Revert mount_path parameter from FastMCP (#1881)

Author: Kludex

README.md

@@ -1078,7 +1078,7 @@ The FastMCP server instance accessible via `ctx.fastmcp` provides access to serv
   - `debug` - Debug mode flag
   - `log_level` - Current logging level
   - `host` and `port` - Server network configuration
-  - `mount_path`, `sse_path`, `streamable_http_path` - Transport paths
+  - `sse_path`, `streamable_http_path` - Transport paths
   - `stateless_http` - Whether the server operates in stateless mode
   - And other configuration options
 
@@ -1614,7 +1614,7 @@ app = Starlette(
 app.router.routes.append(Host('mcp.acme.corp', app=mcp.sse_app()))
 ```
 
-When mounting multiple MCP servers under different paths, you can configure the mount path in several ways:
+You can also mount multiple MCP servers at different sub-paths. The SSE transport automatically detects the mount path via ASGI's `root_path` mechanism, so message endpoints are correctly routed:
 
 ```python
 from starlette.applications import Starlette
@@ -1624,31 +1624,18 @@ from mcp.server.fastmcp import FastMCP
 # Create multiple MCP servers
 github_mcp = FastMCP("GitHub API")
 browser_mcp = FastMCP("Browser")
-curl_mcp = FastMCP("Curl")
 search_mcp = FastMCP("Search")
 
-# Method 1: Configure mount paths via settings (recommended for persistent configuration)
-github_mcp.settings.mount_path = "/github"
-browser_mcp.settings.mount_path = "/browser"
-
-# Method 2: Pass mount path directly to sse_app (preferred for ad-hoc mounting)
-# This approach doesn't modify the server's settings permanently
-
-# Create Starlette app with multiple mounted servers
+# Mount each server at its own sub-path
+# The SSE transport automatically uses ASGI's root_path to construct
+# the correct message endpoint (e.g., /github/messages/, /browser/messages/)
 app = Starlette(
     routes=[
-        # Using settings-based configuration
         Mount("/github", app=github_mcp.sse_app()),
         Mount("/browser", app=browser_mcp.sse_app()),
-        # Using direct mount path parameter
-        Mount("/curl", app=curl_mcp.sse_app("/curl")),
-        Mount("/search", app=search_mcp.sse_app("/search")),
+        Mount("/search", app=search_mcp.sse_app()),
     ]
 )
-
-# Method 3: For direct execution, you can also pass the mount path to run()
-if __name__ == "__main__":
-    search_mcp.run(transport="sse", mount_path="/search")
 ```
 
 For more information on mounting applications in Starlette, see the [Starlette documentation](https://www.starlette.io/routing/#submounting-routes).

docs/migration.md

@@ -116,6 +116,12 @@ result = await session.list_resources(params=PaginatedRequestParams(cursor="next
 result = await session.list_tools(params=PaginatedRequestParams(cursor="next_page_token"))
 ```
 
+### `mount_path` parameter removed from FastMCP
+
+The `mount_path` parameter has been removed from `FastMCP.__init__()`, `FastMCP.run()`, `FastMCP.run_sse_async()`, and `FastMCP.sse_app()`. It was also removed from the `Settings` class.
+
+This parameter was redundant because the SSE transport already handles sub-path mounting via ASGI's standard `root_path` mechanism. When using Starlette's `Mount("/path", app=mcp.sse_app())`, Starlette automatically sets `root_path` in the ASGI scope, and the `SseServerTransport` uses this to construct the correct message endpoint path.
+
 ### Resource URI type changed from `AnyUrl` to `str`
 
 The `uri` field on resource-related types now uses `str` instead of Pydantic's `AnyUrl`. This aligns with the [MCP specification schema](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/draft/schema.ts) which defines URIs as plain strings (`uri: string`) without strict URL validation. This change allows relative paths like `users/me` that were previously rejected.

