Claude Haiku 5.5 changelog

Claude Code makes Haiku 5.5 the default Haiku model

Claude Code 2.1.293 added Claude Haiku 5.5 as the default Haiku model on the Anthropic API.

Claude Haiku 5.5 rejects budget thinking, sampling parameters and prefill

Manual extended thinking, temperature, top_p, top_k and assistant prefill can each return a 400 error on Haiku 5.5, and adaptive thinking is on by default.

Who is affected:

Apps calling claude-haiku-4-5 that plan to switch to Haiku 5.5.

Action needed:

Follow the Haiku 5.5 migration guide before switching model IDs.

New API accounts can no longer edit context around thinking blocks

To counter distillation, the API now checks that a thinking block is sent back with the same system prompt, tools and messages. On Fable 5.1, Opus 5.5, Sonnet 5.5 and Haiku 5.5, accounts created on or after August 31, 2026 get an error unless they opt into dropping the block.

Who is affected:

Agents that compact or rewrite earlier turns, especially on new API accounts.

Action needed:

Use server-side compaction or mid-conversation tool changes, or opt into non-strict mode.