feat: add LANGFLOW_MCP_BASE_URL config for MCP server URL override (#12523)

* feat: add mcp_base_url to config endpoint for MCP server URL override

Add LANGFLOW_MCP_BASE_URL setting that the frontend uses as a fallback
when building MCP server URLs in the UI configuration JSON. This allows
deployments behind reverse proxies to specify the correct external URL.

Priority chain: mcp_base_url > api.defaults.baseURL > window.location.origin

* test: add tests for mcp_base_url config and URL fallback priority

src/backend/base/langflow/api/v1/schemas/__init__.py

@@ -379,6 +379,7 @@ class BaseConfigResponse(BaseModel):
     event_delivery: Literal["polling", "streaming", "direct"]
     voice_mode_available: bool
     frontend_timeout: int
+    mcp_base_url: str
 
 
 class PublicConfigResponse(BaseConfigResponse):
@@ -407,6 +408,7 @@ class PublicConfigResponse(BaseConfigResponse):
             event_delivery=settings.event_delivery,
             voice_mode_available=settings.voice_mode_available,
             frontend_timeout=settings.frontend_timeout,
+            mcp_base_url=settings.mcp_base_url,
             allow_custom_components=settings.allow_custom_components,
         )
 
@@ -460,6 +462,7 @@ class ConfigResponse(BaseConfigResponse):
             public_flow_expiration=settings.public_flow_expiration,
             event_delivery=settings.event_delivery,
             voice_mode_available=settings.voice_mode_available,
+            mcp_base_url=settings.mcp_base_url,
             webhook_auth_enable=auth_settings.WEBHOOK_AUTH_ENABLE,
             default_folder_name=DEFAULT_FOLDER_NAME,
             hide_getting_started_progress=os.getenv("HIDE_GETTING_STARTED_PROGRESS", "").lower() == "true",

src/backend/tests/unit/api/v1/test_endpoints.py

@@ -291,3 +291,41 @@ async def test_get_config_authenticated_returns_full_config(client: AsyncClient,
     assert "auto_saving_interval" in result, "Authenticated response must contain 'auto_saving_interval'"
     assert "health_check_max_retries" in result, "Authenticated response must contain 'health_check_max_retries'"
     assert "feature_flags" in result, "Authenticated response must contain 'feature_flags'"
+
+
+async def test_get_config_returns_mcp_base_url(client: AsyncClient, logged_in_headers: dict):
+    """Test that /config includes mcp_base_url for both authenticated and unauthenticated responses."""
+    # Authenticated
+    response = await client.get("api/v1/config", headers=logged_in_headers)
+    result = response.json()
+    assert response.status_code == status.HTTP_200_OK
+    assert "mcp_base_url" in result, "Authenticated response must contain 'mcp_base_url'"
+    assert isinstance(result["mcp_base_url"], str), "mcp_base_url must be a string"
+
+    # Unauthenticated
+    response = await client.get("api/v1/config")
+    result = response.json()
+    assert response.status_code == status.HTTP_200_OK
+    assert "mcp_base_url" in result, "Public response must contain 'mcp_base_url'"
+    assert isinstance(result["mcp_base_url"], str), "mcp_base_url must be a string"
+
+
+async def test_get_config_mcp_base_url_defaults_to_empty(client: AsyncClient, logged_in_headers: dict):
+    """Test that mcp_base_url defaults to empty string when LANGFLOW_MCP_BASE_URL is not set."""
+    response = await client.get("api/v1/config", headers=logged_in_headers)
+    result = response.json()
+    assert response.status_code == status.HTTP_200_OK
+    assert result["mcp_base_url"] == ""
+
+
+async def test_get_config_mcp_base_url_from_settings(client: AsyncClient, logged_in_headers: dict, monkeypatch):
+    """Test that mcp_base_url reflects the value from settings."""
+    from langflow.services.deps import get_settings_service
+
+    settings_service = get_settings_service()
+    monkeypatch.setattr(settings_service.settings, "mcp_base_url", "https://langflow.example.com")
+
+    response = await client.get("api/v1/config", headers=logged_in_headers)
+    result = response.json()
+    assert response.status_code == status.HTTP_200_OK
+    assert result["mcp_base_url"] == "https://langflow.example.com"

