diff --git a/backend/internal/service/openai_gateway_messages.go b/backend/internal/service/openai_gateway_messages.go
index abf80de75f..f37f2997cd 100644
--- a/backend/internal/service/openai_gateway_messages.go
+++ b/backend/internal/service/openai_gateway_messages.go
@@ -793,7 +793,9 @@ func (s *OpenAIGatewayService) handleAnthropicStreamingResponse(
 			return false
 		}
 
-		isTerminalEvent := isOpenAICompatResponsesTerminalEvent(event.Type)
+		eventType := strings.TrimSpace(event.Type)
+		isBareErrorEvent := eventType == "error"
+		isTerminalEvent := isOpenAICompatResponsesTerminalEvent(eventType) || isBareErrorEvent
 		if isTerminalEvent {
 			if event.Response != nil {
 				if id := strings.TrimSpace(event.Response.ID); id != "" {
@@ -808,7 +810,7 @@ func (s *OpenAIGatewayService) handleAnthropicStreamingResponse(
 			}
 			// cyber_policy 致命不可重试：标记供 handler 事后记录；以 Anthropic SSE error 事件
 			// 回写让客户端感知并停止重试（F4），丢弃后续转换输出。
-			if strings.TrimSpace(event.Type) == "response.failed" {
+			if eventType == "response.failed" || isBareErrorEvent {
 				payloadBytes := []byte(payload)
 				if hit, code, msg := detectOpenAICyberPolicy(payloadBytes); hit {
 					MarkOpsCyberPolicy(c, CyberPolicyMark{
@@ -833,7 +835,10 @@ func (s *OpenAIGatewayService) handleAnthropicStreamingResponse(
 					return true
 				}
 				message := extractOpenAISSEErrorMessage(payloadBytes)
-				if openAIStreamFailedEventShouldFailover(payloadBytes, message) {
+				// Once Anthropic output has started, switching accounts would splice
+				// two model streams together. Surface a proper Anthropic error event
+				// instead of returning a failover error that the handler cannot retry.
+				if !clientOutputStarted && openAIStreamFailedEventShouldFailover(payloadBytes, message) {
 					streamFailoverErr = s.newOpenAIStreamFailoverError(c, account, false, requestID, payloadBytes, message)
 					return true
 				}
diff --git a/backend/internal/service/openai_gateway_messages_failed_response_test.go b/backend/internal/service/openai_gateway_messages_failed_response_test.go
index a03efca42d..51d5fcb7c9 100644
--- a/backend/internal/service/openai_gateway_messages_failed_response_test.go
+++ b/backend/internal/service/openai_gateway_messages_failed_response_test.go
@@ -77,6 +77,84 @@ func TestForwardAsAnthropic_StreamingResponseFailed_ReturnsError(t *testing.T) {
 	require.Contains(t, err.Error(), "upstream response failed")
 }
 
+func TestForwardAsAnthropic_StreamingBareErrorAfterOutputIsVisible(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	body := []byte(`{"model":"gpt-5.4","max_tokens":32,"messages":[{"role":"user","content":"hello"}],"stream":true}`)
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages", bytes.NewReader(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	ssePayload := strings.Join([]string{
+		`data: {"type":"response.created","response":{"id":"resp_bare_error","object":"response","model":"gpt-5.4","status":"in_progress","output":[]}}`,
+		"",
+		`data: {"type":"response.output_text.delta","output_index":0,"content_index":0,"delta":"partial"}`,
+		"",
+		`event: error`,
+		`data: {"type":"error","error":{"type":"server_error","code":"upstream_error","message":"mixed tools failed"}}`,
+		"",
+		`data: [DONE]`,
+		"",
+	}, "\n")
+	upstream := &httpUpstreamRecorder{resp: &http.Response{
+		StatusCode: http.StatusOK,
+		Header:     http.Header{"Content-Type": []string{"text/event-stream"}},
+		Body:       io.NopCloser(strings.NewReader(ssePayload)),
+	}}
+	svc := &OpenAIGatewayService{
+		cfg:          rawChatCompletionsTestConfig(),
+		httpUpstream: upstream,
+	}
+
+	account := rawChatCompletionsTestAccount()
+	_, err := svc.ForwardAsAnthropic(context.Background(), c, account, body, "", "")
+
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "upstream response failed: mixed tools failed")
+	clientStream := rec.Body.String()
+	require.Contains(t, clientStream, `"text":"partial"`)
+	require.Contains(t, clientStream, "event: error")
+	require.Contains(t, clientStream, "mixed tools failed")
+	require.NotContains(t, clientStream, "event: message_stop")
+	require.NotContains(t, err.Error(), "missing terminal event")
+}
+
+func TestForwardAsAnthropic_StreamingBareErrorBeforeOutputFailsOver(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	body := []byte(`{"model":"gpt-5.4","max_tokens":32,"messages":[{"role":"user","content":"hello"}],"stream":true}`)
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages", bytes.NewReader(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	ssePayload := strings.Join([]string{
+		`event: error`,
+		`data: {"type":"error","error":{"type":"server_error","code":"upstream_error","message":"temporary upstream failure"}}`,
+		"",
+		`data: [DONE]`,
+		"",
+	}, "\n")
+	upstream := &httpUpstreamRecorder{resp: &http.Response{
+		StatusCode: http.StatusOK,
+		Header:     http.Header{"Content-Type": []string{"text/event-stream"}},
+		Body:       io.NopCloser(strings.NewReader(ssePayload)),
+	}}
+	svc := &OpenAIGatewayService{
+		cfg:          rawChatCompletionsTestConfig(),
+		httpUpstream: upstream,
+	}
+
+	account := rawChatCompletionsTestAccount()
+	_, err := svc.ForwardAsAnthropic(context.Background(), c, account, body, "", "")
+
+	require.Error(t, err)
+	var failoverErr *UpstreamFailoverError
+	require.True(t, errors.As(err, &failoverErr), "pre-output retryable error must remain failover-safe: %T: %v", err, err)
+	require.Empty(t, rec.Body.String(), "failover path must not commit downstream output")
+}
+
 func TestForwardAsAnthropic_BufferedResponseFailed_Failover(t *testing.T) {
 	gin.SetMode(gin.TestMode)