Claude Sonnet 5.5 changelog

Claude Code makes Sonnet 5.5 the default Sonnet model

Claude Code 2.1.284 added Claude Sonnet 5.5 as the default Sonnet model on the Anthropic API.

Claude Sonnet 5.5 changes thinking and tool use in ways that break Sonnet 5 code

To turn off up-front thinking on Sonnet 5.5 you send between_tools instead of disabled. Forced tool use returns a 400 error. Thinking blocks only work in the account that produced them.

Who is affected:

Code moving from Claude Sonnet 5 to Sonnet 5.5.

Action needed:

Follow the Sonnet 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.