Stop maintaining your own refusal-fallback model list: Opus 5 ships a `"default"` fallbacks mode

Claude Platform Docs·medium signal

The `fallbacks` parameter now supports a `"default"` mode that applies Anthropic's recommended fallback models by refusal category, instead of a hand-maintained model list — use the `server-side-fallback-2026-07-01` beta header, which accepts both the new default mode and explicit lists (the older `server-side-fallback-2026-06-01` header accepts only explicit lists). Requests flagged by safety classifiers on Opus 5 route automatically to another model rather than failing. For production agents this removes a class of brittle glue code where a refusal on one request kills a long-running loop.