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DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

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Claude Code v2.1.221

mcp_tool hooks require a live MCP session and refuse cleanly

You'll notice
Useful2 Signal0
MCP

MCP tool hooks now fail fast with a warning instead of hanging when no session exists.

What

The mcp_tool hook path validates its preconditions before calling instead of attempting a call that cannot succeed.

Details
  • Checks both that the named server is connected or cached and that an MCP session is wired.
  • On failure it returns a structured result with a warning log rather than throwing or hanging.
  • Calls go through the session-level caller with a timeout capped at the MCP timeout and a "mcp_tool hook" context label.
Evidence

Hooks: mcp_tool hook skipped

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

See this entry in the whole of v2.1.221 →