diff --git a/src/mcp/server/streamable_http_manager.py b/src/mcp/server/streamable_http_manager.py
index 50bcd5e79..cca58395e 100644
--- a/src/mcp/server/streamable_http_manager.py
+++ b/src/mcp/server/streamable_http_manager.py
@@ -225,7 +225,7 @@ async def run_server(*, task_status: TaskStatus[None] = anyio.TASK_STATUS_IGNORE
                         read_stream, write_stream = streams
                         task_status.started()
                         try:
-                            # Use a cancel scope for idle timeout — when the
+                            # Use a cancel scope for idle timeout \u2014 when the
                             # deadline passes the scope cancels app.run() and
                             # execution continues after the ``with`` block.
                             # Incoming requests push the deadline forward.
@@ -272,6 +272,7 @@ async def run_server(*, task_status: TaskStatus[None] = anyio.TASK_STATUS_IGNORE
             # Unknown or expired session ID - return 404 per MCP spec
             # TODO: Align error code once spec clarifies
             # See: https://github.com/modelcontextprotocol/python-sdk/issues/1821
+            logger.warning("Rejected request with unknown or expired session ID: %s", request_mcp_session_id)
             error_response = JSONRPCError(
                 jsonrpc="2.0",
                 id=None,
diff --git a/tests/server/test_streamable_http_manager.py b/tests/server/test_streamable_http_manager.py
index 54a898cc5..68a845df4 100644
--- a/tests/server/test_streamable_http_manager.py
+++ b/tests/server/test_streamable_http_manager.py
@@ -1,6 +1,7 @@
 """Tests for StreamableHTTPSessionManager."""
 
 import json
+import logging
 from typing import Any
 from unittest.mock import AsyncMock, patch
 
@@ -269,7 +270,7 @@ async def mock_receive():
 
 
 @pytest.mark.anyio
-async def test_unknown_session_id_returns_404():
+async def test_unknown_session_id_returns_404(caplog: pytest.LogCaptureFixture):
     """Test that requests with unknown session IDs return HTTP 404 per MCP spec."""
     app = Server("test-unknown-session")
     manager = StreamableHTTPSessionManager(app=app)
@@ -299,7 +300,8 @@ async def mock_send(message: Message):
         async def mock_receive():
             return {"type": "http.request", "body": b"{}", "more_body": False}  # pragma: no cover
 
-        await manager.handle_request(scope, mock_receive, mock_send)
+        with caplog.at_level(logging.WARNING, logger="mcp.server.streamable_http_manager"):
+            await manager.handle_request(scope, mock_receive, mock_send)
 
         # Find the response start message
         response_start = next(
@@ -316,6 +318,11 @@ async def mock_receive():
         assert error_data["error"]["code"] == INVALID_REQUEST
         assert error_data["error"]["message"] == "Session not found"
 
+        # Verify warning was logged with the session ID
+        assert any("non-existent-session-id" in record.message for record in caplog.records), (
+            "Should log a warning with the unknown session ID"
+        )
+
 
 @pytest.mark.anyio
 async def test_e2e_streamable_http_server_cleanup():