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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 v2.1.219

Managed-agents docs: mid-session system.message model gate widened, plus two path/method typo fixes

You'll notice
Useful2 Signal0
Elsewhere not in their notes

Mid-session system messages now work with more models, and two doc references were corrected.

sessions.events.list()
What

The documented model gate for mid-session system messages now covers more models, and two incorrect references were corrected.

Details
  • The gate list adds the previous Opus alongside the new one, Sonnet, Fable and Mythos; the rejection error name is unchanged.
  • The polling troubleshooting note now names GET /v1/sessions/{id}/events/stream as the endpoint you may have hit by mistake, where it previously named the paginated endpoint twice.
  • The overview's SDK advice corrects session.events.list() to sessions.events.list().
Evidence

model_does_not_support_mid_conversation_system

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 v2.1.219 →