src/frontend/src/controllers/API/queries/config/use-get-config.ts

@@ -20,6 +20,7 @@ interface BaseConfig {
   event_delivery: EventDeliveryType;
   voice_mode_available: boolean;
   allow_custom_components: boolean;
+  mcp_base_url: string;
 }
 
 // Public config = base config (unauthenticated users get only base fields)
@@ -82,6 +83,7 @@ export const useGetConfig: useQueryFunctionType<
   const setAllowCustomComponents = useUtilityStore(
     (state) => state.setAllowCustomComponents,
   );
+  const setMcpBaseUrl = useUtilityStore((state) => state.setMcpBaseUrl);
 
   const { query } = UseRequestProcessor();
 
@@ -104,6 +106,7 @@ export const useGetConfig: useQueryFunctionType<
       setEventDelivery(data.event_delivery ?? EventDeliveryType.POLLING);
       const allowCustomComponents = data.allow_custom_components ?? true;
       setAllowCustomComponents(allowCustomComponents);
+      setMcpBaseUrl(data.mcp_base_url ?? "");
       recomputeComponentsToUpdateIfNeeded();
 
       // Set authenticated-only fields if present (full config)

src/frontend/src/customization/utils/__tests__/custom-mcp-url.test.ts

@@ -0,0 +1,99 @@
+import { api } from "@/controllers/API/api";
+import { useUtilityStore } from "@/stores/utilityStore";
+import { customGetMCPUrl } from "../custom-mcp-url";
+
+describe("customGetMCPUrl", () => {
+  const originalBaseURL = api.defaults.baseURL;
+
+  afterEach(() => {
+    api.defaults.baseURL = originalBaseURL;
+    useUtilityStore.setState({ mcpBaseUrl: "" });
+  });
+
+  it("uses mcpBaseUrl from store when set", () => {
+    api.defaults.baseURL = "";
+    useUtilityStore.setState({ mcpBaseUrl: "https://custom.example.com" });
+
+    const url = customGetMCPUrl("proj-1");
+
+    expect(url).toBe(
+      "https://custom.example.com/api/v1/mcp/project/proj-1/streamable",
+    );
+  });
+
+  it("mcpBaseUrl takes priority over api.defaults.baseURL", () => {
+    api.defaults.baseURL = "https://api-default.example.com";
+    useUtilityStore.setState({ mcpBaseUrl: "https://override.example.com" });
+
+    const url = customGetMCPUrl("proj-1");
+
+    expect(url).toBe(
+      "https://override.example.com/api/v1/mcp/project/proj-1/streamable",
+    );
+  });
+
+  it("falls back to api.defaults.baseURL when mcpBaseUrl is empty", () => {
+    api.defaults.baseURL = "https://api-default.example.com";
+    useUtilityStore.setState({ mcpBaseUrl: "" });
+
+    const url = customGetMCPUrl("proj-1");
+
+    expect(url).toBe(
+      "https://api-default.example.com/api/v1/mcp/project/proj-1/streamable",
+    );
+  });
+
+  it("falls back to window.location.origin when both are empty", () => {
+    api.defaults.baseURL = "";
+    useUtilityStore.setState({ mcpBaseUrl: "" });
+
+    const url = customGetMCPUrl("proj-1");
+
+    expect(url).toBe(
+      `${window.location.origin}/api/v1/mcp/project/proj-1/streamable`,
+    );
+  });
+
+  it("strips trailing slashes from mcpBaseUrl", () => {
+    useUtilityStore.setState({ mcpBaseUrl: "https://example.com/" });
+
+    const url = customGetMCPUrl("proj-1");
+
+    expect(url).toBe(
+      "https://example.com/api/v1/mcp/project/proj-1/streamable",
+    );
+  });
+
+  it("strips multiple trailing slashes", () => {
+    useUtilityStore.setState({ mcpBaseUrl: "https://example.com///" });
+
+    const url = customGetMCPUrl("proj-1");
+
+    expect(url).toBe(
+      "https://example.com/api/v1/mcp/project/proj-1/streamable",
+    );
+  });
+
+  it("returns SSE URL when transport is sse", () => {
+    useUtilityStore.setState({ mcpBaseUrl: "https://example.com" });
+
+    const url = customGetMCPUrl("proj-1", {}, "sse");
+
+    expect(url).toBe("https://example.com/api/v1/mcp/project/proj-1/sse");
+  });
+
+  it("returns composer URL when useComposer is true and streamableHttpUrl is set", () => {
+    useUtilityStore.setState({ mcpBaseUrl: "https://should-not-use.com" });
+
+    const url = customGetMCPUrl(
+      "proj-1",
+      {
+        useComposer: true,
+        streamableHttpUrl: "https://composer.example.com/streamable",
+      },
+      "streamablehttp",
+    );
+
+    expect(url).toBe("https://composer.example.com/streamable");
+  });
+});

