diff --git a/backend/internal/handler/openai_chat_completions.go b/backend/internal/handler/openai_chat_completions.go
index d58656205d..bbccddf8d5 100644
--- a/backend/internal/handler/openai_chat_completions.go
+++ b/backend/internal/handler/openai_chat_completions.go
@@ -254,10 +254,15 @@ func (h *OpenAIGatewayHandler) ChatCompletions(c *gin.Context) {
 					continue
 				}
 				h.gatewayService.ReportOpenAIAccountScheduleResult(account.ID, false, nil)
-				wroteFallback := h.ensureForwardErrorResponse(c, streamStarted)
+				upstreamErrorAlreadyCommunicated := openAIForwardErrorAlreadyCommunicated(c, writerSizeBeforeForward, err)
+				wroteFallback := false
+				if !upstreamErrorAlreadyCommunicated {
+					wroteFallback = h.ensureForwardErrorResponse(c, streamStarted)
+				}
 				reqLog.Warn("openai_chat_completions.forward_failed",
 					zap.Int64("account_id", account.ID),
 					zap.Bool("fallback_error_response_written", wroteFallback),
+					zap.Bool("upstream_error_response_already_written", upstreamErrorAlreadyCommunicated),
 					zap.Error(err),
 				)
 				return
diff --git a/backend/internal/handler/openai_gateway_handler.go b/backend/internal/handler/openai_gateway_handler.go
index 2a147f2b05..edf503847a 100644
--- a/backend/internal/handler/openai_gateway_handler.go
+++ b/backend/internal/handler/openai_gateway_handler.go
@@ -433,10 +433,15 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 					continue
 				}
 				h.gatewayService.ReportOpenAIAccountScheduleResult(account.ID, false, nil)
-				wroteFallback := h.ensureForwardErrorResponse(c, streamStarted)
+				upstreamErrorAlreadyCommunicated := openAIForwardErrorAlreadyCommunicated(c, writerSizeBeforeForward, err)
+				wroteFallback := false
+				if !upstreamErrorAlreadyCommunicated {
+					wroteFallback = h.ensureForwardErrorResponse(c, streamStarted)
+				}
 				fields := []zap.Field{
 					zap.Int64("account_id", account.ID),
 					zap.Bool("fallback_error_response_written", wroteFallback),
+					zap.Bool("upstream_error_response_already_written", upstreamErrorAlreadyCommunicated),
 					zap.Error(err),
 				}
 				if shouldLogOpenAIForwardFailureAsWarn(c, wroteFallback) {
@@ -1853,6 +1858,37 @@ func shouldLogOpenAIForwardFailureAsWarn(c *gin.Context, wroteFallback bool) boo
 	return c.Writer.Written()
 }
 
+// openAIForwardErrorAlreadyCommunicated reports whether Forward returned an
+// error after it had already written the upstream terminal error response to
+// the client.
+//
+// This matters for Responses streams: upstream may return HTTP 200 with a
+// non-retryable `response.failed` event (for example a policy/safety rejection).
+// The service layer forwards that terminal event verbatim, then returns an
+// error so the caller can log/account for the failed upstream response. The
+// handler must not append its generic fallback `response.failed`, otherwise
+// strict clients may see the useful upstream message replaced by "Upstream
+// request failed" or receive duplicate terminal events.
