The MCPClientError now includes the SSE attempt's SDK error as its cause.

The official source reports this update: @langchain/mcp-adapters@2.0.1. Patch Changes #11804 0552450 Thanks @byhow ! - Keep the SDK error as cause after an HTTP→SSE fallback. When a Streamable HTTP connection falls back to SSE and the SSE attempt fails, the thrown MCPClientError now has the SSE attempt's SDK error (an UnauthorizedError , or the SSE 401) as its cause , one level down as for a direct HTTP or SSE connection, instead of a second MCPClientError . Code that checks UnauthorizedError.isInstance(error.cause) , such as an OAuth login handler, now works for SSE-only servers too. #11845 1b5ac16 Thanks @hntrl !