fix(http): fail closed without auth config

fastapi_mcp/server.py

@@ -3,7 +3,7 @@
 from typing import Dict, Optional, Any, List, Union, Literal, Sequence
 from typing_extensions import Annotated, Doc
 
-from fastapi import FastAPI, Request, APIRouter, params
+from fastapi import FastAPI, Request, APIRouter, params, HTTPException, Depends
 from fastapi.openapi.utils import get_openapi
 from mcp.server.lowlevel.server import Server
 import mcp.types as types
@@ -309,6 +309,17 @@ def _setup_auth(self):
         else:
             logger.info("No auth config provided, skipping auth setup")
 
+    @staticmethod
+    def _fail_closed_auth_dependency():
+        async def require_auth() -> None:
+            raise HTTPException(
+                status_code=401,
+                detail="Authentication required. Configure AuthConfig before exposing MCP tools over HTTP.",
+                headers={"WWW-Authenticate": "Bearer"},
+            )
+
+        return Depends(require_auth)
+
     def mount_http(
         self,
         router: Annotated[
@@ -349,7 +360,7 @@ def mount_http(
         assert isinstance(router, (FastAPI, APIRouter)), f"Invalid router type: {type(router)}"
 
         http_transport = FastApiHttpSessionManager(mcp_server=self.server)
-        dependencies = self._auth_config.dependencies if self._auth_config else None
+        dependencies = self._auth_config.dependencies if self._auth_config else [self._fail_closed_auth_dependency()]
 
         self._register_mcp_endpoints_http(router, http_transport, mount_path, dependencies)
         self._setup_auth()

tests/test_http_real_transport.py

@@ -8,11 +8,12 @@
 import threading
 import coverage
 from typing import AsyncGenerator, Generator
-from fastapi import FastAPI
+from fastapi import FastAPI, Depends
+from fastapi.routing import APIRoute
 import pytest
 import httpx
 import uvicorn
-from fastapi_mcp import FastApiMCP
+from fastapi_mcp import FastApiMCP, AuthConfig
 import mcp.types as types
 
 
@@ -59,6 +60,7 @@ def periodic_save():
         fastapi_app,
         name=SERVER_NAME,
         description="Test description",
+        auth_config=AuthConfig(dependencies=[Depends(lambda: None)]),
     )
     mcp.mount_http()
 
@@ -476,3 +478,29 @@ async def test_http_notification_handling(http_client: httpx.AsyncClient, server
     assert response.status_code == 202
     # Notifications should return empty body
     assert response.content == b"" or response.text == "null"
+
+
+def test_http_fails_closed_without_auth_config(simple_fastapi_app: FastAPI) -> None:
+    mcp = FastApiMCP(simple_fastapi_app)
+    mcp.mount_http()
+
+    route = next(route for route in simple_fastapi_app.routes if isinstance(route, APIRoute) and route.path == "/mcp")
+    assert route.dependant.dependencies
+
+
+@pytest.mark.anyio
+async def test_http_rejects_unauthenticated_request(simple_fastapi_app: FastAPI) -> None:
+    mcp = FastApiMCP(simple_fastapi_app)
+    mcp.mount_http()
+
+    transport = httpx.ASGITransport(app=simple_fastapi_app)
+    async with httpx.AsyncClient(transport=transport, base_url="http://test") as client:
+        response = await client.post(
+            "/mcp",
+            json={"jsonrpc": "2.0", "method": "tools/list", "id": 1},
+            headers={"Accept": "application/json, text/event-stream", "Content-Type": "application/json"},
+        )
+
+    assert response.status_code == 401
+    assert response.headers["www-authenticate"] == "Bearer"
+    assert "Authentication required" in response.json()["detail"]