diff --git a/backend/internal/service/openai_gateway_service.go b/backend/internal/service/openai_gateway_service.go
index 19fad32501..3fdad536d0 100644
--- a/backend/internal/service/openai_gateway_service.go
+++ b/backend/internal/service/openai_gateway_service.go
@@ -3933,6 +3933,11 @@ func (s *OpenAIGatewayService) handleStreamingResponsePassthrough(
 				responseID = extractOpenAIResponseIDFromJSONBytes(dataBytes)
 			}
 			imageCounter.AddSSEData(dataBytes)
+			if sanitizedData, sanitized := sanitizeOpenAIResponseFailedEventForClient(dataBytes, eventType); sanitized {
+				dataBytes = sanitizedData
+				trimmedData = strings.TrimSpace(string(sanitizedData))
+				line = "data: " + string(sanitizedData)
+			}
 			lineStartsClientOutput = forceFlushFailedEvent || openAIStreamDataStartsClientOutput(trimmedData, eventType)
 			if firstTokenMs == nil && lineStartsClientOutput && trimmedData != "[DONE]" {
 				ms := int(time.Since(startTime).Milliseconds())
@@ -4895,6 +4900,11 @@ func (s *OpenAIGatewayService) handleStreamingResponse(ctx context.Context, resp
 				line = "data: " + data
 				eventType = strings.TrimSpace(gjson.GetBytes(dataBytes, "type").String())
 			}
+			if sanitizedData, sanitized := sanitizeOpenAIResponseFailedEventForClient(dataBytes, eventType); sanitized {
+				dataBytes = sanitizedData
+				data = string(sanitizedData)
+				line = "data: " + data
+			}
 			// Replace model in response if needed.
 			// Fast path: most events do not contain model field values.
 			if needModelReplace && mappedModel != "" && strings.Contains(line, mappedModel) {
@@ -5509,6 +5519,37 @@ func extractOpenAISSEErrorMessage(payload []byte) string {
 	return sanitizeUpstreamErrorMessage(strings.TrimSpace(extractUpstreamErrorMessage(payload)))
 }
 
+func sanitizeOpenAIResponseFailedEventForClient(payload []byte, eventType string) ([]byte, bool) {
+	if eventType != "response.failed" || len(payload) == 0 || !gjson.ValidBytes(payload) {
+		return payload, false
+	}
+	if !gjson.GetBytes(payload, "response").Exists() {
+		return payload, false
+	}
+	updated := payload
+	for _, path := range []string{
+		"response.instructions",
+		"response.output",
+		"response.usage",
+		"response.metadata",
+		"response.reasoning",
+		"response.tools",
+		"response.tool_choice",
+		"response.parallel_tool_calls",
+		"response.text",
+		"response.truncation",
+		"response.max_output_tokens",
+		"response.incomplete_details",
+	} {
+		next, err := sjson.DeleteBytes(updated, path)
+		if err != nil {
+			return payload, false
+		}
+		updated = next
+	}
+	return updated, !bytes.Equal(updated, payload)
+}
+
 func (s *OpenAIGatewayService) writeOpenAINonStreamingProtocolError(resp *http.Response, c *gin.Context, message string) error {
 	message = sanitizeUpstreamErrorMessage(strings.TrimSpace(message))
 	if message == "" {
diff --git a/backend/internal/service/openai_gateway_service_test.go b/backend/internal/service/openai_gateway_service_test.go
index 66cfddea6f..abcf53a269 100644
--- a/backend/internal/service/openai_gateway_service_test.go
+++ b/backend/internal/service/openai_gateway_service_test.go
@@ -1368,6 +1368,55 @@ func TestOpenAIStreamingResponseFailedBeforeOutputServerOverloadedCodeReturnsFai
 	require.Empty(t, rec.Body.String())
 }
 
