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Description
Initial Checks
- I confirm that I'm using the latest version of MCP Python SDK
- I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue
Description
When using the low-level Server API (mcp.server.lowlevel.Server
) with stdio transport, sending progress notifications during request handling causes the server to hang indefinitely and never send the final response.
What I am seeing:
- Server successfully sends progress notification via
write_stream.send()
- Server handler returns
CallToolResult
- The final response is never sent to the client
- Client times out waiting for response
- Server remains blocked and time out eventually
Example Code
import asyncio
from mcp.server.lowlevel import Server
from mcp.server.stdio import stdio_server
from mcp.types import *
server = Server("progress-bug-demo")
@server.list_tools()
async def handle_list_tools() -> list[Tool]:
return [Tool(
name="test_tool",
description="Demonstrates the bug",
inputSchema={"type": "object", "properties": {}}
)]
@server.call_tool()
async def handle_call_tool(name: str, arguments: dict) -> list[TextContent]:
ctx = server.request_context
progress_token = ctx.meta.progressToken if ctx.meta else None
if progress_token:
# This causes the server to hang
await ctx.session.send_progress_notification(
progress_token=progress_token,
progress=0.5,
total=1.0,
message="Working..."
)
# This response is never sent due to the hang
return [TextContent(type="text", text="Task completed!")]
async def main():
async with stdio_server() as (read_stream, write_stream):
await server.run(
read_stream,
write_stream,
server.create_initialization_options()
)
if __name__ == "__main__":
asyncio.run(main())
Python & MCP Python SDK
MCP: 1.10.1
Python: 3.12.6
From deeper debugging looks like buffer size is the problem : https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/stdio.py#L57
Have to test locally if increasing it will most likely fix the issue and if its causing more problems than just fixing this.