoningContent,
+      } as Record<number, string>;
+    }
+    const toolCalls = collectIndexedToolCallsFromResponsesPayload(responsePayload);
+    const toolCallDeltas = toolCalls
+      .map((toolCall) => {
+        const canonicalIndex = resolveResponsesToolCallIndex(context, {
+          outputIndex: toolCall.outputIndex,
+          callId: toolCall.id,
+        });
+        const knownTool = context.toolCalls[canonicalIndex] || {};
+        const argumentsDelta = computeNovelResponsesDelta(knownTool.arguments || '', toolCall.arguments);
+        const shouldBackfillId = !!toolCall.id && !knownTool.id;
+        const shouldBackfillName = !!toolCall.name && !knownTool.name;
+        if (!argumentsDelta && !shouldBackfillId && !shouldBackfillName) {
+          return null;
+        }
+        return {
+          index: canonicalIndex,
+          id: toolCall.id,
+          name: toolCall.name,
+          argumentsDelta: argumentsDelta || undefined,
+        };
+      })
+      .filter((item): item is NonNullable<typeof item> => !!item);
     return {
-      finishReason: normalizeStopReason(responsePayload.status) || 'stop',
+      ...(contentDelta || reasoningDelta ? { role: 'assistant' as const } : {}),
+      ...(contentDelta ? { contentDelta } : {}),
+      ...(reasoningDelta ? { reasoningDelta } : {}),
+      ...(responsesReasoning.reasoningSignature ? { reasoningSignature: responsesReasoning.reasoningSignature } : {}),
+      ...(toolCallDeltas.length > 0 ? { toolCallDeltas } : {}),
+      finishReason: toolCallDeltas.length > 0
+        ? 'tool_calls'
+        : (normalizeStopReason(responsePayload.status) || 'stop'),
       done: true,
     };
   }
diff --git a/src/server/transformers/shared/normalized.test.ts b/src/server/transformers/shared/normalized.test.ts
index 6555893..3d3d7df 100644
--- a/src/server/transformers/shared/normalized.test.ts
+++ b/src/server/transformers/shared/normalized.test.ts
@@ -3,8 +3,10 @@ import { describe, expect, it } from 'vitest';
 
 import {
   fromTransformerMetadataRecord,
+  createStreamTransformContext,
   normalizeStopReason,
   normalizeUpstreamFinalResponse,
+  normalizeUpstreamStreamEvent,
   parseDownstreamChatRequest,
   pullSseEventsWithDone,
   serializeFinalResponse,
@@ -79,6 +81,97 @@ describe('shared normalized helpers', () => {
     });
   });
 
+  it('normalizes custom tool calls from responses payloads through the existing tool-call shape', () => {
+    expect(normalizeUpstreamFinalResponse({
+      object: 'response',
+      id: 'resp_custom_tool_1',
+      model: 'gpt-test',
+      created: 123,
+      output: [
+        {
+          type: 'custom_tool_call',
+          call_id: 'call_custom',
+          name: 'MyTool',
+          input: '{"path":"README.md"}',
+        },
+      ],
+      status: 'completed',
+    }, 'fallback-model')).toEqual({
+      id: 'resp_custom_tool_1',
+      model: 'gpt-test',
+      created: 123,
+      content: '',
+      reasoningContent: '',
+      finishReason: 'tool_calls',
+      toolCalls: [{
+        id: 'call_custom',
+        name: 'MyTool',
+        arguments: '{"path":"README.md"}',
+      }],
+    });
+  });
+
+  it('unwraps terminal response.completed envelopes when normalizing final responses', () => {
+    expect(normalizeUpstreamFinalResponse({
+      type: 'response.completed',
+      response: {
+        id: 'resp_terminal_1',
+        model: 'gpt-test',
+        created_at: 123,
+        status: 'completed',
+        output: [
+          {
+            type: 'message',
+            content: [{ type: 'output_text', text: 'hello' }],
+          },
+          {
+            type: 'custom_tool_call',
+            call_id: 'call_custom_1',
+            name: 'Shell',
+            input: '{"command":"pwd"}',
+          },
+        ],
+      },
+    }, 'fallback-model')).toEqual({
+      id: 'resp_terminal_1',
+      model: 'gpt-test',
+      created: 123,
+      content: 'hello',
+      reasoningContent: '',
+      finishReason: 'tool_calls',
+      toolCalls: [{
+        id: 'call_custom_1',
+        name: 'Shell',
+        arguments: '{"command":"pwd"}',
+      }],
+    });
+  });
+
+  it('unwraps terminal response.incomplete envelopes when normalizing final responses', () => {
+    expect(normalizeUpstreamFinalResponse({
+      type: 'response.incomplete',
+      response: {
+        id: 'resp_terminal_2',
+        model: 'gpt-test',
+        created: 456,
+        output: [
+          {
+            type: 'message',
+            content: [{ type: 'output_text', text: 'partial answer' }],
+          },
+        ],
+      },
+    }, 'fallback-model')).toEqual({
+      id: 'resp_terminal_2',
+      model: 'gpt-test',
+      created: 456,
+      content: 'partial answer',
+      reasoningContent: '',
+      finishReason: 'length',
+      toolCalls: [],
+    });
+  });
+
   it('preserves responses reasoning summaries and encrypted reasoning signatures in final normalization', () => {
     expect(normalizeUpstreamFinalResponse({
       object: 'response',
@@ -137,6 +230,142 @@ describe('shared normalized helpers', () => {
     });
   });
 
