Claude Fable 5.1 changelog

Claude Code makes Fable 5.1 the default Fable model

Claude Code 2.1.257 added Claude Fable 5.1 as the default Fable model with a 1M context window at $10 / $50 per million tokens.

Who is affected:

Claude Code users on the fable alias.

What to do:

Update Claude Code. Fable 5.1 needs version 2.1.255 or later.

Fable 5.1 drops forced tool use and binds thinking blocks to history

On Fable 5.1 and Mythos 5.1, tool_choice any and tool return a 400 error. For accounts created on or after August 31, 2026, replaying a thinking block after the system prompt, tools or earlier messages changed also returns a 400 error.

Who is affected:

Code moving from Fable 5 to Fable 5.1, especially agents that rewrite history or force tool calls.

Action needed:

Use tool_choice auto with strict tool use, and stop editing context before thinking blocks.

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.