# AN-2026-0027: Replayed thinking blocks bound to a changed conversation return 400 (accounts created on/after Aug 31, 2026)

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- id: AN-2026-0027 (version 1; cite https://breakwire.org/claim/AN-2026-0027/v/1)
- kind: breaking · status: active · review: editor
- source: <https://platform.claude.com/docs/en/api/errors#:~:text=Thinking%20block%20no%20longer%20matches%20the%20conversation>
- event date: 2026-09-01
- services: anthropic-api
- affected: claude-fable-5-1; claude-opus-5-5; claude-sonnet-5-5; claude-haiku-5-5
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 5, fixed 0, not fixed 0

## Statements (checkable)
0. A replayed thinking block whose preceding system prompt, tools or messages changed is rejected with 400 for new accounts created on or after August 31, 2026, or when prefix_mismatch_behavior is "error".  [✓0 ✗0]
1. Sending thinking.block_binding without the thinking-binding-controls-2026-08-01 beta header returns 400 ending in "block_binding: Extra inputs are not permitted".  [✓0 ✗0]

## Fix
```text
Keep conversation history append-only, or send the thinking-binding-controls-2026-08-01 beta header with thinking.block_binding.prefix_mismatch_behavior: "drop_block".
```

## Error signatures
- substring: `The block is bound to a different conversation`
- substring: `block_binding: Extra inputs are not permitted`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0027/reports_

## History
- v1 2026-10-28 by wire-editor: filed from primary source opened 2026-10-28