Python MCP SDK sse_app() ignores Mount prefix: client 404 on /messages/

Question

When I mount FastMCP's sse_app() under a prefix with Starlette Mount('/mcp', app=mcp.sse_app()) on macOS, the SSE handshake advertises data: /messages/?session_id=... instead of /mcp/messages/..., and the MCP client fails with 404 for url 'http://127.0.0.1:8000/messages/'. Is there a workaround that makes sse_app() respect the mount prefix?

Originally asked on GitHub: https://github.com/modelcontextprotocol/python-sdk/issues/412

AgentArmorHQ team · Accepted Answer

The root cause is that `sse_app()` hardcodes `self.settings.message_path` into `SseServerTransport`, so when Starlette `Mount('/mcp', app=mcp.sse_app())` is used, the SSE handshake still advertises `/messages/` and the client 404s on `http://host/messages/`. The issue is closed, but the thread shows no confirmed fix in a released SDK version; PR #524 and later #540 are referenced as potential fixes.

Workaround the commenter says works: add a `base_path` parameter to `sse_app()` in `src/mcp/server/fastmcp/server.py`:

```python
def sse_app(self, base_path: str = '') -> Starlette:
    sse = SseServerTransport(f'{base_path}{self.settings.message_path}')
```

Then mount explicitly: `Mount('/mcp', mcp.sse_app(base_path='/mcp'))`. The commenter notes it is redundant but works.

A second proposed helper builds the routes directly on the parent Starlette/FastAPI app:

```python
sse = SseServerTransport(f"{base_path}/messages/")
starlette_app.add_route(f"{base_path}/sse", handle_sse)
starlette_app.mount(f"{base_path}/messages/", sse.handle_post_message)
```

The thread does not confirm this helper was tested; one user behind an OpenFaaS gateway that strips the `/function/mcp` prefix still got `404 ... /messages/`. A third workaround: avoid path prefixes entirely by serving MCP on its own hostname using Starlette `Host` routes plus `/etc/hosts` entries (`backend`, `mcp`), so `sse_app()` lives at the root. Multi-level prefixes inserted by a reverse proxy were also reported as failing; client-side handling was requested as feature request #795.

AgentArmorHQ team · Answer

**What @xorcus tried on GitHub** (Worked)

Patched FastMCP's sse_app() to accept base_path='' and used SseServerTransport(f'{base_path}{self.settings.message_path}'), then mounted with Mount('/mcp', mcp.sse_app(base_path='/mcp')). The commenter said it is redundant but works.

Source: https://github.com/modelcontextprotocol/python-sdk/issues/412#issuecomment-2824380370

AgentArmorHQ team · Answer

**What @pztrick tried on GitHub** (Worked)

Served MCP SSE on a dedicated hostname instead of a path prefix: added /etc/hosts entries for 'backend' and 'mcp' and used Starlette Host routes so sse_app() is served at the root of the 'mcp' host.

Source: https://github.com/modelcontextprotocol/python-sdk/issues/412#issuecomment-2794769462

AgentArmorHQ team · Answer

**What @MarkDunne tried on GitHub** (Outcome unclear)

Proposed a register_mcp_router(starlette_app, mcp_server, base_path) helper that creates SseServerTransport(f"{base_path}/messages/") and registers the SSE route and message mount manually. The thread does not record whether this was tested.

Source: https://github.com/modelcontextprotocol/python-sdk/issues/412#issuecomment-2796384734

AgentArmorHQ team · Answer

**What @alexellis tried on GitHub** (Didn't work)

Applied the register_mcp_router helper inside an OpenFaaS function mounted at /function/mcp, where the gateway strips the prefix before the request reaches the app; the client still errored with 404 for http://127.0.0.1:8080/messages/.

Source: https://github.com/modelcontextprotocol/python-sdk/issues/412#issuecomment-2801480631

Python MCP SDK sse_app() ignores Mount prefix: client 404 on /messages/

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Python MCP SDK sse_app() ignores Mount prefix: client 404 on /messages/

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