+func TestOpenAIStreamingResponseFailedAfterOutputSanitizesVerboseResponseForClient(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	cfg := &config.Config{
+		Gateway: config.GatewayConfig{
+			StreamDataIntervalTimeout: 0,
+			StreamKeepaliveInterval:   0,
+			MaxLineSize:               defaultMaxLineSize,
+		},
+	}
+	svc := &OpenAIGatewayService{cfg: cfg}
+
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/", nil)
+
+	longInstructions := strings.Repeat("You are GPT-5.1 running in the Codex CLI. ", 20)
+	failedPayload := fmt.Sprintf(
+		`{"type":"response.failed","response":{"id":"resp_failed","object":"response","created_at":1782446336,"status":"failed","instructions":%q,"output":[{"type":"message","content":[{"type":"output_text","text":"large"}]}],"usage":{"input_tokens":123,"output_tokens":0},"error":{"code":"context_length_exceeded","message":"Your input exceeds the context window of this model. Please adjust your input and try again."}}}`,
+		longInstructions,
+	)
+	resp := &http.Response{
+		StatusCode: http.StatusOK,
+		Body: io.NopCloser(strings.NewReader(strings.Join([]string{
+			"event: response.created",
+			`data: {"type":"response.created","response":{"id":"resp_failed"}}`,
+			"",
+			"event: response.output_text.delta",
+			`data: {"type":"response.output_text.delta","delta":"partial"}`,
+			"",
+			"event: response.failed",
+			"data: " + failedPayload,
+			"",
+		}, "\n"))),
+		Header: http.Header{"X-Request-Id": []string{"rid-failed-after-output"}},
+	}
+
+	_, err := svc.handleStreamingResponse(c.Request.Context(), resp, c, &Account{ID: 1, Platform: PlatformOpenAI, Name: "acc"}, time.Now(), "model", "model")
+	require.Error(t, err)
+
+	body := rec.Body.String()
+	require.Contains(t, body, "event: response.failed")
+	require.Contains(t, body, "context_length_exceeded")
+	require.Contains(t, body, "Your input exceeds the context window")
+	require.NotContains(t, body, "You are GPT-5.1 running in the Codex CLI")
+	require.NotContains(t, body, `"instructions"`)
+	require.NotContains(t, body, `"output"`)
+	require.NotContains(t, body, `"usage"`)
+}
+
 func TestOpenAIStreamingPreambleOnlyMissingTerminalReturnsFailover(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	cfg := &config.Config{
@@ -1707,6 +1756,53 @@ func TestOpenAIStreamingPassthroughResponseFailedBeforeOutputReturnsFailover(t *
 	require.Empty(t, rec.Body.String())
 }
 
+func TestOpenAIStreamingPassthroughResponseFailedAfterOutputSanitizesVerboseResponseForClient(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	cfg := &config.Config{
+		Gateway: config.GatewayConfig{
+			MaxLineSize: defaultMaxLineSize,
+		},
+	}
+	svc := &OpenAIGatewayService{cfg: cfg}
+
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/", nil)
+
+	longInstructions := strings.Repeat("You are GPT-5.1 running in the Codex CLI. ", 20)
+	failedPayload := fmt.Sprintf(
+		`{"type":"response.failed","response":{"id":"resp_pass_failed","object":"response","created_at":1782446336,"status":"failed","instructions":%q,"output":[{"type":"message","content":[{"type":"output_text","text":"large"}]}],"usage":{"input_tokens":123,"output_tokens":0},"error":{"code":"context_length_exceeded","message":"Your input exceeds the context window of this model. Please adjust your input and try again."}}}`,
+		longInstructions,
+	)
+	resp := &http.Response{
+		StatusCode: http.StatusOK,
+		Body: io.NopCloser(strings.NewReader(strings.Join([]string{
+			"event: response.created",
+			`data: {"type":"response.created","response":{"id":"resp_pass_failed"}}`,
+			"",
+			"event: response.output_text.delta",
+			`data: {"type":"response.output_text.delta","delta":"partial"}`,
+			"",
+			"event: response.failed",
+			"data: " + failedPayload,
+			"",
+		}, "\n"))),
+		Header: http.Header{"X-Request-Id": []string{"rid-pass-failed-after-output"}},
+	}
+
+	_, err := svc.handleStreamingResponsePassthrough(c.Request.Context(), resp, c, &Account{ID: 1, Platform: PlatformOpenAI, Name: "acc"}, time.Now(), "", "")
+	require.Error(t, err)
+
+	body := rec.Body.String()
+	require.Contains(t, body, "event: response.failed")
+	require.Contains(t, body, "context_length_exceeded")
+	require.Contains(t, body, "Your input exceeds the context window")
+	require.NotContains(t, body, "You are GPT-5.1 running in the Codex CLI")
+	require.NotContains(t, body, `"instructions"`)
+	require.NotContains(t, body, `"output"`)
+	require.NotContains(t, body, `"usage"`)
+}
+
 func TestOpenAIStreamingPassthroughResponseDoneWithoutDoneMarkerStillSucceeds(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	cfg := &config.Config{