Progress notifications not delivered via SSE in stateless HTTP mode

[hubbard-zlee]

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

Summary

Context.report_progress() notifications are not delivered to clients when using the streamable HTTP transport, while Context.info() and other logging methods work correctly.

Root Cause

In src/mcp/server/fastmcp/server.py, the report_progress method does not pass related_request_id to send_progress_notification():

async def report_progress(self, progress: float, total: float | None = None, message: str | None = None) -> None:
    progress_token = self.request_context.meta.progressToken if self.request_context.meta else None

    if progress_token is None:
        return

    await self.request_context.session.send_progress_notification(
        progress_token=progress_token,
        progress=progress,
        total=total,
        message=message,
        # related_request_id is missing here
    )

The SSE transport uses related_request_id in the message metadata to route notifications to the correct client. Without it, progress notifications are silently dropped.

In contrast, send_log_message (used by info(), warning(), etc.) correctly passes related_request_id:

Fix

Add related_request_id=self.request_id to the send_progress_notification() call:

await self.request_context.session.send_progress_notification(
    progress_token=progress_token,
    progress=progress,
    total=total,
    message=message,
    related_request_id=self.request_id,
)

Reproduction

1. Create a FastMCP server with stateless_http=True
2. In a tool, call both await ctx.info("test") and await ctx.report_progress(0.5, total=1.0, message="test")
3. Observe that info() messages are delivered via SSE but report_progress() messages are not

Environment

• MCP SDK version: 1.26.0
• Python version: 3.11
• Transport: Streamable HTTP (stateless_http=True)

Example Code

Python & MCP Python SDK

- MCP SDK version: 1.26.0
- Python version: 3.11