# AN-2026-0023: Thinking cannot be disabled on Claude Fable 5.1, Mythos 5.1, Fable 5, Mythos 5 and Opus 5.5

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- id: AN-2026-0023 (version 1; cite https://breakwire.org/claim/AN-2026-0023/v/1)
- kind: breaking · status: active · review: editor
- source: <https://platform.claude.com/docs/en/api/errors#:~:text=Thinking%20cannot%20be%20disabled>
- more sources: <https://platform.claude.com/docs/en/release-notes/overview#:~:text=On%20Claude%20Opus%205.5%2C%20thinking%20can>
- event date: 2026-09-22
- services: anthropic-api
- affected: claude-fable-5-1; claude-mythos-5-1; claude-fable-5; claude-mythos-5; claude-opus-5-5; Claude Mythos Preview
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 5, fixed 0, not fixed 0

## Statements (checkable)
0. On these models thinking is always on; thinking: {"type": "disabled"} returns 400 invalid_request_error.  [✓0 ✗0]
1. On Claude Opus 5.5, thinking: {"type": "enabled", ...} also returns 400 (release notes, Sep 22, 2026).  [✓0 ✗0]

## Fix
```text
Omit the thinking field (adaptive thinking runs) and control depth with output_config.effort. To keep thinking text out of responses, set display: "omitted" instead of disabling thinking.
```

## Error signatures
- substring: `"thinking.type.disabled" is not supported for this model. Use "thinking.type.adaptive"`
- substring: `"thinking.type.disabled" is not supported for this model. Thinking defaults to adaptive mode` (Claude Mythos Preview variant)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0023/reports_

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