diff --git a/backend/internal/handler/openai_gateway_handler.go b/backend/internal/handler/openai_gateway_handler.go
index 5319b55d..43999a01 100644
--- a/backend/internal/handler/openai_gateway_handler.go
+++ b/backend/internal/handler/openai_gateway_handler.go
@@ -187,6 +187,11 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 			h.errorResponse(c, http.StatusBadRequest, "invalid_request_error", "previous_response_id must be a response.id (resp_*), not a message id")
 			return
 		}
+		reqLog.Warn("openai.request_validation_failed",
+			zap.String("reason", "previous_response_id_requires_wsv2"),
+		)
+		h.errorResponse(c, http.StatusBadRequest, "invalid_request_error", "previous_response_id is only supported on Responses WebSocket v2")
+		return
 	}
 
 	setOpsRequestContext(c, reqModel, reqStream, body)
@@ -856,7 +861,7 @@ func (h *OpenAIGatewayHandler) validateFunctionCallOutputRequest(c *gin.Context,
 		reqLog.Warn("openai.request_validation_failed",
 			zap.String("reason", "function_call_output_missing_call_id"),
 		)
-		h.errorResponse(c, http.StatusBadRequest, "invalid_request_error", "function_call_output requires call_id or previous_response_id; if relying on history, ensure store=true and reuse previous_response_id")
+		h.errorResponse(c, http.StatusBadRequest, "invalid_request_error", "function_call_output requires call_id on HTTP requests; continuation via previous_response_id is only supported on Responses WebSocket v2")
 		return false
 	}
 	if validation.HasItemReferenceForAllCallIDs {
@@ -866,7 +871,7 @@ func (h *OpenAIGatewayHandler) validateFunctionCallOutputRequest(c *gin.Context,
 	reqLog.Warn("openai.request_validation_failed",
 		zap.String("reason", "function_call_output_missing_item_reference"),
 	)
-	h.errorResponse(c, http.StatusBadRequest, "invalid_request_error", "function_call_output requires item_reference ids matching each call_id, or previous_response_id/tool_call context; if relying on history, ensure store=true and reuse previous_response_id")
+	h.errorResponse(c, http.StatusBadRequest, "invalid_request_error", "function_call_output requires item_reference ids matching each call_id on HTTP requests; continuation via previous_response_id is only supported on Responses WebSocket v2")
 	return false
 }
 
diff --git a/backend/internal/handler/openai_gateway_handler_test.go b/backend/internal/handler/openai_gateway_handler_test.go
index d299fb81..8ecee59a 100644
--- a/backend/internal/handler/openai_gateway_handler_test.go
+++ b/backend/internal/handler/openai_gateway_handler_test.go
@@ -494,6 +494,64 @@ func TestOpenAIResponses_RejectsMessageIDAsPreviousResponseID(t *testing.T) {
 	require.Contains(t, w.Body.String(), "previous_response_id must be a response.id")
 }
 
+func TestOpenAIResponses_RejectsHTTPContinuationPreviousResponseID(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest(http.MethodPost, "/openai/v1/responses", strings.NewReader(
+		`{"model":"gpt-5.1","stream":false,"previous_response_id":"resp_123456","input":[{"type":"input_text","text":"hello"}]}`,
+	))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	groupID := int64(2)
+	c.Set(string(middleware.ContextKeyAPIKey), &service.APIKey{
+		ID:      101,
+		GroupID: &groupID,
+		User:    &service.User{ID: 1},
+	})
+	c.Set(string(middleware.ContextKeyUser), middleware.AuthSubject{
+		UserID:      1,
+		Concurrency: 1,
+	})
+
+	h := newOpenAIHandlerForPreviousResponseIDValidation(t, nil)
+	h.Responses(c)
+
+	require.Equal(t, http.StatusBadRequest, w.Code)
+	require.Contains(t, w.Body.String(), "Responses WebSocket v2")
+	require.Contains(t, w.Body.String(), "previous_response_id")
+}
+
+func TestOpenAIResponses_FunctionCallOutputHTTPGuidanceDoesNotSuggestPreviousResponseReuse(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest(http.MethodPost, "/openai/v1/responses", strings.NewReader(
+		`{"model":"gpt-5.1","stream":false,"input":[{"type":"function_call_output","output":"{}"}]}`,
+	))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	groupID := int64(2)
+	c.Set(string(middleware.ContextKeyAPIKey), &service.APIKey{
+		ID:      101,
+		GroupID: &groupID,
+		User:    &service.User{ID: 1},
+	})
+	c.Set(string(middleware.ContextKeyUser), middleware.AuthSubject{
+		UserID:      1,
+		Concurrency: 1,
+	})
+
+	h := newOpenAIHandlerForPreviousResponseIDValidation(t, nil)
+	h.Responses(c)
+
+	require.Equal(t, http.StatusBadRequest, w.Code)
+	require.Contains(t, w.Body.String(), "Responses WebSocket v2")
+	require.NotContains(t, w.Body.String(), "reuse previous_response_id")
+}
+
 func TestOpenAIResponsesWebSocket_SetsClientTransportWSWhenUpgradeValid(t *testing.T) {
 	gin.SetMode(gin.TestMode)