diff --git a/backend/internal/handler/openai_gateway_compact_body_signal_test.go b/backend/internal/handler/openai_gateway_compact_body_signal_test.go
index e7d054d185..a4bfb90466 100644
--- a/backend/internal/handler/openai_gateway_compact_body_signal_test.go
+++ b/backend/internal/handler/openai_gateway_compact_body_signal_test.go
@@ -117,3 +117,46 @@ func TestNormalizeOpenAIResponsesCompactRequest_SubpathNotPromoted(t *testing.T)
 	require.Equal(t, "/v1/responses/resp_123/cancel", c.Request.URL.Path)
 	require.Equal(t, body, normalized)
 }
+
+// 回归 #3875：body-signal 原始请求 stream:true 时必须标记 client-stream，
+// 供响应写回阶段把上游 unary JSON 合成回 Codex remote compact v2 所需的 SSE。
+func TestNormalizeOpenAIResponsesCompactRequest_BodySignalStreamTrueMarksClientStream(t *testing.T) {
+	h := &OpenAIGatewayHandler{}
+	body := []byte(`{"model":"gpt-5.5","stream":true,"input":[{"type":"compaction_trigger"}]}`)
+	c := newCompactBodySignalTestContext(t, "/v1/responses", body)
+
+	_, ok := h.normalizeOpenAIResponsesCompactRequest(c, zap.NewNop(), body)
+	require.True(t, ok)
+
+	marked, exists := c.Get(service.OpenAICompactClientStreamKeyForTest())
+	require.True(t, exists)
+	require.Equal(t, true, marked)
+}
+
+func TestNormalizeOpenAIResponsesCompactRequest_BodySignalStreamFalseNotMarked(t *testing.T) {
+	h := &OpenAIGatewayHandler{}
+	for name, body := range map[string][]byte{
+		"stream_false":  []byte(`{"model":"gpt-5.5","stream":false,"input":[{"type":"compaction_trigger"}]}`),
+		"stream_absent": []byte(`{"model":"gpt-5.5","input":[{"type":"compaction_trigger"}]}`),
+	} {
+		c := newCompactBodySignalTestContext(t, "/v1/responses", body)
+		_, ok := h.normalizeOpenAIResponsesCompactRequest(c, zap.NewNop(), body)
+		require.True(t, ok, name)
+		require.Equal(t, "/v1/responses/compact", c.Request.URL.Path, name)
+		_, exists := c.Get(service.OpenAICompactClientStreamKeyForTest())
+		require.False(t, exists, "case %s 不应标记 client-stream", name)
+	}
+}
+
+// path-based compact（Codex v1 unary 协议）即使 body 带 stream:true 也不标记，
+// 保持 JSON 写回行为不变。
+func TestNormalizeOpenAIResponsesCompactRequest_PathBasedStreamTrueNotMarked(t *testing.T) {
+	h := &OpenAIGatewayHandler{}
+	body := []byte(`{"model":"gpt-5.5","stream":true,"input":[{"type":"message","role":"user","content":"hello"}]}`)
+	c := newCompactBodySignalTestContext(t, "/v1/responses/compact", body)
+
+	_, ok := h.normalizeOpenAIResponsesCompactRequest(c, zap.NewNop(), body)
+	require.True(t, ok)
+	_, exists := c.Get(service.OpenAICompactClientStreamKeyForTest())
+	require.False(t, exists)
+}
diff --git a/backend/internal/handler/openai_gateway_handler.go b/backend/internal/handler/openai_gateway_handler.go
index f049711b7f..feba261103 100644
--- a/backend/internal/handler/openai_gateway_handler.go
+++ b/backend/internal/handler/openai_gateway_handler.go
@@ -585,7 +585,14 @@ func (h *OpenAIGatewayHandler) normalizeOpenAIResponsesCompactRequest(c *gin.Con
 	if !isCompactRequest && isBareOpenAIResponsesPath(c) && service.HasCompactionTriggerInInput(body) {
 		c.Request.URL.Path = strings.TrimRight(c.Request.URL.Path, "/") + "/compact"
 		isCompactRequest = true
-		reqLog.Info("codex.remote_compact.detected_body_signal")
+		// Codex remote compact v2 的原始请求是流式 /responses：白名单归一化会删除