src/mcp/server/fastmcp/server.py

@@ -76,7 +76,6 @@ class Settings(BaseSettings, Generic[LifespanResultT]):
     # HTTP settings
     host: str
     port: int
-    mount_path: str
     sse_path: str
     message_path: str
     streamable_http_path: str
@@ -138,7 +137,6 @@ def __init__(  # noqa: PLR0913
         log_level: Literal["DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"] = "INFO",
         host: str = "127.0.0.1",
         port: int = 8000,
-        mount_path: str = "/",
         sse_path: str = "/sse",
         message_path: str = "/messages/",
         streamable_http_path: str = "/mcp",
@@ -164,7 +162,6 @@ def __init__(  # noqa: PLR0913
             log_level=log_level,
             host=host,
             port=port,
-            mount_path=mount_path,
             sse_path=sse_path,
             message_path=message_path,
             streamable_http_path=streamable_http_path,
@@ -269,13 +266,11 @@ def session_manager(self) -> StreamableHTTPSessionManager:
     def run(
         self,
         transport: Literal["stdio", "sse", "streamable-http"] = "stdio",
-        mount_path: str | None = None,
     ) -> None:
         """Run the FastMCP server. Note this is a synchronous function.
 
         Args:
             transport: Transport protocol to use ("stdio", "sse", or "streamable-http")
-            mount_path: Optional mount path for SSE transport
         """
         TRANSPORTS = Literal["stdio", "sse", "streamable-http"]
         if transport not in TRANSPORTS.__args__:  # type: ignore  # pragma: no cover
@@ -285,7 +280,7 @@ def run(
             case "stdio":
                 anyio.run(self.run_stdio_async)
             case "sse":  # pragma: no cover
-                anyio.run(lambda: self.run_sse_async(mount_path))
+                anyio.run(self.run_sse_async)
             case "streamable-http":  # pragma: no cover
                 anyio.run(self.run_streamable_http_async)
 
@@ -749,11 +744,11 @@ async def run_stdio_async(self) -> None:
                 self._mcp_server.create_initialization_options(),
             )
 
-    async def run_sse_async(self, mount_path: str | None = None) -> None:  # pragma: no cover
+    async def run_sse_async(self) -> None:  # pragma: no cover
         """Run the server using SSE transport."""
         import uvicorn
 
-        starlette_app = self.sse_app(mount_path)
+        starlette_app = self.sse_app()
 
         config = uvicorn.Config(
             starlette_app,
@@ -779,58 +774,16 @@ async def run_streamable_http_async(self) -> None:  # pragma: no cover
         server = uvicorn.Server(config)
         await server.serve()
 
-    def _normalize_path(self, mount_path: str, endpoint: str) -> str:
-        """
-        Combine mount path and endpoint to return a normalized path.
-
-        Args:
-            mount_path: The mount path (e.g. "/github" or "/")
-            endpoint: The endpoint path (e.g. "/messages/")
-
-        Returns:
-            Normalized path (e.g. "/github/messages/")
-        """
-        # Special case: root path
-        if mount_path == "/":
-            return endpoint
-
-        # Remove trailing slash from mount path
-        if mount_path.endswith("/"):
-            mount_path = mount_path[:-1]
-
-        # Ensure endpoint starts with slash
-        if not endpoint.startswith("/"):
-            endpoint = "/" + endpoint
-
-        # Combine paths
-        return mount_path + endpoint
-
-    def sse_app(self, mount_path: str | None = None) -> Starlette:
+    def sse_app(self) -> Starlette:
         """Return an instance of the SSE server app."""
 
-        # Update mount_path in settings if provided
-        if mount_path is not None:
-            self.settings.mount_path = mount_path
-
-        # Create normalized endpoint considering the mount path
-        normalized_message_endpoint = self._normalize_path(self.settings.mount_path, self.settings.message_path)
-
-        # Set up auth context and dependencies
-
-        sse = SseServerTransport(
-            normalized_message_endpoint,
-            security_settings=self.settings.transport_security,
-        )
+        sse = SseServerTransport(self.settings.message_path, security_settings=self.settings.transport_security)
 
         async def handle_sse(scope: Scope, receive: Receive, send: Send):  # pragma: no cover
             # Add client ID from auth context into request context if available
 
             async with sse.connect_sse(scope, receive, send) as streams:
-                await self._mcp_server.run(
-                    streams[0],
-                    streams[1],
-                    self._mcp_server.create_initialization_options(),
-                )
+                await self._mcp_server.run(streams[0], streams[1], self._mcp_server.create_initialization_options())
             return Response()
 