+func openAIForwardErrorAlreadyCommunicated(c *gin.Context, writerSizeBeforeForward int, err error) bool {
+	if err == nil || c == nil || c.Writer == nil {
+		return false
+	}
+	if c.Writer.Size() == writerSizeBeforeForward {
+		return false
+	}
+
+	msg := strings.TrimSpace(err.Error())
+	for _, prefix := range []string{
+		"upstream response failed:",
+		"non-streaming openai protocol error:",
+	} {
+		if strings.HasPrefix(msg, prefix) {
+			return true
+		}
+	}
+	return false
+}
+
 // errorResponse returns OpenAI API format error response
 func (h *OpenAIGatewayHandler) errorResponse(c *gin.Context, status int, errType, message string) {
 	c.JSON(status, gin.H{
diff --git a/backend/internal/handler/openai_gateway_handler_test.go b/backend/internal/handler/openai_gateway_handler_test.go
index b3fb35eee9..e7605e35e8 100644
--- a/backend/internal/handler/openai_gateway_handler_test.go
+++ b/backend/internal/handler/openai_gateway_handler_test.go
@@ -1579,3 +1579,44 @@ func runOpenAIResponsesWebSocketUsageLogCase(t *testing.T, tc openAIResponsesWSU
 func testStringPtr(v string) *string {
 	return &v
 }
+
+func TestOpenAIForwardErrorAlreadyCommunicated(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	t.Run("upstream response failed after write", func(t *testing.T) {
+		w := httptest.NewRecorder()
+		c, _ := gin.CreateTestContext(w)
+		c.Request = httptest.NewRequest(http.MethodPost, EndpointResponses, nil)
+		before := c.Writer.Size()
+		_, _ = c.Writer.WriteString(`event: response.failed
+data: {"type":"response.failed","error":{"message":"This content was flagged"}}
+
+`)
+
+		reported := openAIForwardErrorAlreadyCommunicated(c, before, errors.New("upstream response failed: This content was flagged"))
+
+		require.True(t, reported)
+	})
+
+	t.Run("no write still needs fallback", func(t *testing.T) {
+		w := httptest.NewRecorder()
+		c, _ := gin.CreateTestContext(w)
+		c.Request = httptest.NewRequest(http.MethodPost, EndpointResponses, nil)
+
+		reported := openAIForwardErrorAlreadyCommunicated(c, c.Writer.Size(), errors.New("upstream response failed: This content was flagged"))
+
+		require.False(t, reported)
+	})
+
+	t.Run("generic error after write still needs fallback", func(t *testing.T) {
+		w := httptest.NewRecorder()
+		c, _ := gin.CreateTestContext(w)
+		c.Request = httptest.NewRequest(http.MethodPost, EndpointResponses, nil)
+		before := c.Writer.Size()
+		_, _ = c.Writer.WriteString(":\n\n")
+
+		reported := openAIForwardErrorAlreadyCommunicated(c, before, errors.New("stream read error: unexpected EOF"))
+
+		require.False(t, reported)
+	})
+}
diff --git a/backend/internal/handler/openai_images.go b/backend/internal/handler/openai_images.go
index 580ecab992..bfa93de977 100644
--- a/backend/internal/handler/openai_images.go
+++ b/backend/internal/handler/openai_images.go
@@ -197,6 +197,7 @@ func (h *OpenAIGatewayHandler) Images(c *gin.Context) {
 
 		service.SetOpsLatencyMs(c, service.OpsRoutingLatencyMsKey, time.Since(routingStart).Milliseconds())
 		forwardStart := time.Now()
+		writerSizeBeforeForward := c.Writer.Size()
 		result, err := func() (*service.OpenAIForwardResult, error) {
 			defer func() {
 				if accountReleaseFunc != nil {
@@ -277,10 +278,15 @@ func (h *OpenAIGatewayHandler) Images(c *gin.Context) {
 					continue
 				}
 				h.gatewayService.ReportOpenAIAccountScheduleResult(account.ID, false, nil)
-				wroteFallback := h.ensureForwardErrorResponse(c, streamStarted)
+				upstreamErrorAlreadyCommunicated := openAIForwardErrorAlreadyCommunicated(c, writerSizeBeforeForward, err)
+				wroteFallback := false
+				if !upstreamErrorAlreadyCommunicated {
+					wroteFallback = h.ensureForwardErrorResponse(c, streamStarted)
+				}
 				fields := []zap.Field{
 					zap.Int64("account_id", account.ID),
 					zap.Bool("fallback_error_response_written", wroteFallback),
+					zap.Bool("upstream_error_response_already_written", upstreamErrorAlreadyCommunicated),
 					zap.Error(err),
 				}
 				if shouldLogOpenAIForwardFailureAsWarn(c, wroteFallback) {