+		// stream 并让上游走 unary JSON，但客户端仍按 SSE 消费响应。记录原始
+		// stream 意图，响应写回阶段据此把 JSON 合成回 SSE（#3875）。
+		clientStream := gjson.GetBytes(body, "stream").Bool()
+		if clientStream {
+			service.MarkOpenAICompactClientStream(c)
+		}
+		reqLog.Info("codex.remote_compact.detected_body_signal", zap.Bool("client_stream", clientStream))
 	}
 	if !isCompactRequest {
 		return body, true
diff --git a/backend/internal/service/openai_compact_stream_bridge.go b/backend/internal/service/openai_compact_stream_bridge.go
new file mode 100644
index 0000000000..ec92e581a4
--- /dev/null
+++ b/backend/internal/service/openai_compact_stream_bridge.go
@@ -0,0 +1,149 @@
+package service
+
+import (
+	"bytes"
+	"encoding/json"
+	"strings"
+
+	"github.com/gin-gonic/gin"
+	"github.com/google/uuid"
+	"github.com/tidwall/gjson"
+	"github.com/tidwall/sjson"
+)
+
+// openAICompactClientStreamKey 标记 body-signal compact 请求（Codex remote
+// compact v2，见 #3777）的原始 body 携带 stream:true。白名单归一化会删除
+// stream 字段并让上游走 unary /responses/compact（JSON），但客户端仍按
+// Responses SSE 协议消费响应：它必须收到 response.output_item.done（其中恰好
+// 一个 type=compaction 的 item）和 response.completed，否则报
+// "stream closed before response.completed" 并无限重连（#3875）。
+const openAICompactClientStreamKey = "openai_compact_client_stream"
+
+// MarkOpenAICompactClientStream 由 handler 在 body-signal 提升时调用，记录
+// 客户端的原始 stream 意图，供响应写回阶段决定是否合成 SSE。
+func MarkOpenAICompactClientStream(c *gin.Context) {
+	if c == nil {
+		return
+	}
+	c.Set(openAICompactClientStreamKey, true)
+}
+
+func OpenAICompactClientStreamKeyForTest() string {
+	return openAICompactClientStreamKey
+}
+
+func openAICompactClientWantsStream(c *gin.Context) bool {
+	if c == nil {
+		return false
+	}
+	value, ok := c.Get(openAICompactClientStreamKey)
+	if !ok {
+		return false
+	}
+	wants, _ := value.(bool)
+	return wants
+}
+
+// writeOpenAICompactSSEBridge 将 unary compact 的最终 JSON 响应按 Codex remote
+// compact v2 的消费协议合成为最小 Responses SSE 流写回客户端。仅当请求被标记
+// 为 body-signal 客户端流式、状态码为 2xx 且 body 是合法 JSON 对象时生效；
+// 返回 false 表示未写出任何内容，调用方应按原路径写回。
+func writeOpenAICompactSSEBridge(c *gin.Context, statusCode int, finalResponse []byte) bool {
+	if c == nil || statusCode < 200 || statusCode >= 300 || !openAICompactClientWantsStream(c) {
+		return false
+	}
+	payload, ok := buildOpenAICompactSSEPayload(finalResponse)
+	if !ok {
+		return false
+	}
+	header := c.Writer.Header()
+	header.Set("Content-Type", "text/event-stream")
+	header.Set("Cache-Control", "no-cache")
+	header.Set("Connection", "keep-alive")
+	header.Set("X-Accel-Buffering", "no")
+	c.Writer.WriteHeader(statusCode)
+	_, _ = c.Writer.Write(payload)
+	c.Writer.Flush()
+	return true
+}
+
+// buildOpenAICompactSSEPayload 把 compact 的 Response JSON 转成 SSE 事件序列：
+// 每个 output[] item 一条 response.output_item.done，最后一条 response.completed
+// 携带完整 response 对象。Codex 的 SSE 解析只从 output_item.done 收集 item，
+// 并要求 response.completed 的 response.id 必填、usage（若存在）必须携带
+// input_tokens/output_tokens/total_tokens 整数字段，否则整条 completed 事件
+// 解析失败，故此处做兜底修补。
+func buildOpenAICompactSSEPayload(finalResponse []byte) ([]byte, bool) {
+	if len(finalResponse) == 0 || !gjson.ValidBytes(finalResponse) {
+		return nil, false
+	}
+	if !gjson.ParseBytes(finalResponse).IsObject() {
+		return nil, false
+	}
