How to mount streamable HTTP app to an existing starlette app

[Liujony]

Describe the bug
Can not mount streamable HTTP app to an existing starlette app with streamable_http_app() nor a brand new StreamableHTTPSessionManager

Or am I doing the wrong way

To Reproduce

1. with streamable_http_app()

application = Starlette(
    debug=True,
    routes=[
        Route("/health", health),
    ],
    middleware=[Middleware(LoggingMiddleware)],
)
mcpServer = FastMCP("TestMCP", stateless_http=True, streamable_http_path="/test")
application.mount("/mcp/test", mcpServer.streamable_http_app())
uvicorn.run(application, host="0.0.0.0", port=8000)

Error Shows
INFO: 127.0.0.1:51977 - "POST /mcp/test/ HTTP/1.1" 404 Not Found

I think this is an issue related to the Starlette, that a sub app can not run lifespan lifespan events in sub-applications #649

2. with StreamableHTTPSessionManager
   Then I try to create a brand new StreamableHTTPSessionManager and mount the handle_request on the app

application = Starlette(
    debug=True,
    routes=[
        Route("/health", health),
    ],
    middleware=[Middleware(LoggingMiddleware)],
)
mcp_server = FastMCP("TestMCP", stateless_http=True, streamable_http_path="/test")
ssesion_manager = StreamableHTTPSessionManager(
    app=mcp_server._mcp_server,
    stateless=True,
    # event_store=event_store,
    # json_response=is_json_response_enabled,
)
exit_stack = AsyncExitStack()
asyncio.run(exit_stack.enter_async_context(session_manager.run()))
application.mount("/mcp", session_manager.handle_request)
uvicorn.run(application, host="0.0.0.0", port=8000)

Error shows

INFO 2025-05-09 18:20:23,993 streamable_http_manager 17158 140704588348864 StreamableHTTP session manager started
INFO 2025-05-09 18:20:23,994 streamable_http_manager 17158 140704588348864 StreamableHTTP session manager shutting down
ERROR 2025-05-09 18:20:23,994 base_events 17158 140704588348864 an error occurred during closing of asynchronous generator <async_generator object StreamableHTTPSessionManager.run at 0x102abd3f0>
asyncgen: <async_generator object StreamableHTTPSessionManager.run at 0x102abd3f0>
  + Exception Group Traceback (most recent call last):
  |   File "/usr/local/lib/python3.13/site-packages/anyio/_backends/_asyncio.py", line 772, in __aexit__
  |     raise BaseExceptionGroup(
  |         "unhandled errors in a TaskGroup", self._exceptions
  |     ) from None
  | BaseExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
  +-+---------------- 1 ----------------
    | Traceback (most recent call last):
    |   File "/usr/local/lib/python3.13/site-packages/mcp/server/streamable_http_manager.py", line 112, in run
    |     yield  # Let the application run
    |     ^^^^^
    | GeneratorExit
    +------------------------------------

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.13/site-packages/mcp/server/streamable_http_manager.py", line 107, in run
    async with anyio.create_task_group() as tg:
               ~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/usr/local/lib/python3.13/site-packages/anyio/_backends/_asyncio.py", line 778, in __aexit__
    if self.cancel_scope.__exit__(type(exc), exc, exc.__traceback__):
       ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/site-packages/anyio/_backends/_asyncio.py", line 457, in __exit__
    raise RuntimeError(
    ...<2 lines>...
    )
RuntimeError: Attempted to exit cancel scope in a different task than it was entered in

which means StreamableHTTP session was not run successfully
And when I try connect mcp server with client is shows it was not run
RuntimeError: Task group is not initialized. Make sure to use run().

Expected behavior
Run Successfully

Package Version

• mcp: 1.8.0

[sidey]

At the section of Streamable HTTP Transport,there are several examples,so here is the right way to mount.
the url should be http://127.0.0.1:3001/echo/mcp

import uvicorn
from mcp.server.fastmcp import FastMCP
from starlette.applications import Starlette
from starlette.routing import Mount

mcp = FastMCP(name="Echo Server", stateless_http=True)

@mcp.tool()
def echo(text: str) -> str:
    """Echo the input text"""
    return text

app = Starlette(routes=[Mount("/echo", app=mcp.streamable_http_app())],
                lifespan=lambda app: mcp.session_manager.run())

if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=3001)

[Liujony]

At the section of Streamable HTTP Transport,there are several examples,so here is the right way to mount. the url should be http://127.0.0.1:3001/echo/mcp

import uvicorn
from mcp.server.fastmcp import FastMCP
from starlette.applications import Starlette
from starlette.routing import Mount

mcp = FastMCP(name="Echo Server", stateless_http=True)

@mcp.tool()
def echo(text: str) -> str:
    """Echo the input text"""
    return text

app = Starlette(routes=[Mount("/echo", app=mcp.streamable_http_app())],
                lifespan=lambda app: mcp.session_manager.run())

if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=3001)

@sidey Your example could work, but doesn't pretty close to problem. I want to mount MULTIPLE mcp server app on my exist Starlette app, and I certainly don't want to put child app's lifespan on main app, it's will be messy in the future. But thx a lot, I think FastMCP already have a solution, I will try it today.

