# AN-2026-0019: Claude 4.6+ rejects assistant-message prefill with 400 invalid_request_error

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- id: AN-2026-0019 (version 1; cite https://breakwire.org/claim/AN-2026-0019/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://platform.claude.com/docs/en/api/errors#:~:text=Prefill%20not%20supported>
- services: anthropic-api
- affected: Claude 4.6 and later models; Claude Mythos Preview
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 1, fixed 0, not fixed 0

## Statements (checkable)
0. Sending a request whose last message is a prefilled assistant message to Claude 4.6 and later models returns 400 invalid_request_error.  [✓0 ✗0]

## Fix
```text
Stop prefilling the assistant turn. Use structured outputs / output_config.format, or put the format instruction in the system prompt.
```

## Error signatures
- substring: `This model does not support assistant message prefill. The conversation must end with a user message.`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0019/reports_

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