+	// SSE 的 data 行不允许出现裸换行：上游 JSON 可能是 pretty-printed 形态，
+	// 嵌入前必须压缩为单行。
+	var compacted bytes.Buffer
+	if err := json.Compact(&compacted, finalResponse); err != nil {
+		return nil, false
+	}
+	response := compacted.Bytes()
+	root := gjson.ParseBytes(response)
+	if strings.TrimSpace(root.Get("id").String()) == "" {
+		next, err := sjson.SetBytes(response, "id", "resp_"+strings.ReplaceAll(uuid.NewString(), "-", ""))
+		if err != nil {
+			return nil, false
+		}
+		response = next
+	}
+	if usage := gjson.GetBytes(response, "usage"); usage.Exists() && !openAICompactUsageParsableByCodex(usage) {
+		next, err := sjson.DeleteBytes(response, "usage")
+		if err != nil {
+			return nil, false
+		}
+		response = next
+	}
+
+	var buf bytes.Buffer
+	outputIndex := 0
+	appendEvent := func(eventType string, data []byte) {
+		buf.WriteString("event: ")
+		buf.WriteString(eventType)
+		buf.WriteString("\ndata: ")
+		buf.Write(data)
+		buf.WriteString("\n\n")
+	}
+	for _, item := range gjson.GetBytes(response, "output").Array() {
+		if !item.IsObject() {
+			continue
+		}
+		event, err := sjson.SetBytes([]byte(`{"type":"response.output_item.done"}`), "output_index", outputIndex)
+		if err != nil {
+			return nil, false
+		}
+		event, err = sjson.SetRawBytes(event, "item", []byte(item.Raw))
+		if err != nil {
+			return nil, false
+		}
+		appendEvent("response.output_item.done", event)
+		outputIndex++
+	}
+
+	completed, err := sjson.SetRawBytes([]byte(`{"type":"response.completed"}`), "response", response)
+	if err != nil {
+		return nil, false
+	}
+	appendEvent("response.completed", completed)
+	return buf.Bytes(), true
+}
+
+func openAICompactUsageParsableByCodex(usage gjson.Result) bool {
+	if !usage.IsObject() {
+		return false
+	}
+	for _, field := range []string{"input_tokens", "output_tokens", "total_tokens"} {
+		if usage.Get(field).Type != gjson.Number {
+			return false
+		}
+	}
+	return true
+}
diff --git a/backend/internal/service/openai_compact_stream_bridge_test.go b/backend/internal/service/openai_compact_stream_bridge_test.go
new file mode 100644
index 0000000000..2eb5ba80ee
--- /dev/null
+++ b/backend/internal/service/openai_compact_stream_bridge_test.go
@@ -0,0 +1,286 @@
+package service
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+	"github.com/tidwall/gjson"
+)
+
+func newCompactBridgeTestContext(t *testing.T, markClientStream bool) (*gin.Context, *httptest.ResponseRecorder) {
+	t.Helper()
+	gin.SetMode(gin.TestMode)
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/responses/compact", nil)
+	if markClientStream {
+		MarkOpenAICompactClientStream(c)
+	}
+	return c, rec
+}
+
+func newCompactBridgeTestService() *OpenAIGatewayService {
+	cfg := &config.Config{}
+	return &OpenAIGatewayService{
+		cfg:           cfg,
+		toolCorrector: NewCodexToolCorrector(),
+	}
+}
+
+// parseCompactBridgeSSE 把合成的 SSE 文本拆成 (eventType, dataJSON) 序列。
+func parseCompactBridgeSSE(t *testing.T, body string) [][2]string {
+	t.Helper()
+	var events [][2]string
+	for _, block := range strings.Split(strings.TrimSpace(body), "\n\n") {
+		lines := strings.Split(block, "\n")
+		require.Len(t, lines, 2, "每个 SSE 事件应为 event+data 两行: %q", block)
+		require.True(t, strings.HasPrefix(lines[0], "event: "), "缺少 event 行: %q", block)
