test: add regression test for issue #2001 (progress related_request_id)

Adds a dedicated regression test file for issue #2001.

Context.report_progress() was silently dropping progress notifications
in stateless HTTP / SSE transports because send_progress_notification()
was called without related_request_id. The fix (already in main via
mcpserver/context.py) passes related_request_id=self.request_id,
consistent with send_log_message().

This test file covers:
- The primary regression: related_request_id is forwarded on every call
- Edge case: no progress_token -> notification is skipped (no-op)
- Edge case: integer progress_token (e.g. 0) works correctly

Closes #2001

Author: kaushik701

tests/issues/test_2001_progress_related_request_id.py

@@ -0,0 +1,117 @@
+"""Regression test for https://github.com/modelcontextprotocol/python-sdk/issues/2001.
+
+Progress notifications not delivered via SSE in stateless HTTP mode.
+
+Root cause: Context.report_progress() was calling send_progress_notification()
+without passing related_request_id.  The SSE / streamable-HTTP transport uses
+that field to route server-initiated messages back to the correct client stream;
+without it notifications are silently dropped.
+
+Fix: pass related_request_id=self.request_id in send_progress_notification(),
+consistent with how send_log_message() already works.
+"""
+
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+from mcp.server.context import ServerRequestContext
+from mcp.server.experimental.request_context import Experimental
+from mcp.server.mcpserver import Context
+
+pytestmark = pytest.mark.anyio
+
+
+async def test_report_progress_passes_related_request_id() -> None:
+      """report_progress must forward request_id as related_request_id.
+
+          Without related_request_id the streamable-HTTP transport cannot route
+              progress notifications to the correct SSE stream; they are silently
+                  dropped.  Regression test for issue #2001.
+                      """
+      mock_session = AsyncMock()
+      mock_session.send_progress_notification = AsyncMock()
+
+    request_context = ServerRequestContext(
+              request_id="req-2001",
+              session=mock_session,
+              meta={"progress_token": "tok-progress"},
+              lifespan_context=None,
+              experimental=Experimental(),
+    )
+
+    ctx = Context(request_context=request_context, mcp_server=MagicMock())
+
+    await ctx.report_progress(25, 100, message="quarter done")
+    await ctx.report_progress(50, 100)
+    await ctx.report_progress(100, 100, message="complete")
+
+    assert mock_session.send_progress_notification.call_count == 3
+
+    mock_session.send_progress_notification.assert_any_call(
+              progress_token="tok-progress",
+              progress=25.0,
+              total=100.0,
+              message="quarter done",
+              related_request_id="req-2001",
+    )
+    mock_session.send_progress_notification.assert_any_call(
+              progress_token="tok-progress",
+              progress=50.0,
+              total=100.0,
+              message=None,
+              related_request_id="req-2001",
+    )
+    mock_session.send_progress_notification.assert_any_call(
+              progress_token="tok-progress",
+              progress=100.0,
+              total=100.0,
+              message="complete",
+              related_request_id="req-2001",
+    )
+
+
+async def test_report_progress_no_token_skips_notification() -> None:
+      """report_progress is a no-op when no progress_token is present."""
+      mock_session = AsyncMock()
+      mock_session.send_progress_notification = AsyncMock()
+
+    request_context = ServerRequestContext(
+              request_id="req-no-token",
+              session=mock_session,
+              meta={},
+              lifespan_context=None,
+              experimental=Experimental(),
+    )
+
+    ctx = Context(request_context=request_context, mcp_server=MagicMock())
+
+    await ctx.report_progress(50, 100)
+
+    mock_session.send_progress_notification.assert_not_called()
+
+
+async def test_report_progress_integer_token() -> None:
+      """report_progress works when progress_token is an integer (e.g. 0)."""
+      mock_session = AsyncMock()
+      mock_session.send_progress_notification = AsyncMock()
+
+    request_context = ServerRequestContext(
+              request_id="req-int-token",
+              session=mock_session,
+              meta={"progress_token": 0},
+              lifespan_context=None,
+              experimental=Experimental(),
+    )
+
+    ctx = Context(request_context=request_context, mcp_server=MagicMock())
+
+    await ctx.report_progress(1, 10)
+
+    mock_session.send_progress_notification.assert_awaited_once_with(
+              progress_token=0,
+              progress=1.0,
+              total=10.0,
+              message=None,
+              related_request_id="req-int-token",
+    )