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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 3 Aug 2026

Subagent frontmatter MCP servers connect through the shared bounded dial

You'll notice

Useful3 Signal0

MCP

A hung MCP server listed in an agent's frontmatter can no longer stall subagent startup.

What

MCP servers declared in an agent's frontmatter are connected through the shared dial helper with a timeout and abort signal, so a hung server can no longer stall subagent startup indefinitely.

Details

previously the code awaited connectToServer directly with no bound
on timeout it logs a warning and returns no client
if the server entry was newly created, the connection is still cleaned up should it land after the timeout

Evidence

subagent MCP server connect

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 →