diff --git a/backend/internal/handler/openai_gateway_handler.go b/backend/internal/handler/openai_gateway_handler.go
index 1695eccc1b..adecb57a7c 100644
--- a/backend/internal/handler/openai_gateway_handler.go
+++ b/backend/internal/handler/openai_gateway_handler.go
@@ -567,7 +567,7 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 		// Forward request
 		service.SetOpsLatencyMs(c, service.OpsRoutingLatencyMsKey, time.Since(routingStart).Milliseconds())
 		forwardStart := time.Now()
-		// 用扣除 compact 心跳字节的口径快照：心跳注释不构成语义响应，
+		// 用扣除非语义心跳字节的口径快照：心跳注释不构成语义响应，
 		// 不能因心跳字节变化而放弃 failover 换号（#3887）。
 		writerSizeBeforeForward := service.OpenAICompactKeepaliveAdjustedWrittenSize(c)
 		// 跨 passthrough 边界的 failover：从 Kiro 等透传账号切到 Bedrock 等非透传账号前，
@@ -618,7 +618,9 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 						h.handleFailoverExhausted(c, failoverErr, true)
 						return
 					}
-					if failoverErr.SafeToFailoverAfterWrite && c.Writer.Written() {
+					// openAIForwardMayFailover 已确认写出的字节不含语义输出，
+					// 但重试耗尽时仍须按已提交的 SSE 响应返回流内错误。
+					if c.Writer.Written() {
 						streamStarted = true
 					}
 					if failoverErr.ShouldReportAccountScheduleFailure() {
diff --git a/backend/internal/service/openai_capacity_shed_test.go b/backend/internal/service/openai_capacity_shed_test.go
index e68a48db1c..70153a0d0b 100644
--- a/backend/internal/service/openai_capacity_shed_test.go
+++ b/backend/internal/service/openai_capacity_shed_test.go
@@ -94,6 +94,11 @@ func TestOpenAIStreamErrorFrameDoesNotStartClientOutput(t *testing.T) {
 		{`{"type":"response.failed","response":{"error":{"code":"server_is_overloaded"}}}`, "response.failed", false},
 		{`{"type":"response.created","response":{"id":"resp_1"}}`, "response.created", false},
 		{`{"type":"response.in_progress","response":{"id":"resp_1"}}`, "response.in_progress", false},
+		{`{"type":"response.output_item.added","item":{"type":"reasoning","summary":[]}}`, "response.output_item.added", false},
+		{`{"type":"response.output_item.added","item":{"type":"reasoning","encrypted_content":"ciphertext"}}`, "response.output_item.added", true},
+		{`{"type":"response.reasoning_summary_part.added","part":{"type":"summary_text","text":""}}`, "response.reasoning_summary_part.added", false},
+		{`{"type":"response.reasoning_summary_part.added","part":{"type":"summary_text","text":"thinking"}}`, "response.reasoning_summary_part.added", true},
+		{`{"type":"response.content_part.added","part":{"type":"output_text","text":""}}`, "response.content_part.added", false},
 		{`{"type":"response.output_text.delta","delta":"hi"}`, "response.output_text.delta", true},
 		{`[DONE]`, "", true},
 	}
@@ -102,6 +107,69 @@ func TestOpenAIStreamErrorFrameDoesNotStartClientOutput(t *testing.T) {
 	}
 }
 
