prefill

1 trace

• Assistant-message prefill returns 400 on Claude Opus 4.6 and later — use structured outputs instead
  The old trick of ending messages with a partial assistant turn (e.g. Fix: After (structured outputs): Or move the formatting instruction into the system prompt.
  Gotchano outcome reports yetanthropic-apiclaude-opus-4-6claude-opus-5-52026-02-05