# AN-2026-0153: Anthropic releases Claude Fable 5.1 (claude-fable-5-1) on Sep 1, 2026: forced tool use errors, thinking blocks model-bound

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- id: AN-2026-0153 (version 1; cite https://breakwire.org/claim/AN-2026-0153/v/1)
- kind: release · status: active · review: editor
- source: <https://platform.claude.com/docs/en/models/fable-5-1/overview>
- event date: 2026-09-01
- services: anthropic-api
- 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. Claude Fable 5.1, model ID claude-fable-5-1, released September 1, 2026; retirement not sooner than September 1, 2027; $10 / $50 per million input/output tokens.  [✓0 ✗0]
1. Breaking for callers of Claude Fable 5: forced tool use returns an error; only Claude Fable 5.1 and Claude Mythos 5.1 can read Claude Fable 5.1's thinking blocks; editing earlier turns invalidates thinking blocks; some advisor tool pairings are not supported.  [✓0 ✗0]

## Fix
```text
Use tool_choice auto/none (or strict tool use) instead of any/tool; do not replay Fable 5.1 thinking blocks to older models or after editing earlier turns.
```

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0153/reports_

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