Add mount_path support for proper SSE endpoint routing with multiple FastMCP servers #540
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This commit adds a mount_path attribute to FastMCP settings to support properly constructing SSE endpoints when servers are mounted under sub-paths. The _normalize_path helper method combines mount paths with endpoints to ensure correct session_uri generation. This fixes issues when running multiple FastMCP servers under different path prefixes in a single Starlette application.
Tests to verify proper handling of mount paths when creating SSE applications. Ensures path normalization works correctly for different path combinations.
This commit adds documentation for mounting multiple FastMCP servers under different paths, showing how to configure mount_path for each server instance.
Format code to comply with line length limit (88 characters).
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I wish Starlette allowed some reflection on where a route was mounted. This kind of handshake could benefit from that, and I'm guessing the Starlette folks never thought it would be needed. That mount point duplication in settings just seems superfluous, but looking at the Starlette internals, I couldn't see a way of getting at those values either. Perhaps asking them nicely to add this capability would work. |
@richardhundt I agree that Starlette's lack of reflection on mount points is a bit unfortunate. While we wait for that capability, I was thinking of adding a syntactic sugar method that could help avoid the duplication: def get_mount(self, mount_path: Optional[str] = None) -> Mount:
"""
Configure mount_path and return a Mount object for Starlette routing.
Args:
mount_path: Optional override for the mount_path setting
Returns:
Starlette Mount object configured for this FastMCP server
"""
# Set mount_path setting
if mount_path:
self.settings.mount_path = mount_path
# Return configured Mount object
return Mount(self.settings.mount_path, app=self.sse_app())This would allow for more concise code: # Current approach
github_mcp.settings.mount_path = "/github"
routes = [Mount("/github", app=github_mcp.sse_app())]
# Proposed approach
routes = [github_mcp.get_mount("/github")]What do you think? This could help prevent mount path mismatches and simplify the API. |
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Looks like this will all be moot soon. The updated specification reduces all communication to just a single endpoint supporting both SSE and transactional (request/response) style HTTP interactions (controlled by the Accept header). It complicates the clients a bit because they need to distinguish SSE streaming from transactional responses, but that's manageable and solves all of these routing issues. https://modelcontextprotocol.io/specification/2025-03-26/basic/transports#streamable-http |
Add mount_path capability to FastMCP for proper SSE endpoint handling when mounted under sub-paths
Motivation and Context
When multiple FastMCP servers are mounted under different sub-paths in a single Starlette application, the session URIs were being generated incorrectly. FastMCP would create session URIs that didn't include the mount path, causing clients to send messages to the wrong endpoint. This change fixes the issue by
adding a
mount_pathsetting that properly normalizes URIs.How Has This Been Tested?
Breaking Changes
None. This is backwards compatible with existing code. The new
mount_pathsetting defaults to "/".Types of changes
Checklist
Additional context
The issue occurs because the SseServerTransport class creates a session URI without considering that the server might be mounted under a sub-path. The fix adds a mount_path setting to FastMCP and implements a path normalization helper method that combines the mount path with the message path to ensure correct
URI generation.
This solution maintains clean separation between FastMCP and SseServerTransport without requiring changes to the SseServerTransport class itself.