+  it('treats response.reasoning_summary_text.done as reasoning-only stream output', () => {
+    const context = createStreamTransformContext('gpt-test');
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.reasoning_summary_text.done',
+      item_id: 'rs_1',
+      output_index: 0,
+      summary_index: 0,
+      text: 'plan first',
+    }, context, 'fallback-model')).toEqual({
+      reasoningDelta: 'plan first',
+    });
+  });
+
+  it('normalizes terminal-only responses output_item.done message content into visible stream content', () => {
+    const context = createStreamTransformContext('gpt-test');
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.output_item.done',
+      output_index: 0,
+      item: {
+        id: 'msg_1',
+        type: 'message',
+        role: 'assistant',
+        status: 'completed',
+        content: [{ type: 'output_text', text: 'hello' }],
+      },
+    }, context, 'fallback-model')).toEqual({
+      role: 'assistant',
+      contentDelta: 'hello',
+    });
+  });
+
+  it('normalizes terminal-only responses output_item.done tool metadata into tool deltas', () => {
+    const context = createStreamTransformContext('gpt-test');
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.output_item.done',
+      output_index: 0,
+      item: {
+        id: 'fc_1',
+        type: 'function_call',
+        call_id: 'call_1',
+        name: 'Glob',
+        arguments: '{"pattern":"README*"}',
+        status: 'completed',
+      },
+    }, context, 'fallback-model')).toEqual({
+      toolCallDeltas: [{
+        index: 0,
+        id: 'call_1',
+        name: 'Glob',
+        argumentsDelta: '{"pattern":"README*"}',
+      }],
+    });
+  });
+
+  it('normalizes terminal-only custom tool responses into tool deltas and final tool calls', () => {
+    const context = createStreamTransformContext('gpt-test');
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.output_item.done',
+      output_index: 0,
+      item: {
+        id: 'ct_1',
+        type: 'custom_tool_call',
+        call_id: 'call_custom_1',
+        name: 'MyTool',
+        input: '{"foo":"bar"}',
+        status: 'completed',
+      },
+    }, context, 'fallback-model')).toEqual({
+      toolCallDeltas: [{
+        index: 0,
+        id: 'call_custom_1',
+        name: 'MyTool',
+        argumentsDelta: '{"foo":"bar"}',
+      }],
+    });
+
+    expect(normalizeUpstreamFinalResponse({
+      object: 'response',
+      id: 'resp_custom_tool_1',
+      model: 'gpt-test',
+      created: 321,
+      output: [
+        {
+          id: 'ct_1',
+          type: 'custom_tool_call',
+          call_id: 'call_custom_1',
+          name: 'MyTool',
+          input: '{"foo":"bar"}',
+        },
+      ],
+      status: 'completed',
+    }, 'fallback-model')).toEqual({
+      id: 'resp_custom_tool_1',
+      model: 'gpt-test',
+      created: 321,
+      content: '',
+      reasoningContent: '',
+      finishReason: 'tool_calls',
+      toolCalls: [{
+        id: 'call_custom_1',
+        name: 'MyTool',
+        arguments: '{"foo":"bar"}',
+      }],
+    });
+  });
+
+  it('normalizes terminal-only responses output payloads carried on response.completed', () => {
+    const context = createStreamTransformContext('gpt-test');
+
+    expect(normalizeUpstreamStreamEvent({
+      type: 'response.completed',
+      response: {
+        id: 'resp_done_only',
+        model: 'gpt-test',
+        status: 'completed',
+        output: [
+          {
+            id: 'msg_1',
+            type: 'message',
+            role: 'assistant',
+            status: 'completed',
+            content: [{ type: 'output_text', text: 'hello' }],
+          },
+        ],
+      },
+    }, context, 'fallback-model')).toMatchObject({
+      contentDelta: 'hello',
+      finishReason: 'stop',
+      done: true,
+    });
+  });
+
   it('maps claude tools, tool choice, metadata, and reasoning when parsing downstream requests', () => {
     const result = parseDownstreamChatRequest({
       model: 'gpt-5',