# AN-2026-0884: Claude Sonnet 5.5 / Haiku 5.5 thinking blocks are bound to the account that produced them: another account's blocks are silently dropped (reason organization_binding_mismatch)

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- id: AN-2026-0884 (version 1; cite https://breakwire.org/claim/AN-2026-0884/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://platform.claude.com/docs/en/build-with-claude/preserved-thinking#account-bound-thinking>
- more sources: <https://platform.claude.com/docs/en/release-notes/overview#:~:text=Thinking%20blocks%20that%20Claude%20Sonnet%205.5%20produces>
- event date: 2026-09-28
- services: anthropic-api, anthropic-google-cloud
- affected: claude-sonnet-5-5; claude-haiku-5-5
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. Anthropic's preserved-thinking docs (opened 2026-11-05) say thinking blocks that Claude Sonnet 5.5 or Claude Haiku 5.5 produces work only in the account that produced them or in an account linked to it; when another account sends one of these blocks, the API drops the block before the model sees it, the request succeeds, and the model answers without that reasoning. Blocks from earlier models are not affected.  [✓0 ✗0]
1. The Claude Platform release notes add the same rule for Claude Sonnet 5.5 under September 28, 2026 (the Sonnet 5.5 launch entry; the line was detected by Breakwire's monitor on 2026-11-05).  [✓0 ✗0]
2. On the Claude API and Claude Models on Google Cloud (Google managed), with the thinking-binding-controls-2026-08-01 beta header, each dropped block is listed in the response's input_transformations as a thinking_dropped entry with reason "organization_binding_mismatch"; without the header the drop is silent.  [✓0 ✗0]
3. The same page says the API can also leave out a thinking block produced by another account that is no longer in good standing, for Claude Fable and Mythos models, Opus 5, Opus 5.5, Sonnet 5.5 and Haiku 5.5; the request still succeeds.  [✓0 ✗0]

## Fix
```text
Do not hand a Sonnet 5.5 / Haiku 5.5 conversation (with its thinking blocks) to a different, unlinked Anthropic account or API-key org (e.g. key pools or gateways spanning orgs, or a user's own key replaying your history). Keep a conversation on one account, or start the other account from a summary of the task state. Send the thinking-binding-controls-2026-08-01 beta header and alert on input_transformations entries with reason organization_binding_mismatch to see the drops.
```

## Error signatures
- code: `organization_binding_mismatch` (input_transformations reason documented on platform.claude.com preserved-thinking (account-bound-thinking section))

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0884/reports_

## History
- v1 2026-11-05 by wire-editor: filed from primary source opened 2026-11-05 (hourly run 01:00 EST; review candidates 274/278)