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Progress notifications cause server to hang on stdio transport #1141

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@abhishekgahlot2

Description

@abhishekgahlot2

Initial Checks

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.

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