         # Create routes

tests/server/fastmcp/test_server.py

@@ -49,85 +49,23 @@ async def test_create_server(self):
         assert mcp.icons[0].src == "https://example.com/icon.png"
 
     @pytest.mark.anyio
-    async def test_normalize_path(self):
-        """Test path normalization for mount paths."""
-        mcp = FastMCP()
-
-        # Test root path
-        assert mcp._normalize_path("/", "/messages/") == "/messages/"
-
-        # Test path with trailing slash
-        assert mcp._normalize_path("/github/", "/messages/") == "/github/messages/"
-
-        # Test path without trailing slash
-        assert mcp._normalize_path("/github", "/messages/") == "/github/messages/"
-
-        # Test endpoint without leading slash
-        assert mcp._normalize_path("/github", "messages/") == "/github/messages/"
-
-        # Test both with trailing/leading slashes
-        assert mcp._normalize_path("/api/", "/v1/") == "/api/v1/"
-
-    @pytest.mark.anyio
-    async def test_sse_app_with_mount_path(self):
-        """Test SSE app creation with different mount paths."""
-        # Test with default mount path
-        mcp = FastMCP()
-        with patch.object(mcp, "_normalize_path", return_value="/messages/") as mock_normalize:
-            mcp.sse_app()
-            # Verify _normalize_path was called with correct args
-            mock_normalize.assert_called_once_with("/", "/messages/")
-
-        # Test with custom mount path in settings
-        mcp = FastMCP()
-        mcp.settings.mount_path = "/custom"
-        with patch.object(mcp, "_normalize_path", return_value="/custom/messages/") as mock_normalize:
-            mcp.sse_app()
-            # Verify _normalize_path was called with correct args
-            mock_normalize.assert_called_once_with("/custom", "/messages/")
-
-        # Test with mount_path parameter
-        mcp = FastMCP()
-        with patch.object(mcp, "_normalize_path", return_value="/param/messages/") as mock_normalize:
-            mcp.sse_app(mount_path="/param")
-            # Verify _normalize_path was called with correct args
-            mock_normalize.assert_called_once_with("/param", "/messages/")
+    async def test_sse_app_returns_starlette_app(self):
+        """Test that sse_app returns a Starlette application with correct routes."""
+        from starlette.applications import Starlette
 
-    @pytest.mark.anyio
-    async def test_starlette_routes_with_mount_path(self):
-        """Test that Starlette routes are correctly configured with mount path."""
-        # Test with mount path in settings
-        mcp = FastMCP()
-        mcp.settings.mount_path = "/api"
+        mcp = FastMCP("test", host="0.0.0.0")  # Use 0.0.0.0 to avoid auto DNS protection
         app = mcp.sse_app()
 
-        # Find routes by type
-        sse_routes = [r for r in app.routes if isinstance(r, Route)]
-        mount_routes = [r for r in app.routes if isinstance(r, Mount)]
+        assert isinstance(app, Starlette)
 
         # Verify routes exist
-        assert len(sse_routes) == 1, "Should have one SSE route"
-        assert len(mount_routes) == 1, "Should have one mount route"
-
-        # Verify path values
-        assert sse_routes[0].path == "/sse", "SSE route path should be /sse"
-        assert mount_routes[0].path == "/messages", "Mount route path should be /messages"
-
-        # Test with mount path as parameter
-        mcp = FastMCP()
-        app = mcp.sse_app(mount_path="/param")
-
-        # Find routes by type
         sse_routes = [r for r in app.routes if isinstance(r, Route)]
         mount_routes = [r for r in app.routes if isinstance(r, Mount)]
 
-        # Verify routes exist
         assert len(sse_routes) == 1, "Should have one SSE route"
         assert len(mount_routes) == 1, "Should have one mount route"
-
-        # Verify path values
-        assert sse_routes[0].path == "/sse", "SSE route path should be /sse"
-        assert mount_routes[0].path == "/messages", "Mount route path should be /messages"
+        assert sse_routes[0].path == "/sse"
+        assert mount_routes[0].path == "/messages"
 
     @pytest.mark.anyio
     async def test_non_ascii_description(self):