# AN-2026-0024: Claude Sonnet 5.5: use thinking "between_tools" instead of "disabled"; effort limits apply

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- id: AN-2026-0024 (version 1; cite https://breakwire.org/claim/AN-2026-0024/v/1)
- kind: breaking · status: active · review: editor
- source: <https://platform.claude.com/docs/en/api/errors#:~:text=To%20turn%20thinking%20off%20on%20this%20model%2C%20send>
- more sources: <https://platform.claude.com/docs/en/release-notes/overview#:~:text=Code%20written%20for%20Claude%20Sonnet%205%20can%20break%20on%20Claude%20Sonnet%205.5%20in%20five%20ways>
- event date: 2026-09-28
- services: anthropic-api
- affected: claude-sonnet-5-5; claude-haiku-5-5 (effort limits with thinking disabled)
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 6, fixed 0, not fixed 0

## Statements (checkable)
0. On Claude Sonnet 5.5, thinking: {"type": "disabled"} returns 400; the lowest setting is thinking: {"type": "between_tools"}.  [✓0 ✗0]
1. At xhigh or max effort, between_tools returns 400.  [✓0 ✗0]
2. With between_tools, a per-message output_config.effort that differs from the level in effect returns 400.  [✓0 ✗0]
3. Sending between_tools to any model other than Claude Sonnet 5.5 returns 400.  [✓0 ✗0]

## Fix
```text
On Sonnet 5.5 send thinking: {"type": "between_tools"} at effort high or below, keep effort constant across the conversation, or use adaptive thinking (omit thinking) to vary effort per turn. Do not send between_tools to other models.
```

## Error signatures
- substring: `To turn thinking off on this model, send "thinking": {"type": "between_tools"} instead of {"type": "disabled"}`
- substring: `is not supported when thinking is disabled on this model. Use effort`
- substring: `effort cannot change when thinking is disabled on this model`
- substring: `"thinking.type.between_tools" is not supported for this model.`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0024/reports_

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