+func TestOpenAIStreamMetadataPreambleAndMessageOnlyOverloadFailOver(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	largeMetadata := strings.Repeat("x", 16*1024)
+	stream := strings.Join([]string{
+		"event: response.created",
+		`data: {"type":"response.created","response":{"id":"resp_1","metadata":{"padding":"` + largeMetadata + `"}}}`,
+		"",
+		"event: response.output_item.added",
+		`data: {"type":"response.output_item.added","item":{"type":"reasoning","summary":[]}}`,
+		"",
+		"event: response.reasoning_summary_part.added",
+		`data: {"type":"response.reasoning_summary_part.added","part":{"type":"summary_text","text":""}}`,
+		"",
+		"event: error",
+		`data: {"type":"error","error":{"type":"service_unavailable_error","message":"Our servers are currently overloaded. Please try again later."}}`,
+		"",
+	}, "\n")
+
+	tests := []struct {
+		name string
+		run  func(*OpenAIGatewayService, *gin.Context, *http.Response, *Account) error
+	}{
+		{
+			name: "native",
+			run: func(svc *OpenAIGatewayService, c *gin.Context, resp *http.Response, account *Account) error {
+				_, err := svc.handleStreamingResponse(c.Request.Context(), resp, c, account, time.Now(), "model", "model")
+				return err
+			},
+		},
+		{
+			name: "passthrough",
+			run: func(svc *OpenAIGatewayService, c *gin.Context, resp *http.Response, account *Account) error {
+				_, err := svc.handleStreamingResponsePassthrough(c.Request.Context(), resp, c, account, time.Now(), "model", "model")
+				return err
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			svc := &OpenAIGatewayService{cfg: &config.Config{Gateway: config.GatewayConfig{MaxLineSize: defaultMaxLineSize}}}
+			rec := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(rec)
+			c.Request = httptest.NewRequest(http.MethodPost, "/", nil)
+			resp := &http.Response{
+				StatusCode: http.StatusOK,
+				Body:       io.NopCloser(strings.NewReader(stream)),
+				Header:     http.Header{"X-Request-Id": []string{"rid-message-only-overload"}},
+			}
+			account := &Account{ID: 1, Platform: PlatformOpenAI, Type: AccountTypeOAuth, Name: "acc"}
+
+			err := tt.run(svc, c, resp, account)
+			require.Error(t, err)
+			var failoverErr *UpstreamFailoverError
+			require.ErrorAs(t, err, &failoverErr)
+			require.True(t, failoverErr.RetryableOnSameAccount)
+			require.True(t, failoverErr.RequestScopedTransient)
+			require.False(t, c.Writer.Written())
+			require.Empty(t, rec.Body.String())
+		})
+	}
+}
+
 // 回归用例（真实上游降载序列）：created → in_progress → error 帧 → response.failed。
 // 期望仍然走 pre-output failover（同账号重试 + 请求级瞬时标记），且不向客户端写出任何字节。
 func TestOpenAIStreamCapacityShedErrorFramePrecedingFailedStillFailsOver(t *testing.T) {
@@ -211,6 +279,18 @@ func TestSanitizeOpenAICapacityShedErrorCodeForClient(t *testing.T) {
 			wantChanged: true,
 			wantContain: `"code":"server_error"`,
 		},
+		{
+			name:        "failed事件只有过载文案时补充code",
+			payload:     `{"type":"response.failed","response":{"error":{"message":"Our servers are currently overloaded. Please try again later."}}}`,
+			wantChanged: true,
+			wantContain: `"code":"server_error"`,
+		},
+		{
+			name:        "error帧只有过载文案时补充code",
+			payload:     `{"type":"error","error":{"message":"Server is overloaded. Please try again later."}}`,
+			wantChanged: true,
+			wantContain: `"code":"server_error"`,
+		},
 		{
 			name:        "rate_limit不改写",
 			payload:     `{"type":"response.failed","response":{"error":{"code":"rate_limit_exceeded","message":"try again in 3s"}}}`,
diff --git a/backend/internal/service/openai_compact_sse_keepalive.go b/backend/internal/service/openai_compact_sse_keepalive.go
index 70e6af9454..360c1d55a5 100644
--- a/backend/internal/service/openai_compact_sse_keepalive.go
+++ b/backend/internal/service/openai_compact_sse_keepalive.go
@@ -161,21 +161,28 @@ func OpenAICompactKeepaliveAdjustedWrittenSize(c *gin.Context) int {
 	if c == nil || c.Writer == nil {
 		return -1
 	}
-	value, ok := c.Get(openAICompactSSEKeepaliveKey)
-	if !ok {
-		return c.Writer.Size()
+	streamKeepaliveBytes := 0
+	if value, ok := c.Get(openAIStreamKeepaliveBytesKey); ok {
+		streamKeepaliveBytes, _ = value.(int)
 	}
-	k, ok := value.(*openAICompactSSEKeepalive)
-	if !ok || k == nil {
-		return c.Writer.Size()
+	size := c.Writer.Size()
+	compactKeepaliveBytes := 0
+	if value, ok := c.Get(openAICompactSSEKeepaliveKey); ok {
+		if k, valid := value.(*openAICompactSSEKeepalive); valid && k != nil {
+			k.mu.Lock()
+			size = k.writer.Size()
+			compactKeepaliveBytes = k.bytes
+			k.mu.Unlock()
+		}
 	}
-	k.mu.Lock()
-	defer k.mu.Unlock()
-	size := k.writer.Size()
 	if size < 0 {
 		return size
 	}
-	if real := size - k.bytes; real > 0 {
+	keepaliveBytes := compactKeepaliveBytes + streamKeepaliveBytes
+	if keepaliveBytes <= 0 {
+		return size
+	}
+	if real := size - keepaliveBytes; real > 0 {
 		return real
 	}
 	return -1
diff --git a/backend/internal/service/openai_compact_sse_keepalive_test.go b/backend/internal/service/openai_compact_sse_keepalive_test.go
index 95ee4ce76c..3703ea9f2a 100644
--- a/backend/internal/service/openai_compact_sse_keepalive_test.go
+++ b/backend/internal/service/openai_compact_sse_keepalive_test.go
@@ -71,6 +71,20 @@ func TestOpenAICompactSSEKeepalive_StopBeforeFirstBeatKeepsWriterUntouched(t *te
 	require.False(t, StopOpenAICompactSSEKeepaliveCommitted(c))
 }
 
+func TestOpenAIAdjustedWrittenSizeExcludesResponsesStreamKeepalive(t *testing.T) {
+	c, rec := newCompactBridgeTestContext(t, false)
+	n, err := c.Writer.Write([]byte(":\n\n"))
+	require.NoError(t, err)
+	recordOpenAIStreamKeepaliveBytes(c, n)
+
+	require.Equal(t, -1, OpenAICompactKeepaliveAdjustedWrittenSize(c))
+
+	_, err = c.Writer.Write([]byte("data: semantic\n\n"))
+	require.NoError(t, err)
+	require.Equal(t, len("data: semantic\n\n"), OpenAICompactKeepaliveAdjustedWrittenSize(c))
+	require.Equal(t, ":\n\ndata: semantic\n\n", rec.Body.String())
+}
+
 // 心跳已提交后，2xx 桥接续写事件而不重复提交响应头。
 func TestWriteOpenAICompactSSEBridge_AfterKeepaliveCommitAppendsEvents(t *testing.T) {
 	c, rec := newCompactBridgeTestContext(t, true)
diff --git a/backend/internal/service/openai_first_output_timeout_test.go b/backend/internal/service/openai_first_output_timeout_test.go
index 799f7af12b..6783de90d1 100644
--- a/backend/internal/service/openai_first_output_timeout_test.go
+++ b/backend/internal/service/openai_first_output_timeout_test.go
@@ -542,7 +542,7 @@ func TestOpenAINativeFirstOutputScannerAllowsLargeEventAfterSemanticBoundary(t *
 	require.Equal(t, "request-large-image", rec.Result().Header.Get("X-Request-Id"))
 }
 
