# AN-2026-0609: Anthropic server-side fallback (fallbacks: "default", beta server-side-fallback-2026-07-01): a refused request can be answered by a different model; check response.model and usage.iterations

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- id: AN-2026-0609 (version 1; cite https://breakwire.org/claim/AN-2026-0609/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback#:~:text=fallback_message>
- more sources: <https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback.md> <https://platform.claude.com/docs/en/build-with-claude/fallback-credit>
- 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 5.1/5, Opus 5.5/5, Sonnet 5.5 and Haiku 5.5 include safety classifiers; a decline is a normal response, not an error, with stop_reason "refusal" and stop_details.category.  [✓0 ✗0]
1. With fallbacks set to "default" and the server-side-fallback-2026-07-01 beta header, the API retries a declined request on the fallback model Anthropic recommends for the refusal category (up to three custom fallback models can be named); categories without a recommendation keep the refusal. Haiku 5.5 has no server-side fallback.  [✓0 ✗0]
2. The docs detect a fallback by a fallback_message entry in usage.iterations combined with stop_reason not being "refusal", and read response.model.  [✓0 ✗0]
3. When fallback is used, the triggering refusal is billed too if it arrived mid-stream or is in a billed category (bio, frontier_llm, reasoning_extraction).  [✓0 ✗0]

## Fix
```text
Log response.model and usage.iterations; treat stop_reason "refusal" as a non-error outcome to handle explicitly; only enable fallbacks where answering from another model is acceptable.
```

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0609/reports_

## History
- v1 2026-10-29 by wire-editor: filed from primary source opened 2026-10-28