Impact note · revision 1.0
Edited conversation prefixes can invalidate thinking
Fable 5.1 thinking blocks are bound to the system prompt, tools, and message history that produced them.
Bottom line
Where the prefix check is enforced, changing earlier messages, the system prompt, or the tools array can reject replayed thinking with a 400 or drop it under the configured mismatch behavior.
Operator action
What to do
Keep histories append-only, compare consecutive request prefixes, choose error or drop_block deliberately, and monitor input_transformations during migration testing.
- When
- Before migrating a client-managed multi-turn conversation
- Evidence
- Officially confirmed
- Last verified
Who is affected
- Client-managed Messages API histories
- Custom compaction
- Dynamic system prompts
- Dynamic tool arrays
- claude-fable-5-1
What changed
- New accounts created on or after August 31, 2026 are documented as enforcing prefix binding by default.
- Older accounts can opt into the same check with the thinking-binding-controls beta header.
- A permanent mismatch is not repaired by retrying the unchanged request body.
Migration checks
- Capture two consecutive request bodies from a representative multi-turn session.
- Compare system, tools, and the shared messages prefix byte-for-byte.
- Test with prefix_mismatch_behavior set to error in continuous integration.
- Use server-side compaction or strip stale thinking blocks at a client-side compaction boundary.
What we observed
The bundled Node.js preflight was executed against six frozen synthetic fixtures. It checks only the encoded request-shape and prefix rules. No Anthropic API request was made, and no production behavior was reproduced.
Inspect the local preflight →What remains unknown
- No live Anthropic API request was made for this local pilot.
- Default enforcement depends on account creation date and explicit beta configuration.
Primary evidence
- Migrating to Claude Fable 5.1 and Claude Mythos 5.1Anthropic primary source · checked Sep 11, 2026
- Claude API errorsAnthropic primary source · checked Sep 11, 2026
Revision and corrections
- Initial pilot edition; no material correction recorded.
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