-func TestOpenAINativeFirstOutputTimeoutDisabledPreservesKeepaliveFlush(t *testing.T) {
+func TestOpenAINativeFirstOutputTimeoutDisabledKeepsPreamblePrivateAcrossKeepalive(t *testing.T) {
 	svc := &OpenAIGatewayService{cfg: &config.Config{Gateway: config.GatewayConfig{
 		StreamKeepaliveInterval: 1,
 		MaxLineSize:             defaultMaxLineSize,
@@ -552,7 +552,7 @@ func TestOpenAINativeFirstOutputTimeoutDisabledPreservesKeepaliveFlush(t *testin
 		defer func() { _ = pw.Close() }()
 		_, _ = pw.Write([]byte("data: {\"type\":\"response.created\",\"response\":{\"id\":\"resp_stalled\"}}\n\n"))
 		_, _ = pw.Write([]byte("data: {\"type\":\"response.in_progress\",\"response\":{\"id\":\"resp_stalled\"}}\n\n"))
-		time.Sleep(1100 * time.Millisecond)
+		time.Sleep(2100 * time.Millisecond)
 	}()
 	rec := httptest.NewRecorder()
 	c, _ := gin.CreateTestContext(rec)
@@ -561,10 +561,11 @@ func TestOpenAINativeFirstOutputTimeoutDisabledPreservesKeepaliveFlush(t *testin
 
 	_, err := svc.handleStreamingResponse(c.Request.Context(), resp, c, &Account{ID: 1, Platform: PlatformOpenAI}, time.Now(), "model", "model")
 
-	require.Error(t, err)
+	var failoverErr *UpstreamFailoverError
+	require.ErrorAs(t, err, &failoverErr)
 	require.Contains(t, rec.Body.String(), ":\n\n")
-	require.Contains(t, rec.Body.String(), "response.created")
-	require.Contains(t, rec.Body.String(), "response.in_progress")
+	require.NotContains(t, rec.Body.String(), "response.created")
+	require.NotContains(t, rec.Body.String(), "response.in_progress")
 }
 
 func TestOpenAINativeFirstOutputFailoverKeepsAttemptHeadersPrivateAfterKeepaliveCommit(t *testing.T) {
diff --git a/backend/internal/service/openai_gateway_passthrough.go b/backend/internal/service/openai_gateway_passthrough.go
index 1fac0440f6..e9789d6b01 100644
--- a/backend/internal/service/openai_gateway_passthrough.go
+++ b/backend/internal/service/openai_gateway_passthrough.go
@@ -809,6 +809,19 @@ type openaiNonStreamingResultPassthrough struct {
 	imageOutputSizes []string
 }
 
+const openAIStreamKeepaliveBytesKey = "openai_stream_keepalive_bytes"
+
+func recordOpenAIStreamKeepaliveBytes(c *gin.Context, written int) {
+	if c == nil || written <= 0 {
+		return
+	}
+	current := 0
+	if value, ok := c.Get(openAIStreamKeepaliveBytesKey); ok {
+		current, _ = value.(int)
+	}
+	c.Set(openAIStreamKeepaliveBytesKey, current+written)
+}
+
 func openAIStreamClientOutputStarted(c *gin.Context, localStarted bool) bool {
 	if localStarted {
 		return true
@@ -831,6 +844,85 @@ func openAIStreamEventIsPreamble(eventType string) bool {
 	}
 }
 
