mount_path does not work with sse_app

[kinch-tech]

while i start mcp server with mcp.run, provided with mount_path, then client get bad response with 404。

the endpoint for handling post message should be normalized_message_endpoint, not the original message_path

        routes.append(
            Mount(
                self.settings.message_path,
                app=sse.handle_post_message,
            )
        )

[adamsaimi]

Hello i opened a merge request, there was no 1.9.x branch available so I targeted main.

[felixweinberger]

Hi @kinch-tech thanks for this submission could you provide a full code snippet to reproduce the issue so we can debug?

[felixweinberger]

Closing this via #1881, which removes the mount_path parameter entirely.

The mount_path parameter was redundant because the SSE transport already handles sub-path mounting automatically via ASGI's standard root_path mechanism. When you use Starlette's Mount(), it sets root_path in the ASGI scope, and SseServerTransport already uses this to construct the correct message endpoint path (see sse.py lines 148-161).

The fix: Just remove any mount_path configuration and rely on ASGI's built-in mechanism:

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

github_mcp = FastMCP("GitHub API")
browser_mcp = FastMCP("Browser")

# This just works - no mount_path needed
app = Starlette(
    routes=[
        Mount("/github", app=github_mcp.sse_app()),
        Mount("/browser", app=browser_mcp.sse_app()),
    ]
)

The SSE transport will automatically return /github/messages/ and /browser/messages/ as the message endpoints to clients.