Streamable HTTP Trailing Slash Compatibility

Author: chi2liu

src/mcp/server/fastmcp/server.py

@@ -92,7 +92,7 @@ class Settings(BaseSettings, Generic[LifespanResultT]):
     mount_path: str = "/"  # Mount path (e.g. "/github", defaults to root path)
     sse_path: str = "/sse"
     message_path: str = "/messages/"
-    streamable_http_path: str = "/mcp/"
+    streamable_http_path: str = "/mcp"
 
     # StreamableHTTP settings
     json_response: bool = False
@@ -830,6 +830,18 @@ async def handle_streamable_http(
                 )
             )
 
+        # Always mount both /mcp and /mcp/ for full compatibility, regardless of default
+        _main_path = self.settings.streamable_http_path
+        if _main_path.endswith("/"):
+            _alt_path = _main_path.rstrip("/")
+        else:
+            _alt_path = _main_path + "/"
+        if _alt_path != _main_path:
+            if self._auth_server_provider:
+                routes.append(Mount(_alt_path, app=RequireAuthMiddleware(handle_streamable_http, required_scopes)))
+            else:
+                routes.append(Mount(_alt_path, app=handle_streamable_http))
+
         routes.extend(self._custom_starlette_routes)
 
         return Starlette(