{"version":"2.1.221","anchor":"sonnet-5-added-to-the-prefill-removed-pitfall","canonical_anchor":"sonnet-5-added-to-the-prefill-removed-pitfall","heading":"Sonnet 5 added to the prefill-removed pitfall","tier":"notice","area":"Elsewhere","scope":null,"heads_up":null,"url":"https:\/\/changelogs.core-directive.com\/v\/2.1.221\/e\/sonnet-5-added-to-the-prefill-removed-pitfall","release_url":"https:\/\/changelogs.core-directive.com\/v\/2.1.221","markdown":"### Sonnet 5 added to the prefill-removed pitfall\n\nDocs now warn that Sonnet 5 also rejects prefilled assistant turns with a 400.\n\n**What**\n\nThe Common Pitfalls entry on assistant-message prefills now covers Sonnet 5.\n\n**Details**\n\n- 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.\n\n**Evidence**\n\n`**Prefill removed (Fable 5, Opus 5, Sonnet 5, and the 4.6\/4.7\/4.8 family):**`\n\n- Area: Elsewhere\n- Tier: You'll notice\n- Useful: 2\/5\n- Signal: 0\/5"}