Enhance FastMCP server with port override functionality during run method

Author: yeoyujie

README.md

@@ -502,7 +502,17 @@ from mcp.server.fastmcp import FastMCP
 mcp = FastMCP("My App")
 
 if __name__ == "__main__":
+    # Run with default settings
     mcp.run()
+    
+    # Override port
+    mcp.run(port=3000)
+    
+    # Specify transport and port
+    mcp.run(transport="streamable-http", port=8080)
+    
+    # SSE with custom mount path and port
+    mcp.run(transport="sse", mount_path="/api", port=9000)
 ```
 
 Run it with:
@@ -512,8 +522,12 @@ python server.py
 mcp run server.py
 ```
 
-Note that `mcp run` or `mcp dev` only supports server using FastMCP and not the low-level server variant.
+The `run()` method accepts these parameters:
+- `transport`: Transport protocol ("stdio", "sse", or "streamable-http")
+- `mount_path`: Optional mount path for SSE transport
+- `port`: Optional port override (defaults to settings.port or 8000)
 
+Note that `mcp run` or `mcp dev` only supports server using FastMCP and not the low-level server variant.
 ### Streamable HTTP Transport
 
 > **Note**: Streamable HTTP transport is superseding SSE transport for production deployments.

src/mcp/server/fastmcp/server.py

@@ -206,14 +206,18 @@ def run(
         self,
         transport: Literal["stdio", "sse", "streamable-http"] = "stdio",
         mount_path: str | None = None,
+        port: int | 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
+            port: Optional port to run the server on
         """
         TRANSPORTS = Literal["stdio", "sse", "streamable-http"]
+        if port is not None:
+            self.settings.port = port  # override port if provided
         if transport not in TRANSPORTS.__args__:  # type: ignore
             raise ValueError(f"Unknown transport: {transport}")
 

tests/server/fastmcp/test_server.py

@@ -850,3 +850,36 @@ def prompt_fn(name: str) -> str:
         async with client_session(mcp._mcp_server) as client:
             with pytest.raises(McpError, match="Missing required arguments"):
                 await client.get_prompt("prompt_fn")
+
+
+@pytest.fixture
+def server_port() -> int:
+    import socket
+
+    with socket.socket() as s:
+        s.bind(("127.0.0.1", 0))
+        return s.getsockname()[1]
+
+
+class TestServerTransports:
+    """Test port overrides during run."""
+
+    @pytest.mark.anyio
+    async def test_port_override_default_port(self, server_port):
+        """Test that the port argument overrides default port."""
+        mcp = FastMCP()
+
+        with patch("anyio.run") as mock_anyio_run:
+            mcp.run(port=server_port)
+            assert mcp.settings.port == server_port
+            mock_anyio_run.assert_called_once_with(mcp.run_stdio_async)
+
+    @pytest.mark.anyio
+    async def test_port_inheritance_without_override(self, server_port):
+        """Test that existing settings.port is preserved when no port override is provided."""
+        mcp = FastMCP(port=server_port)
+
+        with patch("anyio.run") as mock_anyio_run:
+            mcp.run()
+            assert mcp.settings.port == server_port
+            mock_anyio_run.assert_called_once_with(mcp.run_stdio_async)