diff --git a/backend/internal/service/openai_gateway_forward.go b/backend/internal/service/openai_gateway_forward.go
index 9a4c2389e3..4f2b102140 100644
--- a/backend/internal/service/openai_gateway_forward.go
+++ b/backend/internal/service/openai_gateway_forward.go
@@ -472,13 +472,29 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 		if decodeErr != nil {
 			return nil, decodeErr
 		}
+		// Responses OAuth 与 Chat 兼容入口保持一致：纯文本 system 可以无损提升后删除，
+		// JSON object 模式仍需在 input 中保留 JSON 指令供上游兼容校验。
+		omitPromotedSystemMessages := !strings.EqualFold(
+			strings.TrimSpace(gjson.GetBytes(body, "text.format.type").String()),
+			"json_object",
+		)
 		codexResult := codexTransformResult{}
 		if compatMessagesBridge {
-			codexResult = applyCodexOAuthTransformWithOptions(decoded, codexOAuthTransformOptions{IsCodexCLI: isCodexCLI, IsCompact: isCompactRequest, SkipDefaultInstructions: true, PreserveToolCallIDs: true})
+			codexResult = applyCodexOAuthTransformWithOptions(decoded, codexOAuthTransformOptions{
+				IsCodexCLI:                          isCodexCLI,
+				IsCompact:                           isCompactRequest,
+				SkipDefaultInstructions:             true,
+				PreserveToolCallIDs:                 true,
+				OmitPromotedSystemMessagesFromInput: omitPromotedSystemMessages,
+			})
 			ensureCodexOAuthInstructionsField(decoded)
 			markDecodedModified()
 		} else {
-			codexResult = applyCodexOAuthTransform(decoded, isCodexCLI, isCompactRequest)
+			codexResult = applyCodexOAuthTransformWithOptions(decoded, codexOAuthTransformOptions{
+				IsCodexCLI:                          isCodexCLI,
+				IsCompact:                           isCompactRequest,
+				OmitPromotedSystemMessagesFromInput: omitPromotedSystemMessages,
+			})
 		}
 		if codexResult.Error != nil {
 			c.JSON(http.StatusBadRequest, gin.H{"error": gin.H{"type": "invalid_request_error", "message": codexResult.Error.Error()}})
diff --git a/backend/internal/service/openai_oauth_passthrough_test.go b/backend/internal/service/openai_oauth_passthrough_test.go
index fb9a6fee27..fa750fa5b5 100644
--- a/backend/internal/service/openai_oauth_passthrough_test.go
+++ b/backend/internal/service/openai_oauth_passthrough_test.go
@@ -132,6 +132,44 @@ func TestOpenAIGatewayService_ResponsesUnknownModelDoesNotFallbackToGPT54(t *tes
 	require.True(t, rec.Code >= http.StatusBadRequest)
 }
 
+func TestOpenAIGatewayService_OAuthResponsesPromotesSystemMessageWithoutDuplication(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	const systemPrompt = "Unique system prefix for Responses token accounting."
+	const existingInstructions = "Existing instructions."
+	body := []byte(`{"model":"gpt-5.4","stream":false,"instructions":"` + existingInstructions + `","input":[{"role":"system","content":"` + systemPrompt + `"},{"role":"user","content":"hello"}]}`)
+	rec := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(rec)
+	c.Request = httptest.NewRequest(http.MethodPost, "/v1/responses", bytes.NewReader(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	upstream := &httpUpstreamRecorder{err: errors.New("stop after capture")}
+	svc := &OpenAIGatewayService{cfg: &config.Config{}, httpUpstream: upstream}
+	account := &Account{
+		ID:          124,
+		Name:        "openai-oauth",
+		Platform:    PlatformOpenAI,
+		Type:        AccountTypeOAuth,
+		Concurrency: 1,
+		Credentials: map[string]any{
+			"access_token":       "oauth-token",
+			"chatgpt_account_id": "chatgpt-acc",
+		},
+		Status:      StatusActive,
+		Schedulable: true,
+	}
+
+	result, err := svc.Forward(context.Background(), c, account, body)
+
+	require.Error(t, err)
+	require.Nil(t, result)
+	require.NotEmpty(t, upstream.lastBody)
+	require.Equal(t, systemPrompt+"\n\n"+existingInstructions, gjson.GetBytes(upstream.lastBody, "instructions").String())
+	require.Equal(t, int64(1), gjson.GetBytes(upstream.lastBody, "input.#").Int())
+	require.Equal(t, "user", gjson.GetBytes(upstream.lastBody, "input.0.role").String())
+	require.Equal(t, 1, strings.Count(string(upstream.lastBody), systemPrompt))
+}
+
 func TestOpenAIGatewayService_NativeResponsesBodyModificationPreservesHTMLChars(t *testing.T) {
 	gin.SetMode(gin.TestMode)