[jasny6969]

At the section of Streamable HTTP Transport,there are several examples,so here is the right way to mount. the url should be http://127.0.0.1:3001/echo/mcp

import uvicorn
from mcp.server.fastmcp import FastMCP
from starlette.applications import Starlette
from starlette.routing import Mount

mcp = FastMCP(name="Echo Server", stateless_http=True)

@mcp.tool()
def echo(text: str) -> str:
    """Echo the input text"""
    return text

app = Starlette(routes=[Mount("/echo", app=mcp.streamable_http_app())],
                lifespan=lambda app: mcp.session_manager.run())

if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=3001)

@sidey Your example could work, but doesn't pretty close to problem. I want to mount MULTIPLE mcp server app on my exist Starlette app, and I certainly don't want to put child app's lifespan on main app, it's will be messy in the future. But thx a lot, I think FastMCP already have a solution, I will try it today.

Did you find a working solution to this problem?

[Liujony]

@jasny6969 I did find a working solution, but I actually use the way I originally didn't want to use before.

The FastMCP's method is simply pass the sub-app's lifespan to the main starlette app. So it wouldn't work when I need to mount multiple mcp server.

Docs: fastapi-integration

from fastmcp import FastMCP
from fastapi import FastAPI
from starlette.routing import Mount

# Create your FastMCP server as well as any tools, resources, etc.
mcp = FastMCP("MyServer")

# Create the ASGI app
mcp_app = mcp.http_app(path='/mcp')

# Create a FastAPI app and mount the MCP server
app = FastAPI(lifespan=mcp_app.lifespan)
app.mount("/mcp-server", mcp_app)

So I have to do it on my own, I pass all the start-up function of the mcp server out and wrap them up in a lifespan function, then add it to the main app's lifespan

def registerAllMCPsForRouteAndLifespan() -> tuple[List[BaseRoute], List[Callable[..., AsyncIterator[None]]]]:

    MCPServersToBeRegister = [FastMCP("test1"), FastMCP("test2"), FastMCP("test3"), ......]
    mcpRoutes: List[BaseRoute] = list()
    lifeSpans: List[Callable[..., AsyncIterator[None]]] = list()

    for server in MCPServersToBeRegister:
        mcpRoutes.append(Mount("/mcp/"+server.name(), app=server.streamable_http_app()))
        lifeSpans.append(server.session_manager.run)
    return mcpRoutes, lifeSpans



def createLifespansFunc(lifespans: List[Callable[..., AsyncIterator[None]]]) -> Coroutine[Any, Any, None]:
    async def lifespan(app):
        logger.info(f"Lifespan Begin", extra={"timestamp": time.time()})
        async with AsyncExitStack() as stack:
            for func in lifespans:
                await stack.enter_async_context(func())
            logger.info(f"Finish Lifespan init", extra={"timestamp": time.time()})
            yield
            logger.info(f"All MCP Session Manager closed", extra={"timestamp": time.time()})
    return lifespan



def main():
    host = os.getenv('MCP_SERVER_ARRE')
    port = int(os.getenv('MCP_SERVER_PORT'))

    allMCPRoutes, withLifeSpans = registerAllMCPsForRouteAndLifespan()
    application = Starlette(
        debug=True,
        routes=[
            *allMCPRoutes
        ],
        middleware=[Middleware(LoggingMiddleware)],
        lifespan=createLifespansFunc(withLifeSpans)
    )
    uvicorn.run(application, host=host, port=port)

And it work perfectly

[LiangYang666]

@Liujony @jasny6969
You can use this method to mount multiple SSE or streamable HTTP applications. Note that for streamable HTTP applications, you need to pass them into the combine_lifespans function.

from contextlib import AsyncExitStack, asynccontextmanager

import uvicorn
from starlette.applications import Starlette
from starlette.routing import Mount

from mcp.server.fastmcp import FastMCP

mcp0 = FastMCP()
mcp1 = FastMCP()
mcp2 = FastMCP()

def combine_lifespans(*lifespans):
    @asynccontextmanager
    async def combined_lifespan(app):
        async with AsyncExitStack() as stack:
            for l in lifespans:
                ctx = l(app)
                await stack.enter_async_context(ctx)
            yield
    return combined_lifespan

main_app = Starlette(
    routes=[
        Mount("/server0/", app=mcp0.sse_app()),
        Mount("/server1/", app=mcp1.streamable_http_app()),
        Mount("/server2/", app=mcp2.streamable_http_app())],
    lifespan=combine_lifespans(lambda app: mcp1.session_manager.run(),
                               lambda app: mcp2.session_manager.run())
)

if __name__ == "__main__":
    uvicorn.run(main_app, host="127.0.0.1", port=8000)

[felixweinberger]

I believe the answers above have addressed the original issue here.