+func openAIStreamAddedEventStartsClientOutput(payload []byte, eventType string) bool {
+	if len(payload) == 0 || !gjson.ValidBytes(payload) {
+		return true
+	}
+
+	switch strings.TrimSpace(eventType) {
+	case "response.output_item.added":
+		item := gjson.GetBytes(payload, "item")
+		if !item.Exists() || !item.IsObject() {
+			return true
+		}
+		switch strings.TrimSpace(item.Get("type").String()) {
+		case "reasoning":
+			if item.Get("encrypted_content").String() != "" {
+				return true
+			}
+			summary := item.Get("summary")
+			if !summary.IsArray() {
+				return false
+			}
+			for _, part := range summary.Array() {
+				if strings.TrimSpace(part.Get("type").String()) != "summary_text" || part.Get("text").String() != "" {
+					return true
+				}
+			}
+			return false
+		case "message":
+			content := item.Get("content")
+			if !content.IsArray() {
+				return false
+			}
+			for _, part := range content.Array() {
+				switch strings.TrimSpace(part.Get("type").String()) {
+				case "output_text":
+					if part.Get("text").String() != "" {
+						return true
+					}
+				case "refusal":
+					if part.Get("refusal").String() != "" {
+						return true
+					}
+				default:
+					return true
+				}
+			}
+			return false
+		case "function_call":
+			return item.Get("arguments").String() != ""
+		case "custom_tool_call":
+			return item.Get("input").String() != ""
+		case "compaction":
+			return item.Get("encrypted_content").String() != ""
+		default:
+			return true
+		}
+	case "response.content_part.added":
+		part := gjson.GetBytes(payload, "part")
+		if !part.Exists() || !part.IsObject() {
+			return true
+		}
+		switch strings.TrimSpace(part.Get("type").String()) {
+		case "output_text":
+			return part.Get("text").String() != ""
+		case "refusal":
+			return part.Get("refusal").String() != ""
+		default:
+			return true
+		}
+	case "response.reasoning_summary_part.added":
+		part := gjson.GetBytes(payload, "part")
+		if !part.Exists() || !part.IsObject() || strings.TrimSpace(part.Get("type").String()) != "summary_text" {
+			return true
+		}
+		return part.Get("text").String() != ""
+	default:
+		return true
+	}
+}
+
 func openAIStreamDataStartsClientOutput(data, eventType string) bool {
 	trimmed := strings.TrimSpace(data)
 	if trimmed == "" {
@@ -847,6 +939,8 @@ func openAIStreamDataStartsClientOutput(data, eventType string) bool {
 		// （content_policy / invalid_request 等）维持原样转发，保留上游错误细节。
 		payload := []byte(trimmed)
 		return !openAIStreamFailedEventShouldFailover(payload, extractOpenAISSEErrorMessage(payload))
+	case "response.output_item.added", "response.content_part.added", "response.reasoning_summary_part.added":
+		return openAIStreamAddedEventStartsClientOutput([]byte(trimmed), eventType)
 	}
 	return !openAIStreamEventIsPreamble(eventType)
 }
@@ -925,9 +1019,13 @@ func isOpenAIUpstreamCapacityShedEvent(payload []byte) bool {
 	switch openAIStreamFailedEventErrorCode(payload) {
 	case "server_is_overloaded", "slow_down":
 		return true
-	default:
-		return false
 	}
+	for _, path := range []string{"response.error.message", "error.message", "message"} {
+		if isOpenAICapacityShedMessage(gjson.GetBytes(payload, path).String()) {
+			return true
+		}
+	}
+	return false
 }
 
 // openAICapacityShedRetryableClientCode 是把上游容量降载错误转发给客户端时改写
@@ -950,9 +1048,12 @@ func sanitizeOpenAICapacityShedErrorCodeForClient(payload []byte) ([]byte, bool)
 	updated := payload
 	changed := false
 	for _, path := range []string{"response.error.code", "error.code"} {
-		switch strings.ToLower(strings.TrimSpace(gjson.GetBytes(updated, path).String())) {
-		case "server_is_overloaded", "slow_down":
-		default:
+		parent := strings.TrimSuffix(path, ".code")
+		if !gjson.GetBytes(updated, parent).Exists() {
+			continue
+		}
+		code := strings.ToLower(strings.TrimSpace(gjson.GetBytes(updated, path).String()))
+		if code != "" && code != "server_is_overloaded" && code != "slow_down" {
 			continue
 		}
 		next, err := sjson.SetBytes(updated, path, openAICapacityShedRetryableClientCode)
@@ -985,7 +1086,7 @@ func openAIStreamFailedEventSemanticStatus(payload []byte, message string) int {
 		return http.StatusUnauthorized
 	case strings.Contains(combined, "permission") || strings.Contains(combined, "forbidden") || strings.Contains(combined, "access denied"):
 		return http.StatusForbidden
-	case code == "server_is_overloaded" || code == "slow_down":
+	case isOpenAIUpstreamCapacityShedEvent(payload):
 		return http.StatusServiceUnavailable
 	default:
 		return http.StatusBadGateway
@@ -1119,6 +1220,16 @@ func openAIStreamFailedEventShouldFailover(payload []byte, message string) bool
 	return true
 }
 