+		require.True(t, strings.HasPrefix(lines[1], "data: "), "缺少 data 行: %q", block)
+		events = append(events, [2]string{
+			strings.TrimPrefix(lines[0], "event: "),
+			strings.TrimPrefix(lines[1], "data: "),
+		})
+	}
+	return events
+}
+
+func TestBuildOpenAICompactSSEPayload_EmitsItemsAndCompleted(t *testing.T) {
+	finalResponse := []byte(`{
+		"id":"resp_compact_1",
+		"object":"response",
+		"model":"gpt-5.1-codex",
+		"status":"completed",
+		"output":[
+			{"id":"cmp_1","type":"compaction","status":"completed","encrypted_content":"compact-payload","summary":[{"type":"summary_text","text":"compact summary"}],"opaque":{"kept":true}},
+			{"id":"msg_1","type":"message","role":"assistant","content":[{"type":"output_text","text":"done"}]}
+		],
+		"usage":{"input_tokens":9,"output_tokens":4,"total_tokens":13}
+	}`)
+
+	payload, ok := buildOpenAICompactSSEPayload(finalResponse)
+	require.True(t, ok)
+
+	events := parseCompactBridgeSSE(t, string(payload))
+	require.Len(t, events, 3)
+
+	require.Equal(t, "response.output_item.done", events[0][0])
+	first := events[0][1]
+	require.Equal(t, "response.output_item.done", gjson.Get(first, "type").String())
+	require.Equal(t, int64(0), gjson.Get(first, "output_index").Int())
+	require.Equal(t, "compaction", gjson.Get(first, "item.type").String())
+	require.Equal(t, "cmp_1", gjson.Get(first, "item.id").String())
+	require.Equal(t, "compact-payload", gjson.Get(first, "item.encrypted_content").String())
+	require.Equal(t, "compact summary", gjson.Get(first, "item.summary.0.text").String())
+	require.True(t, gjson.Get(first, "item.opaque.kept").Bool(), "item 原始字段必须逐字节保留")
+
+	require.Equal(t, "response.output_item.done", events[1][0])
+	require.Equal(t, int64(1), gjson.Get(events[1][1], "output_index").Int())
+	require.Equal(t, "message", gjson.Get(events[1][1], "item.type").String())
+
+	require.Equal(t, "response.completed", events[2][0])
+	completed := events[2][1]
+	require.Equal(t, "response.completed", gjson.Get(completed, "type").String())
+	require.Equal(t, "resp_compact_1", gjson.Get(completed, "response.id").String())
+	require.Equal(t, int64(13), gjson.Get(completed, "response.usage.total_tokens").Int())
+	require.Len(t, gjson.Get(completed, "response.output").Array(), 2)
+}
+
+func TestBuildOpenAICompactSSEPayload_InjectsMissingResponseID(t *testing.T) {
+	payload, ok := buildOpenAICompactSSEPayload([]byte(`{"output":[{"type":"compaction","encrypted_content":"x"}]}`))
+	require.True(t, ok)
+
+	events := parseCompactBridgeSSE(t, string(payload))
+	require.Len(t, events, 2)
+	completed := events[1][1]
+	// Codex 的 ResponseCompleted 解析要求 response.id 为非空 string，缺失时必须注入。
+	id := gjson.Get(completed, "response.id").String()
+	require.True(t, strings.HasPrefix(id, "resp_"), "缺失 id 必须注入 resp_* 兜底: %q", id)
+	require.NotEqual(t, "resp_", id)
+}
+
+func TestBuildOpenAICompactSSEPayload_DropsMalformedUsage(t *testing.T) {
+	payload, ok := buildOpenAICompactSSEPayload([]byte(`{
+		"id":"resp_1",
+		"output":[{"type":"compaction","encrypted_content":"x"}],
+		"usage":{"prompt_tokens":9,"completion_tokens":4}
+	}`))
+	require.True(t, ok)
+
+	events := parseCompactBridgeSSE(t, string(payload))
+	completed := events[len(events)-1][1]
+	// usage 缺少 Codex 必需的整数字段时必须整体删除，否则 completed 事件解析失败。
