speed: "fast" is not supported on Opus 4.7

What changed under your feet

speed: "fast" worked on the Opus generation before 4.7, works again on the generations after it, and errors specifically on Opus 4.7 itself. If you're testing model versions in sequence during an upgrade, this is the kind of gap that's easy to misdiagnose as a general deprecation rather than what it actually is — a single-generation removal that got reversed on the very next release.

Why 4.7 is the odd one out

Fast mode was pulled for the 4.7 release and reintroduced afterward, which makes it one of the few capabilities in the current lineup that isn't a straight line from older to newer — it's present, then absent, then present again. Treating capability as monotonically increasing across model generations is a reasonable default assumption almost everywhere else in this lineup; this is the specific exception to that pattern, alongside the cache-minimum table, which also doesn't move in one direction as models get newer.

The practical fix

If your target is Opus 4.7 specifically, drop the speed field rather than setting it to "fast" — the request runs at standard speed instead, with no fast-mode option available on this version at all. If you're not locked to 4.7 for a specific reason, moving to the next generation restores fast mode along with the rest of its capability set, and is very likely the simpler fix if you're mid-migration anyway rather than maintaining 4.7 long-term.

Don't assume fast-mode pricing carries over

Where fast mode is available, its per-token pricing is not the same as standard-speed pricing on the same model, and that premium isn't necessarily consistent from one generation to the next either. Confirm the fast-mode rate for whichever model you land on rather than assuming it matches what you budgeted for a different generation — see that model's own page under model reference for its specific figures.