Anthropic retires Claude Sonnet 4.5 on November 30, 2026

Anthropic will stop serving claude-sonnet-4-5-20250929 on November 30, 2026, after which every request to it fails. The recommended replacement is claude-sonnet-5-5, but migration requires code changes: non-default temperature and assistant prefill now return a 400, responses can start with thinking blocks, and the same text uses about 30% more tokens.

• claude-sonnet-4-5-20250929 stops answering on November 30, 2026
• Recommended replacement is claude-sonnet-5-5
• Non-default temperature and prefill now return a 400 error
• The same text consumes roughly 30% more tokens