Progress notifications cause server to hang on stdio transport

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.

Start with the low-level Server API and the stdio transport implementation at src/mcp/server/stdio.py:57. Run the provided progress-notification example with MCP 1.10.1 to confirm where the send blocks, then inspect the stream buffer behavior. Done means progress notifications and the final CallToolResult both reach the client without the server hanging.