+	require.False(t, gjson.Get(completed, "response.usage").Exists())
+}
+
+func TestBuildOpenAICompactSSEPayload_KeepsWellFormedUsage(t *testing.T) {
+	payload, ok := buildOpenAICompactSSEPayload([]byte(`{
+		"id":"resp_1",
+		"output":[{"type":"compaction","encrypted_content":"x"}],
+		"usage":{"input_tokens":9,"output_tokens":4,"total_tokens":13,"input_tokens_details":{"cached_tokens":2}}
+	}`))
+	require.True(t, ok)
+
+	events := parseCompactBridgeSSE(t, string(payload))
+	completed := events[len(events)-1][1]
+	require.Equal(t, int64(9), gjson.Get(completed, "response.usage.input_tokens").Int())
+	require.Equal(t, int64(2), gjson.Get(completed, "response.usage.input_tokens_details.cached_tokens").Int())
+}
+
+func TestBuildOpenAICompactSSEPayload_RejectsNonJSONObject(t *testing.T) {
+	for name, body := range map[string][]byte{
+		"empty":     nil,
+		"sse_text":  []byte("data: {\"type\":\"response.completed\"}\n\n"),
+		"array":     []byte(`[{"id":"resp_1"}]`),
+		"non_json":  []byte("upstream said no"),
+		"bare_true": []byte("true"),
+	} {
+		_, ok := buildOpenAICompactSSEPayload(body)
+		require.False(t, ok, "case %s 不应被合成为 SSE", name)
+	}
+}
+
+func TestWriteOpenAICompactSSEBridge_RequiresMarkAndSuccessStatus(t *testing.T) {
+	finalResponse := []byte(`{"id":"resp_1","output":[{"type":"compaction","encrypted_content":"x"}]}`)
+
+	// 未标记 client stream：不写出，走原 JSON 路径。
+	c, rec := newCompactBridgeTestContext(t, false)
+	require.False(t, writeOpenAICompactSSEBridge(c, http.StatusOK, finalResponse))
+	require.Zero(t, rec.Body.Len())
+
+	// 标记但上游非 2xx：错误响应保持 JSON 原样（Codex 依赖 HTTP 状态码走重试）。
+	c, rec = newCompactBridgeTestContext(t, true)
+	require.False(t, writeOpenAICompactSSEBridge(c, http.StatusBadGateway, finalResponse))
+	require.Zero(t, rec.Body.Len())
+
+	// 标记且 2xx：合成 SSE。
+	c, rec = newCompactBridgeTestContext(t, true)
+	require.True(t, writeOpenAICompactSSEBridge(c, http.StatusOK, finalResponse))
+	require.Equal(t, http.StatusOK, rec.Code)
+	require.Equal(t, "text/event-stream", rec.Header().Get("Content-Type"))
+	require.Contains(t, rec.Body.String(), "event: response.completed")
+}
+
+// 回归 #3875：body-signal 提升后的 compact 请求，上游返回 unary JSON，
+// 客户端（Codex remote compact v2）必须收到 SSE 事件流而非 JSON 文档，
+// 否则报 "stream closed before response.completed" 并无限重连。
+func TestHandleNonStreamingResponse_CompactClientStreamBridgesToSSE(t *testing.T) {
+	svc := newCompactBridgeTestService()
+	c, rec := newCompactBridgeTestContext(t, true)
+	resp := &http.Response{
+		StatusCode: http.StatusOK,
+		Header:     http.Header{"Content-Type": []string{"application/json"}},
+		Body: io.NopCloser(strings.NewReader(`{
+			"id":"resp_compact_json",
+			"object":"response",
+			"model":"gpt-5.1-codex",
+			"status":"completed",
+			"output":[{"id":"cmp_1","type":"compaction","status":"completed","encrypted_content":"compact-payload"}],
+			"usage":{"input_tokens":9,"output_tokens":4,"total_tokens":13}
+		}`)),
+	}
+
+	result, err := svc.handleNonStreamingResponse(context.Background(), resp, c, &Account{ID: 1, Type: AccountTypeOAuth}, "gpt-5.5", "gpt-5.5")
+	require.NoError(t, err)
+	require.NotNil(t, result)
+
+	require.Equal(t, "text/event-stream", rec.Header().Get("Content-Type"))
