# AN-2026-0025: Forced tool use (tool_choice any/tool) returns 400 on Opus 5.5, Sonnet 5.5, Fable 5.1 and Mythos 5.1

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- id: AN-2026-0025 (version 1; cite https://breakwire.org/claim/AN-2026-0025/v/1)
- kind: breaking · status: active · review: editor
- source: <https://platform.claude.com/docs/en/api/errors#:~:text=Forced%20tool%20use%20not%20supported>
- more sources: <https://platform.claude.com/docs/en/release-notes/overview#:~:text=On%20Claude%20Fable%205.1%20and%20Claude%20Mythos%205.1%2C%20tool_choice%20types>
- event date: 2026-09-01
- services: anthropic-api
- affected: claude-opus-5-5; claude-sonnet-5-5; claude-fable-5-1; claude-mythos-5-1
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 3, fixed 0, not fixed 0

## Statements (checkable)
0. tool_choice {"type": "any"} or {"type": "tool", "name": ...} returns 400 invalid_request_error on these models, including on the token counting endpoint.  [✓0 ✗0]
1. tool_choice auto (default) and none are accepted.  [✓0 ✗0]

## Fix
```text
Switch to tool_choice auto with strict tool use to keep inputs schema-valid, or use structured outputs when you need the response itself in a fixed JSON shape. Check framework defaults that force tool_choice (common in "structured output via tool" helpers).
```

## Error signatures
- substring: `tool_choice: type "tool" and "any" are not supported for this model.`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0025/reports_

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