ensure message path is built correctly, and introduce a hacky fix for mounting on APIRouter

Author: shahar4499

examples/shared/setup.py

@@ -6,7 +6,7 @@
 class LoggingConfig(BaseModel):
     LOGGER_NAME: str = "fastapi_mcp"
     LOG_FORMAT: str = "%(levelprefix)s %(asctime)s\t[%(name)s] %(message)s"
-    LOG_LEVEL: str = logging.getLevelName(logging.INFO)
+    LOG_LEVEL: str = logging.getLevelName(logging.DEBUG)
 
     version: int = 1
     disable_existing_loggers: bool = False

fastapi_mcp/server.py

@@ -159,11 +159,20 @@ def mount(self, router: Optional[FastAPI | APIRouter] = None, mount_path: str =
         if not router:
             router = self.fastapi
 
-        # Create SSE transport for MCP messages
-        sse_transport = FastApiSseTransport(f"{mount_path}/messages/")
+        # Build the base path correctly for the SSE transport
+        if isinstance(router, FastAPI):
+            base_path = router.root_path
+        elif isinstance(router, APIRouter):
+            base_path = self.fastapi.root_path + router.prefix
+        else:
+            raise ValueError(f"Invalid router type: {type(router)}")
+
+        messages_path = f"{base_path}{mount_path}/messages/"
+
+        sse_transport = FastApiSseTransport(messages_path)
 
         # Route for MCP connection
-        @router.get(mount_path, include_in_schema=False)
+        @router.get(mount_path, include_in_schema=False, operation_id="mcp_connection")
         async def handle_mcp_connection(request: Request):
             async with sse_transport.connect_sse(request.scope, request.receive, request._send) as (reader, writer):
                 await self.server.run(
@@ -173,10 +182,18 @@ async def handle_mcp_connection(request: Request):
                 )
 
         # Route for MCP messages
-        @router.post(f"{mount_path}/messages/", include_in_schema=False)
+        @router.post(f"{mount_path}/messages/", include_in_schema=False, operation_id="mcp_messages")
         async def handle_post_message(request: Request):
             return await sse_transport.handle_fastapi_post_message(request)
 
+        # HACK: If we got a router and not a FastAPI instance, we need to re-include the router so that
+        # FastAPI will pick up the new routes we added. The problem with this approach is that we assume
+        # that the router is a sub-router of self.fastapi, which may not always be the case.
+        #
+        # TODO: Find a better way to do this.
+        if isinstance(router, APIRouter):
+            self.fastapi.include_router(router)
+
         logger.info(f"MCP server listening at {mount_path}")
 
     async def _execute_api_tool(