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FastMCP 4.0.1 makes ClientGroup reentrant, and LangChain shipped a private workaround for it the same day
FastMCP v4.0.1 (2026-09-02, PR #4975) makes `ClientGroup` reference-count its context the way `Client` already does, so entering a connected group from a nested block or concurrent task reuses existing connections instead of raising. LangChain's 1.4.0a4, cut hours later on 2026-09-02, contains commits `fix(sdk): add _ReentrantClientGroup` followed by `fix(sdk): use latest fastmcp and rm reentrant impl` and `fix(langchain): drive MCP elicitation via member session for fastmcp 4.0.1`. That is a downstream framework carrying a private shim for two days and deleting it the moment upstream landed the real fix.
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