refusals-and-fallback changedbuild-with-claude/refusals-and-fallback
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627627A few rules apply to the `fallbacks` list:
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629629* Entries are tried in order. Each must be distinct from the other entries and from the requested model.
630* Each entry must be one of the requested model's permitted targets. With the beta header set, that list is published as `allowed_fallback_models` on the model's entry in the [Models API](https://platform.claude.com/docs/en/api/models/list).
630* Each entry must be one of the requested model's permitted targets. With the beta header set, that list is published as `allowed_fallback_models` on the model's entry in the [Models API](https://platform.claude.com/docs/en/api/beta/models/list).
631631* Each entry names a `model` and can override `max_tokens`, `thinking`, `output_config`, and `speed` for that attempt only.
632632* The request must be valid as a direct request to every model named. If a fallback model does not support a feature the request uses, the API rejects the request up front. Under the `2026-07-01` header, `thinking: {"type": "between_tools"}` is the exception: when a Claude Sonnet 5.5 request falls back to Claude Sonnet 5, the fallback attempt runs with `thinking: {"type": "disabled"}` and `display: "omitted"`. Under `2026-06-01`, set `thinking` on the Claude Sonnet 5 entry, or the request is rejected up front.
633633* As with the default mode, only a safety classifier decline triggers the fallback. A rate limit, overload, or server error on the requested model is returned to you as-is.
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