Claude Code v2.1.221 ·
Sonnet 5 added to the prefill-removed pitfall
Docs now warn that Sonnet 5 also rejects prefilled assistant turns with a 400.
You'll noticeTier: how much it should matter to you
2Useful: my rating, 1 to 5
0Signal: worth watching, 1 to 5
ElsewhereArea: what it touches
ImprovementsKind: in v2.1.221,
ImprovementsSection of the release
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 →