+func openAIStreamErrorEventShouldFailover(payload []byte, message string) bool {
+	if strings.TrimSpace(gjson.GetBytes(payload, "type").String()) != "error" {
+		return false
+	}
+	if isOpenAIContextWindowError(message, payload) {
+		return false
+	}
+	return isOpenAITransientProcessingError(http.StatusBadRequest, message, payload)
+}
+
 func openAIStreamFailedEventRetryableOnSameAccount(account *Account, payload []byte, message string) bool {
 	if account == nil {
 		return false
@@ -1347,6 +1458,25 @@ func (s *OpenAIGatewayService) handleStreamingResponsePassthrough(
 				}
 			}
 			eventType := strings.TrimSpace(gjson.Get(trimmedData, "type").String())
+			if eventType == "error" && !openAIStreamClientOutputStarted(c, clientOutputStarted) {
+				errorMessage := extractOpenAISSEErrorMessage(dataBytes)
+				if status, errType, errMsg, matched := applyOpenAIStreamFailedErrorPassthroughRule(c, account.Platform, dataBytes, errorMessage); matched {
+					s.recordOpenAIStreamUpstreamError(c, account, true, upstreamRequestID, "http_error", dataBytes, errorMessage)
+					MarkResponseCommitted(c)
+					c.Writer.Header().Set("Content-Type", "application/json; charset=utf-8")
+					c.JSON(status, gin.H{
+						"error": gin.H{
+							"type":    errType,
+							"message": errMsg,
+						},
+					})
+					return resultWithUsage(), fmt.Errorf("upstream error event: passthrough rule matched message=%s", errMsg)
+				}
+				if openAIStreamErrorEventShouldFailover(dataBytes, errorMessage) {
+					return resultWithUsage(),
+						s.newOpenAIStreamFailoverError(c, account, true, upstreamRequestID, dataBytes, errorMessage, resp.Header)
+				}
+			}
 			if eventType == "response.failed" {
 				failedMessage = extractOpenAISSEErrorMessage(dataBytes)
 				// response.failed 自带上游已消耗的 usage（input token 通常已扣）；必须先解析
diff --git a/backend/internal/service/openai_gateway_response_handling.go b/backend/internal/service/openai_gateway_response_handling.go
index 82cd89f695..98475a4a33 100644
--- a/backend/internal/service/openai_gateway_response_handling.go
+++ b/backend/internal/service/openai_gateway_response_handling.go
@@ -54,8 +54,9 @@ func (s *OpenAIGatewayService) handleStreamingResponseWithReasoning(ctx context.
 		firstOutputTimeout = s.openAIFirstOutputTimeout(reasoningEffort)
 	}
 	guardFirstOutput := firstOutputTimeout > 0
+	stageFirstOutput := account != nil && account.Platform == PlatformOpenAI
 	var attemptResponseHeaders http.Header
-	if guardFirstOutput {
+	if stageFirstOutput {
 		if s.responseHeaderFilter != nil {
 			attemptResponseHeaders = responseheaders.FilterHeaders(resp.Header, s.responseHeaderFilter)
 		} else if requestID := strings.TrimSpace(resp.Header.Get("x-request-id")); requestID != "" {
@@ -66,8 +67,8 @@ func (s *OpenAIGatewayService) handleStreamingResponseWithReasoning(ctx context.
 	}
 	// x-codex-turn-state 不在通用响应头白名单内，按 Codex 协议显式回传：
 	// 客户端会在同回合的后续请求中回带（openai_codex_turn_state.go）。
-	// 首输出守卫模式下只暂存，溯源在 applyAttemptResponseHeaders 真正提交时记录。
-	if guardFirstOutput {
+	// OpenAI 首个语义输出前只暂存，溯源在 applyAttemptResponseHeaders 真正提交时记录。
+	if stageFirstOutput {
 		stageOpenAICodexTurnState(&attemptResponseHeaders, resp.Header)
 	} else {
 		s.relayOpenAICodexTurnState(c, account, resp.Header)
@@ -80,12 +81,12 @@ func (s *OpenAIGatewayService) handleStreamingResponseWithReasoning(ctx context.
 	c.Header("X-Accel-Buffering", "no")
 
