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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 v2.1.224

SendMessage records why a message was not delivered

Under the hood
Useful2 Signal3
Agents

Undelivered agent messages now record why, such as an unreachable recipient or a failed mailbox write.

What

The tool that sends messages between agents now reports a delivery outcome to telemetry instead of failing silently, using not_reachable when the recipient is not in the roster or cannot be resolved and mailbox_write_failed when the write itself fails.

Evidence

mailbox_write_failed

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

Related

Other releases about the same thing. Found by shared names or similar wording; neither means one caused the other.

See this entry in the whole of v2.1.224 →