fix: avoid default codex instructions for chat bridge

backend/internal/service/openai_gateway_chat_completions.go

@@ -172,7 +172,12 @@ func (s *OpenAIGatewayService) ForwardAsChatCompletions(
 		if err := json.Unmarshal(responsesBody, &reqBody); err != nil {
 			return nil, fmt.Errorf("unmarshal for codex transform: %w", err)
 		}
-		codexResult := applyCodexOAuthTransform(reqBody, false, false)
+		codexResult := applyCodexOAuthTransformWithOptions(reqBody, codexOAuthTransformOptions{
+			SkipDefaultInstructions: !isResponsesShape,
+		})
+		if !isResponsesShape {
+			ensureCodexOAuthInstructionsField(reqBody)
+		}
 		if codexResult.NormalizedModel != "" {
 			upstreamModel = codexResult.NormalizedModel
 		}

backend/internal/service/openai_gateway_chat_completions_test.go

@@ -181,6 +181,47 @@ func TestForwardAsChatCompletions_APIKeyPropagatesPromptCacheKeyInResponsesBody(
 	require.Equal(t, generateSessionUUID(isolateOpenAISessionID(99, "cache-key-123")), upstream.lastReq.Header.Get("session_id"))
 }
 
+func TestForwardAsChatCompletions_OAuthDoesNotInjectDefaultInstructions(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	body := []byte(`{"model":"gpt-5.4","messages":[{"role":"user","content":"hello"}],"stream":false}`)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/chat/completions", bytes.NewReader(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	upstream := &httpUpstreamRecorder{resp: &http.Response{
+		StatusCode: http.StatusBadRequest,
+		Header:     http.Header{"Content-Type": []string{"application/json"}, "x-request-id": []string{"rid_chat_no_default_instructions"}},
+		Body:       io.NopCloser(strings.NewReader(`{"error":{"type":"invalid_request_error","message":"stop before response parsing"}}`)),
+	}}
+
+	svc := &OpenAIGatewayService{
+		cfg:          &config.Config{},
+		httpUpstream: upstream,
+	}
+	account := &Account{
+		ID:          3,
+		Name:        "openai-oauth",
+		Platform:    PlatformOpenAI,
+		Type:        AccountTypeOAuth,
+		Concurrency: 1,
+		Credentials: map[string]any{
+			"access_token":       "oauth-token",
+			"chatgpt_account_id": "chatgpt-acc",
+		},
+	}
+
+	result, err := svc.ForwardAsChatCompletions(context.Background(), c, account, body, "", "gpt-5.4")
+	require.Error(t, err)
+	require.Nil(t, result)
+	require.NotNil(t, upstream.lastReq)
+	require.Equal(t, chatgptCodexURL, upstream.lastReq.URL.String())
+	require.True(t, gjson.GetBytes(upstream.lastBody, "instructions").Exists())
+	require.Equal(t, "", gjson.GetBytes(upstream.lastBody, "instructions").String())
+	require.NotContains(t, string(upstream.lastBody), "Communicate with the user by streaming thinking")
+}
+
 func TestForwardAsChatCompletions_ClientDisconnectDrainsUpstreamUsage(t *testing.T) {
 	gin.SetMode(gin.TestMode)