 	// Pass through other headers
-	if !guardFirstOutput && resp.Header.Get("x-request-id") != "" {
+	if !stageFirstOutput && resp.Header.Get("x-request-id") != "" {
 		v := resp.Header.Get("x-request-id")
 		c.Header("x-request-id", v)
 	}
 	applyAttemptResponseHeaders := func() {
-		if !guardFirstOutput || len(attemptResponseHeaders) == 0 || c.Writer.Written() {
+		if !stageFirstOutput || len(attemptResponseHeaders) == 0 || c.Writer.Written() {
 			return
 		}
 		for key, values := range attemptResponseHeaders {
@@ -117,7 +118,7 @@ func (s *OpenAIGatewayService) handleStreamingResponseWithReasoning(ctx context.
 	firstOutputProgressObserved := false
 	bufferedWriter := bufio.NewWriterSize(w, 4*1024)
 	var firstOutputStage *openAIFirstOutputStage
-	if guardFirstOutput {
+	if stageFirstOutput {
 		firstOutputStage = newDefaultOpenAIFirstOutputStage()
 		defer func() {
 			if err := firstOutputStage.Close(); err != nil {
@@ -126,19 +127,19 @@ func (s *OpenAIGatewayService) handleStreamingResponseWithReasoning(ctx context.
 		}()
 	}
 	writePendingString := func(value string) (int, error) {
-		if firstOutputStage != nil && !firstOutputProgressObserved && !firstOutputStage.closed {
+		if firstOutputStage != nil && !firstOutputStage.closed {
 			return firstOutputStage.WriteString(value)
 		}
 		return bufferedWriter.WriteString(value)
 	}
 	pendingBytes := func() int64 {
-		if firstOutputStage != nil && !firstOutputProgressObserved && !firstOutputStage.closed {
+		if firstOutputStage != nil && !firstOutputStage.closed {
 			return firstOutputStage.Buffered()
 		}
 		return int64(bufferedWriter.Buffered())
 	}
 	flushBuffered := func() error {
-		if firstOutputStage != nil && !firstOutputProgressObserved && !firstOutputStage.closed {
+		if firstOutputStage != nil && !firstOutputStage.closed {
 			if err := firstOutputStage.CommitTo(w); err != nil {
 				return err
 			}
@@ -155,11 +156,11 @@ func (s *OpenAIGatewayService) handleStreamingResponseWithReasoning(ctx context.
 	imageCounter := newOpenAIImageOutputCounter()
 	responseID := ""
 	var firstOutputScanGuard atomic.Bool
-	firstOutputScanGuard.Store(guardFirstOutput)
+	firstOutputScanGuard.Store(stageFirstOutput)
 	scanner := bufio.NewScanner(resp.Body)
 	scanBuf := getSSEScannerBuf64K()
 	scanner.Buffer(scanBuf[:0], maxLineSize)
-	if guardFirstOutput {
+	if stageFirstOutput {
 		scanner.Split(openAIFirstOutputDynamicScanLines(&firstOutputScanGuard))
 	}
 	documentScanner := newOpenAISSEJSONDocumentScanner(scanner)
@@ -250,7 +251,7 @@ func (s *OpenAIGatewayService) handleStreamingResponseWithReasoning(ctx context.
 	eventStartsVisibleOutput := false
 	eventShouldFlush := false
 	handlePendingWriteError := func(err error) {
-		if firstOutputStage != nil && !firstOutputProgressObserved && !firstOutputStage.closed {
+		if firstOutputStage != nil && !firstOutputStage.closed {
 			message := "OpenAI first-output staging failed"
 			if errors.Is(err, errOpenAIFirstOutputStageLimit) {
 				message = "OpenAI first-output staging limit exceeded"
@@ -350,7 +351,7 @@ func (s *OpenAIGatewayService) handleStreamingResponseWithReasoning(ctx context.
 		lastDownstreamWriteAt = time.Now()
 	}
 	finalizeStream := func() (*openaiStreamingResult, error) {
-		if guardFirstOutput && eventInProgress {
+		if stageFirstOutput && eventInProgress {
 			// EOF dispatches the final SSE event even without a trailing blank line.
 			completeGuardedEvent(true)
 		}
@@ -392,7 +393,7 @@ func (s *OpenAIGatewayService) handleStreamingResponseWithReasoning(ctx context.
 			failoverErr.SafeToFailoverAfterWrite = true
 			return resultWithUsage(), failoverErr, true
 		}
-		if errors.Is(scanErr, bufio.ErrTooLong) && guardFirstOutput && !firstOutputProgressObserved {
+		if errors.Is(scanErr, bufio.ErrTooLong) && stageFirstOutput && !firstOutputProgressObserved {
 			logger.LegacyPrintf("service.openai_gateway", "SSE line too long before first output: account=%d max_size=%d error=%v", account.ID, maxLineSize, scanErr)
 			failoverErr := s.newOpenAIStreamFailoverError(
 				c, account, false, upstreamRequestID, nil,
@@ -455,6 +456,26 @@ func (s *OpenAIGatewayService) handleStreamingResponseWithReasoning(ctx context.
 				responseID = extractOpenAIResponseIDFromJSONBytes(dataBytes)
 			}
 			forceFlushFailedEvent := false
+			if eventType == "error" && !openAIStreamClientOutputStarted(c, clientOutputStarted) {
+				errorMessage := extractOpenAISSEErrorMessage(dataBytes)
+				if status, errType, errMsg, matched := applyOpenAIStreamFailedErrorPassthroughRule(c, account.Platform, dataBytes, errorMessage); matched {
+					s.recordOpenAIStreamUpstreamError(c, account, false, upstreamRequestID, "http_error", dataBytes, errorMessage)
+					MarkResponseCommitted(c)
+					c.Writer.Header().Set("Content-Type", "application/json; charset=utf-8")
+					c.JSON(status, gin.H{
+						"error": gin.H{
+							"type":    errType,
+							"message": errMsg,
+						},
+					})
+					streamEarlyErr = fmt.Errorf("upstream error event: passthrough rule matched message=%s", errMsg)
+					return
+				}
+				if openAIStreamErrorEventShouldFailover(dataBytes, errorMessage) {
+					streamEarlyErr = s.newOpenAIStreamFailoverError(c, account, false, upstreamRequestID, dataBytes, errorMessage, resp.Header)
+					return
+				}
+			}
 			if eventType == "response.failed" {
 				failedMessage = extractOpenAISSEErrorMessage(dataBytes)
 				// response.failed 自带上游已消耗的 usage（input token 通常已扣）；必须先解析
@@ -559,9 +580,12 @@ func (s *OpenAIGatewayService) handleStreamingResponseWithReasoning(ctx context.
 			}
 			startsClientOutput := forceFlushFailedEvent || openAIStreamDataStartsClientOutput(data, eventType)
 			startsVisibleOutput := openAIStreamDataStartsVisibleOutput(data, eventType)
-			if guardFirstOutput {
+			if stageFirstOutput {
 				eventStartsClientOutput = eventStartsClientOutput || startsClientOutput
 				eventStartsVisibleOutput = eventStartsVisibleOutput || startsVisibleOutput
+				if startsClientOutput {
+					firstOutputScanGuard.Store(false)
+				}
 			}
 			if startsClientOutput && !openAIStreamEventTypeIsTerminal(eventType) {
 				responsesSemanticOutputSeen = true
@@ -607,7 +631,7 @@ func (s *OpenAIGatewayService) handleStreamingResponseWithReasoning(ctx context.
 		}
 