+	events := parseCompactBridgeSSE(t, rec.Body.String())
+	require.Len(t, events, 2)
+	require.Equal(t, "response.output_item.done", events[0][0])
+	require.Equal(t, "compaction", gjson.Get(events[0][1], "item.type").String())
+	require.Equal(t, "response.completed", events[1][0])
+	require.Equal(t, "resp_compact_json", gjson.Get(events[1][1], "response.id").String())
+
+	// 计费与响应元数据不受写回形态影响。
+	require.NotNil(t, result.usage)
+	require.Equal(t, 9, result.usage.InputTokens)
+	require.Equal(t, 4, result.usage.OutputTokens)
+	require.Equal(t, "resp_compact_json", result.responseID)
+}
+
+// 回归防护：path-based compact（Codex v1 unary 协议、链式 sub2api）未标记
+// client stream，必须保持 v0.1.146 以来的 JSON 写回行为。
+func TestHandleNonStreamingResponse_PathBasedCompactStaysJSON(t *testing.T) {
+	svc := newCompactBridgeTestService()
+	c, rec := newCompactBridgeTestContext(t, false)
+	resp := &http.Response{
+		StatusCode: http.StatusOK,
+		Header:     http.Header{"Content-Type": []string{"application/json"}},
+		Body: io.NopCloser(strings.NewReader(`{
+			"id":"resp_compact_json",
+			"output":[{"id":"cmp_1","type":"compaction","encrypted_content":"compact-payload"}],
+			"usage":{"input_tokens":9,"output_tokens":4,"total_tokens":13}
+		}`)),
+	}
+
+	result, err := svc.handleNonStreamingResponse(context.Background(), resp, c, &Account{ID: 1, Type: AccountTypeOAuth}, "gpt-5.5", "gpt-5.5")
+	require.NoError(t, err)
+	require.NotNil(t, result)
+
+	require.NotContains(t, rec.Header().Get("Content-Type"), "text/event-stream")
+	body := rec.Body.String()
+	require.Equal(t, "resp_compact_json", gjson.Get(body, "id").String())
+	require.Equal(t, "compaction", gjson.Get(body, "output.0.type").String())
+}
+
+// 上游对 compact 返回 SSE（如链式网关）时，最终响应经 SSE→JSON 提取后，
+// 对 client-stream 请求同样必须再合成回 SSE。
+func TestHandleSSEToJSON_CompactClientStreamBridgesToSSE(t *testing.T) {
+	svc := newCompactBridgeTestService()
+	c, rec := newCompactBridgeTestContext(t, true)
+	upstreamSSE := strings.Join([]string{
+		`data: {"type":"response.completed","response":{"id":"resp_compact_sse","object":"response","model":"gpt-5.1-codex","status":"completed","output":[{"id":"cmp_sse_1","type":"compaction","status":"completed","encrypted_content":"compact-sse-payload"}],"usage":{"input_tokens":3,"output_tokens":2,"total_tokens":5}}}`,
+		"",
+	}, "\n")
+	resp := &http.Response{
+		StatusCode: http.StatusOK,
+		Header:     http.Header{"Content-Type": []string{"text/event-stream"}},
+		Body:       io.NopCloser(strings.NewReader(upstreamSSE)),
+	}
+
+	result, err := svc.handleNonStreamingResponse(context.Background(), resp, c, &Account{ID: 1, Type: AccountTypeOAuth}, "gpt-5.5", "gpt-5.5")
+	require.NoError(t, err)
+	require.NotNil(t, result)
+
+	require.Equal(t, "text/event-stream", rec.Header().Get("Content-Type"))
+	events := parseCompactBridgeSSE(t, rec.Body.String())
+	require.Len(t, events, 2)
+	require.Equal(t, "response.output_item.done", events[0][0])
+	require.Equal(t, "compact-sse-payload", gjson.Get(events[0][1], "item.encrypted_content").String())
+	require.Equal(t, "response.completed", events[1][0])
+	require.Equal(t, "resp_compact_sse", gjson.Get(events[1][1], "response.id").String())
+}
+
+// 透传分支（OAuth passthrough）同样命中桥接。
+func TestHandleNonStreamingResponsePassthrough_CompactClientStreamBridgesToSSE(t *testing.T) {
