diff --git a/src/mcp/server/streamable_http_manager.py b/src/mcp/server/streamable_http_manager.py
index 50bcd5e79..d822f791e 100644
--- a/src/mcp/server/streamable_http_manager.py
+++ b/src/mcp/server/streamable_http_manager.py
@@ -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(f"Rejected request with unknown or expired session ID: {request_mcp_session_id}")
             error_response = JSONRPCError(
                 jsonrpc="2.0",
                 id=None,