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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

Sonnet 5 added to the prefill-removed pitfall

You'll notice
Useful2 Signal0
Elsewhere

Docs now warn that Sonnet 5 also rejects prefilled assistant turns with a 400.

What

The Common Pitfalls entry on assistant-message prefills now covers Sonnet 5.

Details
  • Sonnet 5 joins Fable 5, Opus 5, and the 4.6/4.7/4.8 family in the list of models that return a 400 for last-assistant-turn prefills.
Evidence

Prefill removed (Fable 5, Opus 5, Sonnet 5, and the 4.6/4.7/4.8 family):

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 →