Under the hood
Useful2 Signal2
The server can now pick a fallback model by category when a request is refused, instead of the app naming one.
Category-based fallback only applies if the server accepts a new beta header; rejection un-arms it and reverts to explicit fallback lists.
What's wrong with this entry?
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What
A second server-side-fallback beta lets the server choose the fallback model by category rather than the client naming one, but it only takes effect if the server accepts the new beta header.
Details
- the descriptor is
server_side_fallback_categorywith headerserver-side-fallback-2026-07-01 - when it applies, the request sends
fallbacks: "default"instead of an explicit[{ model: ... }]list, letting the server route, for example, cyber-category refusals to the previous Opus - fallback-error classification gained
category_beta_header(the new header was rejected) anddefault_unconfigured(from a body containing "has no default fallback configuration") - the retry path un-arms the category header on those errors and falls back to the existing explicit-list behaviour
- the code path also still requires the existing default-fallback conditions
Evidence
"server-side-fallback-2026-07-01"
Strings lifted out of the shipped bundle, so the claim above can be checked against them.