mount_path does not work with sse_app #783

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1. adamsaimi commented on Jun 21, 2025

   

   adamsaimi

   on Jun 21, 2025

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

   added

   bugSomething isn't workingSomething isn't working

   needs reproneeds additional information to be able to reproduce bugneeds additional information to be able to reproduce bug

   on Oct 3, 2025
3. felixweinberger commented on Oct 3, 2025

   

   felixweinberger

   on Oct 3, 2025

   Contributor

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   Hi @kinch-tech thanks for this submission could you provide a full code snippet to reproduce the issue so we can debug?
4. felixweinberger commented on Jan 16, 2026

   

   felixweinberger

   on Jan 16, 2026

   Contributor

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   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.
5. felixweinberger

   closed this as completedon Jan 16, 2026