diff --git a/src/server/transformers/openai/responses/conversion.test.ts b/src/server/transformers/openai/responses/conversion.test.ts
index 25c93d0..e3206bc 100644
--- a/src/server/transformers/openai/responses/conversion.test.ts
+++ b/src/server/transformers/openai/responses/conversion.test.ts
@@ -863,6 +863,51 @@ describe('convertOpenAiBodyToResponsesBody', () => {
     ]);
   });
 
+  it('drops tool outputs whose prior tool call cannot be converted into an OpenAI-compatible tool call', () => {
+    const result = convertResponsesBodyToOpenAiBody(
+      {
+        model: 'gpt-5',
+        input: [
+          {
+            type: 'function_call',
+            call_id: 'call_invalid',
+            name: '   ',
+            arguments: '{"plan":true}',
+          },
+          {
+            type: 'function_call_output',
+            call_id: 'call_invalid',
+            output: 'unsupported call',
+          },
+          {
+            type: 'message',
+            role: 'assistant',
+            content: [
+              {
+                type: 'output_text',
+                text: 'continue normally',
+              },
+            ],
+          },
+        ],
+      },
+      'gpt-5',
+      false,
+    );
+
+    expect(result.messages).toEqual([
+      {
+        role: 'assistant',
+        content: [
+          {
+            type: 'text',
+            text: 'continue normally',
+          },
+        ],
+      },
+    ]);
+  });
+
   it('shortens long MCP tool names consistently across tools, tool_choice and assistant tool calls', () => {
     const sharedSuffix = 'server__execute_super_long_nested_tool_name_that_needs_shortening';
     const firstName = `mcp__alpha_workspace__${sharedSuffix}`;
diff --git a/src/server/transformers/openai/responses/conversion.ts b/src/server/transformers/openai/responses/conversion.ts
index c2507c7..c7ba482 100644
--- a/src/server/transformers/openai/responses/conversion.ts
+++ b/src/server/transformers/openai/responses/conversion.ts
@@ -844,9 +844,14 @@ export function convertResponsesBodyToOpenAiBody(
   const input = stripOrphanedResponsesToolOutputs(normalizedBody.input);
   let functionCallIndex = 0;
   let pendingToolCalls: OpenAiToolCall[] = [];
+  const emittedToolCallIds = new Set<string>();
 
   const flushPendingToolCalls = () => {
     if (pendingToolCalls.length <= 0) return;
+    for (const toolCall of pendingToolCalls) {
+      const callId = asTrimmedString(toolCall.id);
+      if (callId) emittedToolCallIds.add(callId);
+    }
     messages.push({
       role: 'assistant',
       content: '',
@@ -894,7 +899,9 @@ export function convertResponsesBodyToOpenAiBody(
 
     if (itemType === 'function_call_output' || itemType === 'custom_tool_call_output') {
       flushPendingToolCalls();
-      pushToolOutputMessage(item.call_id ?? item.id, item.output ?? item.content);
+      const toolCallId = asTrimmedString(item.call_id ?? item.id);
+      if (!toolCallId || !emittedToolCallIds.has(toolCallId)) return;
+      pushToolOutputMessage(toolCallId, item.output ?? item.content);
       return;
     }
 
@@ -924,7 +931,9 @@ export function convertResponsesBodyToOpenAiBody(
     const content = toOpenAiMessageContent(item.content ?? item.input ?? item);
 
     if (normalizedRole === 'tool') {
-      pushToolOutputMessage(item.tool_call_id ?? item.call_id ?? item.id, item.content);
+      const toolCallId = asTrimmedString(item.tool_call_id ?? item.call_id ?? item.id);
+      if (!toolCallId || !emittedToolCallIds.has(toolCallId)) return;
+      pushToolOutputMessage(toolCallId, item.content);
       return;
     }