diff --git a/backend/internal/service/openai_gateway_service.go b/backend/internal/service/openai_gateway_service.go
index 01b25800..2c9840fd 100644
--- a/backend/internal/service/openai_gateway_service.go
+++ b/backend/internal/service/openai_gateway_service.go
@@ -4922,20 +4922,22 @@ func (s *OpenAIGatewayService) handleNonStreamingResponse(ctx context.Context, r
 	if isEventStreamResponse(resp.Header) {
 		return s.handleSSEToJSON(resp, c, body, originalModel, mappedModel)
 	}
+	bodyLooksLikeSSE := bytes.Contains(body, []byte("data:")) || bytes.Contains(body, []byte("event:"))
+
 	// For OAuth accounts, also fall back to a body-content heuristic because
 	// the upstream may omit the Content-Type header while still sending SSE.
 	// This heuristic is NOT applied to API-key accounts to avoid false
 	// positives on JSON responses that coincidentally contain "data:" or
 	// "event:" in their text content.
-	if account.Type == AccountTypeOAuth {
-		bodyLooksLikeSSE := bytes.Contains(body, []byte("data:")) || bytes.Contains(body, []byte("event:"))
-		if bodyLooksLikeSSE {
-			return s.handleSSEToJSON(resp, c, body, originalModel, mappedModel)
-		}
+	if account.Type == AccountTypeOAuth && bodyLooksLikeSSE {
+		return s.handleSSEToJSON(resp, c, body, originalModel, mappedModel)
 	}
 
 	usageValue, usageOK := extractOpenAIUsageFromJSONBytes(body)
 	if !usageOK {
+		if bodyLooksLikeSSE {
+			return s.handleSSEToJSON(resp, c, body, originalModel, mappedModel)
+		}
 		return nil, fmt.Errorf("parse response: invalid json response")
 	}
 	usage := &usageValue
diff --git a/backend/internal/service/openai_gateway_service_test.go b/backend/internal/service/openai_gateway_service_test.go
index ef35aa1a..8bed920d 100644
--- a/backend/internal/service/openai_gateway_service_test.go
+++ b/backend/internal/service/openai_gateway_service_test.go
@@ -2281,6 +2281,35 @@ func TestHandleSSEToJSON_CompletedEventReturnsJSON(t *testing.T) {
 	require.NotContains(t, rec.Body.String(), "data:")
 }
 
+func TestHandleNonStreamingResponse_APIKeyFallsBackToSSEBodyWhenContentTypeIsWrong(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/responses", nil)
+
+	svc := &OpenAIGatewayService{cfg: &config.Config{}}
+	resp := &http.Response{
+		StatusCode: http.StatusOK,
+		Header:     http.Header{"Content-Type": []string{"application/json"}},
+		Body: io.NopCloser(strings.NewReader(strings.Join([]string{
+			`data: {"type":"response.output_text.delta","delta":"hel"}`,
+			`data: {"type":"response.output_text.delta","delta":"lo"}`,
+			`data: {"type":"response.completed","response":{"id":"resp_api_key_sse","object":"response","model":"gpt-5.4","status":"completed","output":[],"usage":{"input_tokens":3,"output_tokens":2,"total_tokens":5}}}`,
+			`data: [DONE]`,
+		}, "\n"))),
+	}
+	account := &Account{ID: 1, Type: AccountTypeAPIKey}
+
+	result, err := svc.handleNonStreamingResponse(context.Background(), resp, c, account, "gpt-5.4", "gpt-5.4")
+	require.NoError(t, err)
+	require.NotNil(t, result)
+	require.Equal(t, 3, result.InputTokens)
+	require.Equal(t, 2, result.OutputTokens)
+	require.NotContains(t, rec.Body.String(), "data:")
+	require.Equal(t, "resp_api_key_sse", gjson.Get(rec.Body.String(), "id").String())
+	require.Equal(t, "hello", gjson.Get(rec.Body.String(), "output.0.content.0.text").String())
+}
+
 func TestHandleSSEToJSON_ReconstructsImageGenerationOutputItemDone(t *testing.T) {
 	gin.SetMode(gin.TestMode)
 	rec := httptest.NewRecorder()