src/frontend/src/customization/utils/custom-mcp-url.ts

@@ -1,5 +1,6 @@
 import { api } from "@/controllers/API/api";
 import type { MCPTransport } from "@/controllers/API/queries/mcp/use-patch-install-mcp";
+import { useUtilityStore } from "@/stores/utilityStore";
 
 type ComposerConnectionOptions = {
   useComposer?: boolean;
@@ -26,7 +27,12 @@ export const customGetMCPUrl = (
     }
   }
 
-  const apiHost = api.defaults.baseURL || window.location.origin;
+  const configBaseUrl = useUtilityStore.getState().mcpBaseUrl;
+  const apiHost = (
+    configBaseUrl ||
+    api.defaults.baseURL ||
+    window.location.origin
+  ).replace(/\/+$/, "");
   const baseUrl = `${apiHost}/api/v1/mcp/project/${projectId}`;
   return transport === "streamablehttp"
     ? `${baseUrl}/streamable`

src/frontend/src/stores/utilityStore.ts

@@ -64,4 +64,6 @@ export const useUtilityStore = create<UtilityStoreType>((set, get) => ({
   allowCustomComponents: true,
   setAllowCustomComponents: (allowCustomComponents: boolean) =>
     set({ allowCustomComponents }),
+  mcpBaseUrl: "",
+  setMcpBaseUrl: (mcpBaseUrl: string) => set({ mcpBaseUrl }),
 }));

src/frontend/src/types/zustand/utility/index.ts

@@ -38,4 +38,6 @@ export type UtilityStoreType = {
   setHideGettingStartedProgress: (hideGettingStartedProgress: boolean) => void;
   allowCustomComponents: boolean;
   setAllowCustomComponents: (allowCustomComponents: boolean) => void;
+  mcpBaseUrl: string;
+  setMcpBaseUrl: (mcpBaseUrl: string) => void;
 };

src/lfx/src/lfx/services/settings/base.py

@@ -92,6 +92,11 @@ class Settings(BaseSettings):
     If not provided, a hash of the database URL will be used. Useful when multiple Langflow
     instances share the same database and need coordinated migration locking."""
 
+    mcp_base_url: str = ""
+    """External base URL used to build MCP server URLs in the UI configuration JSON
+    (e.g. 'https://langflow.example.com'). When empty, the frontend falls back to
+    the browser's window.location.origin."""
+
     mcp_server_timeout: int = 20
     """The number of seconds to wait before giving up on a lock to released or establishing a connection to the
     database."""