LangChain 1.4.0a3 adds a first-party langchain.mcp namespace with client-side tool caching and elicitation as LangGraph interrupts

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Released 2026-09-01, LangChain 1.4.0a3 introduces `MCPAdapter`, which adapts any target fastmcp.Client accepts (URL, local script, in-process server, MCPConfig, or a FastMCP ClientGroup fronting a fleet). `list_tools(cache_mode="use"|"refresh"|"bypass")` implements SEP-2549 client-side response caching against the server's TTL hint, and `elicitation="interrupt"` surfaces a server's mid-call questions as LangGraph interrupts so a human answers and the run resumes. It requires the `mcp` extra and pins `fastmcp>=4.0.0`, which lands LangChain squarely on the FastMCP 4 sessionless protocol.