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Title: [Fix] SSE client to support MCP server mounted under a base path on a different ASGI server by rkondra-eightfold · Pull Request #524 · modelcontextprotocol/python-sdk · GitHub

Open Graph Title: [Fix] SSE client to support MCP server mounted under a base path on a different ASGI server by rkondra-eightfold · Pull Request #524 · modelcontextprotocol/python-sdk

X Title: [Fix] SSE client to support MCP server mounted under a base path on a different ASGI server by rkondra-eightfold · Pull Request #524 · modelcontextprotocol/python-sdk

Description: What does this PR do? Fixes incorrect resolution of the /messages endpoint URL in the SSE client when the FastAPI app is mounted under a base path (e.g., /mcp). You can now provide a server_mount_path to ensure correct endpoint resolution. Motivation and Context Problem # Create a FastAPI app app = FastAPI() # Create an MCP server and link it to the FastAPI app server = FastMCP('test_app') # Mount the server's SSE API on /mcp app.mount('/mcp', server.sse_app()) The server sends a relative endpoint like /messages?session_id=..., the client resolves it using urljoin(base_url, relative_path) When the app is mounted (e.g., /mcp/sse), this strips the subpath and constructs an invalid endpoint (e.g., http://host/messages?session_id=... instead of http://host/mcp/messages?session_id=...). Fix This PR: Extracts the origin and mounted path prefix from the SSE base URL Correctly reassembles the endpoint URL using that prefix async with sse_client(base_url, server_mount_path='/mcp') as (read, write): async with ClientSession(read, write) as session: await session.initialize() # Initialize the session print('✅ Connection established!') How Has This Been Tested? server: from fastapi import FastAPI from models import User from mcp.server.fastmcp import FastMCP # Create a FastAPI app app = FastAPI() # Create an MCP server and link it to the FastAPI app server = FastMCP('test_app') # Mount the server's SSE API on /mcp app.mount('/mcp', server.sse_app()) # Start the server when this file is run directly if __name__ == "__main__": import uvicorn uvicorn.run(app, host="0.0.0.0", port=8000) client: import asyncio from mcp import ClientSession from mcp.client.sse import sse_client async def run(): base_url = 'http://0.0.0.0:8000/mcp/sse' # Make sure the URL is correct for the SSE connection print(f'🔗 Connecting to SSE MCP server at {base_url}...') try: async with sse_client(base_url, server_mount_path='/mcp') as (read, write): async with ClientSession(read, write) as session: await session.initialize() # Initialize the session print('✅ Connection established!') except Exception as e: print(f'[ERROR] Connection to SSE server failed: {e}') # Run the test if __name__ == '__main__': print('🚀 Running MCP SSE client tests...\n') asyncio.run(run()) This test should pass now. Breaking Changes None identified, backward compatible Types of changes Bug fix (non-breaking change which fixes an issue) New feature (non-breaking change which adds functionality) Breaking change (fix or feature that would cause existing functionality to change) Documentation update Checklist I have read the MCP Documentation My code follows the repository's style guidelines New and existing tests pass locally I have added appropriate error handling I have added or updated documentation as needed Additional context NA

Open Graph Description: What does this PR do? Fixes incorrect resolution of the /messages endpoint URL in the SSE client when the FastAPI app is mounted under a base path (e.g., /mcp). You can now provide a server_mount_...

X Description: What does this PR do? Fixes incorrect resolution of the /messages endpoint URL in the SSE client when the FastAPI app is mounted under a base path (e.g., /mcp). You can now provide a server_mount_...

Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/524

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