 		// A blank line dispatches a guarded event from the attempt-local stage.
-		if guardFirstOutput && line == "" {
+		if stageFirstOutput && line == "" {
 			if !clientDisconnected {
 				if _, err := writePendingString("\n"); err != nil {
 					handlePendingWriteError(err)
@@ -672,7 +696,7 @@ func (s *OpenAIGatewayService) handleStreamingResponseWithReasoning(ctx context.
 	events := make(chan scanEvent, openAIFirstOutputEventQueueSize(guardFirstOutput))
 	done := make(chan struct{})
 	sendEvent := func(ev scanEvent) bool {
-		if guardFirstOutput {
+		if firstOutputScanGuard.Load() {
 			ev.processed = make(chan struct{})
 		}
 		select {
@@ -716,7 +740,7 @@ func (s *OpenAIGatewayService) handleStreamingResponseWithReasoning(ctx context.
 		select {
 		case ev, ok := <-events:
 			if !ok {
-				if guardFirstOutput && eventInProgress {
+				if stageFirstOutput && eventInProgress {
 					// EOF dispatches the final SSE event even without a trailing blank
 					// line. Do not synthesize extra bytes on the downstream wire.
 					completeGuardedEvent(true)
@@ -783,10 +807,12 @@ func (s *OpenAIGatewayService) handleStreamingResponseWithReasoning(ctx context.
 			if time.Since(lastDownstreamWriteAt) < keepaliveInterval {
 				continue
 			}
-			if guardFirstOutput {
+			if stageFirstOutput {
 				// Bypass attempt-local buffered frames. The stable SSE headers may be
 				// committed here, but account headers remain private until semantic output.
-				if _, err := w.Write([]byte(":\n\n")); err != nil {
+				n, err := w.Write([]byte(":\n\n"))
+				recordOpenAIStreamKeepaliveBytes(c, n)
+				if err != nil {
 					clientDisconnected = true
 					logger.LegacyPrintf("service.openai_gateway", "Client disconnected during streaming, continuing to drain upstream for billing")
 					continue
diff --git a/backend/internal/service/openai_gateway_service_codex_cli_only_test.go b/backend/internal/service/openai_gateway_service_codex_cli_only_test.go
index 846529828b..c8acab0358 100644
--- a/backend/internal/service/openai_gateway_service_codex_cli_only_test.go
+++ b/backend/internal/service/openai_gateway_service_codex_cli_only_test.go
@@ -287,6 +287,24 @@ func TestIsOpenAITransientProcessingError(t *testing.T) {
 		[]byte(`{"error":{"code":"slow_down","message":"Please retry later."}}`),
 	))
 
+	require.True(t, isOpenAITransientProcessingError(
+		http.StatusBadRequest,
+		"",
+		[]byte(`{"error":{"message":"Our servers are currently overloaded. Please try again later."}}`),
+	))
+
+	require.True(t, isOpenAITransientProcessingError(
+		http.StatusServiceUnavailable,
+		"Server is overloaded. Please try again later.",
+		nil,
+	))
+
+	require.True(t, isOpenAITransientProcessingError(
+		http.StatusBadGateway,
+		"",
+		[]byte(`{"error":{"message":"Our servers are currently overloaded. Please try again later."}}`),
+	))
+
 	require.True(t, isOpenAITransientProcessingError(
 		http.StatusBadRequest,
 		"",
diff --git a/backend/internal/service/openai_gateway_service_test.go b/backend/internal/service/openai_gateway_service_test.go
index 617c07c066..c87da09555 100644
--- a/backend/internal/service/openai_gateway_service_test.go
+++ b/backend/internal/service/openai_gateway_service_test.go
@@ -204,17 +204,17 @@ func TestFailoverOpenAIUpstreamHTTPError_NilContextSkipsTempUnschedulablePolicy(
 			"temp_unschedulable_enabled": true,
 			"temp_unschedulable_rules": []any{map[string]any{
 				"error_code":       float64(http.StatusBadRequest),
-				"keywords":         []any{"servers are currently overloaded"},
+				"keywords":         []any{"custom temporary outage"},
 				"duration_minutes": float64(1),
 			}},
 		},
 	}
-	body := []byte(`{"error":{"message":"Our servers are currently overloaded."}}`)
+	body := []byte(`{"error":{"message":"Custom temporary outage."}}`)
 	resp := &http.Response{StatusCode: http.StatusBadRequest, Header: http.Header{}}
 
 	got := svc.failoverOpenAIUpstreamHTTPError(
 		context.Background(), nil, account, resp, body,
-		"Our servers are currently overloaded.", "gpt-5.4",
+		"Custom temporary outage.", "gpt-5.4",
 	)
 
 	require.Nil(t, got)
@@ -2288,7 +2288,7 @@ func TestOpenAIStreamingMissingTerminalEventReturnsIncompleteError(t *testing.T)
 
 	go func() {
 		defer func() { _ = pw.Close() }()
-		_, _ = pw.Write([]byte("data: {\"type\":\"response.output_item.added\",\"item\":{\"type\":\"message\"},\"output_index\":0}\n\n"))
+		_, _ = pw.Write([]byte("data: {\"type\":\"response.output_text.delta\",\"delta\":\"partial\",\"output_index\":0}\n\n"))
 	}()
 
 	_, err := svc.handleStreamingResponse(c.Request.Context(), resp, c, &Account{ID: 1}, time.Now(), "model", "model")
@@ -2320,7 +2320,7 @@ func TestOpenAIStreamingPassthroughMissingTerminalEventReturnsIncompleteError(t
 
 	go func() {
 		defer func() { _ = pw.Close() }()
-		_, _ = pw.Write([]byte("data: {\"type\":\"response.output_item.added\",\"item\":{\"type\":\"message\"},\"output_index\":0}\n\n"))
+		_, _ = pw.Write([]byte("data: {\"type\":\"response.output_text.delta\",\"delta\":\"partial\",\"output_index\":0}\n\n"))
 	}()
 
