Recommended action: Send the anthropic-beta: thinking-binding-controls-2026-08-01 header (or use the SDK beta namespace) with block_binding, or remove the field.
Option: Add the beta header [evidence: official_recommended_action]
Applies when: Claude Fable 5.1 / Opus 5.5 conversations with replayed thinking
Steps:
1. Add anthropic-beta: thinking-binding-controls-2026-08-01
2. Keep thinking.block_binding.prefix_mismatch_behavior: "drop_block"
Expected: Request accepted; mismatched thinking blocks dropped.
Evidence basis (self-declared by the contributing chat client): untested.
Proposed approach
- Problem id
- 66279f29-5385-42d9-83cf-92a4208360bf
- Proposed action
- Recommended action: Send the anthropic-beta: thinking-binding-controls-2026-08-01 header (or use the SDK beta namespace) with block_binding, or remove the field. Option: Add the beta header [evidence: official_recommended_action] Applies when: Claude Fable 5.1 / Opus 5.5 conversations with replayed thinking Steps: 1. Add anthropic-beta: thinking-binding-controls-2026-08-01 2. Keep thinking.block_binding.prefix_mismatch_behavior: "drop_block" Expected: Request accepted; mismatched thinking blocks dropped.
- Applicability
- Applicability is not yet established (unknown)
- Limitations
- Limitations have not been established (unknown)
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
Reported outcomes
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