You'll noticeTier: how much it should matter to you
3Useful: my rating, 1 to 5
2Signal: worth watching, 1 to 5
SessionsArea: what it touches
ImprovementsKind: in v2.1.232,
ImprovementsSection of the release
What
The session id given by --resume or by the launch config is validated before use. A valid id resumes as before; a non-empty id that does not parse now takes the fork and replay path rather than silently resuming. v2.1.231 resumed on any non-empty id.
Details
Accepted forms are a plain UUID, or a prefixed identifier matching ^[A-Za-z0-9][A-Za-z0-9_]*$ whose trailing 01-prefixed base58 segment decodes to a 32-hex-digit UUID.
Evidence
/^[A-Za-z0-9][A-Za-z0-9_]*$/.test(e)
Strings lifted out of the shipped bundle, so the claim above can be checked against them.
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Confirmed sinceAnthropic's documentation has since written up --resume, on Claude Code changelog.* Improved `--continue` / `--resume`: the conversation appears immediately instead of waiting for SessionStart hooks, and the first message no longer re-reads the whole transcriptchangelogsee the edit
How sure we are
One source agreesOne thing we can check says the same as this entry.
Anthropic's documentation agreesAnthropic's documentation has since written up --resume, on Claude Code changelog.