 	_, err := svc.handleStreamingResponsePassthrough(c.Request.Context(), resp, c, &Account{ID: 1}, time.Now(), "", "")
diff --git a/backend/internal/service/openai_gateway_upstream_errors.go b/backend/internal/service/openai_gateway_upstream_errors.go
index 00a62f3f3e..b4ecf61286 100644
--- a/backend/internal/service/openai_gateway_upstream_errors.go
+++ b/backend/internal/service/openai_gateway_upstream_errors.go
@@ -117,7 +117,7 @@ func isOpenAIInstructionsRequiredError(upstreamStatusCode int, upstreamMsg strin
 }
 
 func isOpenAITransientProcessingError(upstreamStatusCode int, upstreamMsg string, upstreamBody []byte) bool {
-	if upstreamStatusCode != http.StatusBadRequest && upstreamStatusCode != http.StatusServiceUnavailable {
+	if upstreamStatusCode < http.StatusBadRequest {
 		return false
 	}
 
@@ -132,6 +132,15 @@ func isOpenAITransientProcessingError(upstreamStatusCode int, upstreamMsg string
 	if len(upstreamBody) > 0 && hasOpenAIServerOverloadedCode(upstreamBody) {
 		return true
 	}
+	if isOpenAICapacityShedMessage(upstreamMsg) ||
+		isOpenAICapacityShedMessage(gjson.GetBytes(upstreamBody, "error.message").String()) ||
+		isOpenAICapacityShedMessage(gjson.GetBytes(upstreamBody, "response.error.message").String()) ||
+		isOpenAICapacityShedMessage(string(upstreamBody)) {
+		return true
+	}
+	if upstreamStatusCode != http.StatusBadRequest && upstreamStatusCode != http.StatusServiceUnavailable {
+		return false
+	}
 	if upstreamStatusCode != http.StatusBadRequest {
 		return false
 	}
@@ -164,6 +173,13 @@ func isOpenAITransientProcessingError(upstreamStatusCode int, upstreamMsg string
 	return match(string(upstreamBody))
 }
 
+func isOpenAICapacityShedMessage(text string) bool {
+	lower := strings.ToLower(strings.TrimSpace(text))
+	return strings.Contains(lower, "server is overloaded") ||
+		strings.Contains(lower, "servers are overloaded") ||
+		strings.Contains(lower, "servers are currently overloaded")
+}
+
 func isOpenAIContextWindowError(upstreamMsg string, upstreamBody []byte) bool {
 	match := func(text string) bool {
 		lower := strings.ToLower(strings.TrimSpace(text))
diff --git a/backend/internal/service/openai_visible_ttft_test.go b/backend/internal/service/openai_visible_ttft_test.go
index 8fd243c39e..4d811dfbb3 100644
--- a/backend/internal/service/openai_visible_ttft_test.go
+++ b/backend/internal/service/openai_visible_ttft_test.go
@@ -71,11 +71,38 @@ func TestOpenAIResponsesTTFTStartsAtCompletedImage(t *testing.T) {
 	}
 }
 
-func TestOpenAINativeProgressDisarmsTimeoutWithoutStartingTTFT(t *testing.T) {
-	result := runSyntheticVisibleTTFTStream(t, false, 1200*time.Millisecond, 1,
-		`{"type":"response.output_text.delta","delta":"test output"}`)
-	require.NotNil(t, result.firstTokenMs)
-	require.GreaterOrEqual(t, *result.firstTokenMs, 1100)
+func TestOpenAINativeMetadataDoesNotDisarmFirstOutputTimeout(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	svc := &OpenAIGatewayService{cfg: &config.Config{Gateway: config.GatewayConfig{
+		MaxLineSize:                     defaultMaxLineSize,
+		OpenAIFirstOutputTimeoutSeconds: 1,
+	}}}
+	reader, writer := io.Pipe()
+	writerDone := make(chan struct{})
+	go func() {
+		defer close(writerDone)
+		defer func() { _ = writer.Close() }()
+		_, _ = io.WriteString(writer, "data: {\"type\":\"response.created\",\"response\":{\"id\":\"resp_test\"}}\n\n")
+		_, _ = io.WriteString(writer, "data: {\"type\":\"response.output_item.added\",\"item\":{\"id\":\"item_test\",\"type\":\"reasoning\",\"summary\":[]}}\n\n")
+		time.Sleep(1200 * time.Millisecond)
+	}()
+
+	recorder := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(recorder)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/responses", nil)
+	resp := &http.Response{StatusCode: http.StatusOK, Header: http.Header{}, Body: reader}
+	account := &Account{ID: 1, Name: "account_test", Platform: PlatformOpenAI}
+
+	_, err := svc.handleStreamingResponse(context.Background(), resp, c, account, time.Now(), "test-model", "test-model")
+	var failoverErr *UpstreamFailoverError
+	require.ErrorAs(t, err, &failoverErr)
+	require.True(t, failoverErr.SafeToFailoverAfterWrite)
+	require.Empty(t, recorder.Body.String())
+	select {
+	case <-writerDone:
+	case <-time.After(time.Second):
+		t.Fatal("synthetic upstream writer did not exit")
+	}
 }
 
 func runSyntheticVisibleTTFTStream(t *testing.T, passthrough bool, visibleDelay time.Duration, timeoutSeconds int, visibleEvent string) *openaiStreamingResult {