+	svc := newCompactBridgeTestService()
+	c, rec := newCompactBridgeTestContext(t, true)
+	resp := &http.Response{
+		StatusCode: http.StatusOK,
+		Header:     http.Header{"Content-Type": []string{"application/json"}},
+		Body: io.NopCloser(strings.NewReader(`{
+			"id":"resp_compact_pt",
+			"output":[{"id":"cmp_pt_1","type":"compaction","encrypted_content":"compact-pt-payload"}],
+			"usage":{"input_tokens":7,"output_tokens":3,"total_tokens":10}
+		}`)),
+	}
+
+	result, err := svc.handleNonStreamingResponsePassthrough(context.Background(), resp, c, "gpt-5.5", "")
+	require.NoError(t, err)
+	require.NotNil(t, result)
+
+	require.Equal(t, "text/event-stream", rec.Header().Get("Content-Type"))
+	events := parseCompactBridgeSSE(t, rec.Body.String())
+	require.Len(t, events, 2)
+	require.Equal(t, "compaction", gjson.Get(events[0][1], "item.type").String())
+	require.Equal(t, "resp_compact_pt", gjson.Get(events[1][1], "response.id").String())
+	require.NotNil(t, result.usage)
+	require.Equal(t, 7, result.usage.InputTokens)
+}
diff --git a/backend/internal/service/openai_gateway_passthrough.go b/backend/internal/service/openai_gateway_passthrough.go
index 635b2ac950..3a56ab5c61 100644
--- a/backend/internal/service/openai_gateway_passthrough.go
+++ b/backend/internal/service/openai_gateway_passthrough.go
@@ -1088,7 +1088,9 @@ func (s *OpenAIGatewayService) handleNonStreamingResponsePassthrough(
 	if originalModel != "" && mappedModel != "" && originalModel != mappedModel {
 		body = s.replaceModelInResponseBody(body, mappedModel, originalModel)
 	}
-	c.Data(resp.StatusCode, contentType, body)
+	if !writeOpenAICompactSSEBridge(c, resp.StatusCode, body) {
+		c.Data(resp.StatusCode, contentType, body)
+	}
 	return &openaiNonStreamingResultPassthrough{
 		OpenAIUsage:      usage,
 		usage:            usage,
@@ -1151,7 +1153,9 @@ func (s *OpenAIGatewayService) handlePassthroughSSEToJSON(resp *http.Response, c
 			contentType = "text/event-stream"
 		}
 	}
-	c.Data(resp.StatusCode, contentType, body)
+	if !writeOpenAICompactSSEBridge(c, resp.StatusCode, body) {
+		c.Data(resp.StatusCode, contentType, body)
+	}
 
 	return &openaiNonStreamingResultPassthrough{
 		OpenAIUsage:      usage,
diff --git a/backend/internal/service/openai_gateway_response_handling.go b/backend/internal/service/openai_gateway_response_handling.go
index b814ff96f3..cb4b780cdf 100644
--- a/backend/internal/service/openai_gateway_response_handling.go
+++ b/backend/internal/service/openai_gateway_response_handling.go
@@ -825,7 +825,9 @@ func (s *OpenAIGatewayService) handleNonStreamingResponse(ctx context.Context, r
 		}
 	}
 
-	c.Data(resp.StatusCode, contentType, body)
+	if !writeOpenAICompactSSEBridge(c, resp.StatusCode, body) {
+		c.Data(resp.StatusCode, contentType, body)
+	}
 
 	return &openaiNonStreamingResult{
 		OpenAIUsage:      usage,
@@ -907,7 +909,9 @@ func (s *OpenAIGatewayService) handleSSEToJSON(resp *http.Response, c *gin.Conte
 			contentType = "text/event-stream"
 		}
 	}
-	c.Data(resp.StatusCode, contentType, body)
+	if !writeOpenAICompactSSEBridge(c, resp.StatusCode, body) {
+		c.Data(resp.StatusCode, contentType, body)
+	}
 
 	return &openaiNonStreamingResult{
 		OpenAIUsage:      usage,