The line in the response object
You call the API. You pass model: “claude-fable-5”. You get back a completion, a token count, and a field that reads “model”: “claude-opus-4-8”.
Nothing errored. Nothing retried. The request was classified before generation began, matched a sensitive category, and was handed to a different set of weights entirely. Anthropic documented this when it brought Fable 5 back on July 1: blocked requests are sent to Opus 4.8 instead, and the user is notified. The switch happens at the API layer, and the response object names the model that actually ran.
That is the well-behaved version. It is also, as far as I can tell, the only version in wide deployment that tells you the truth in-band.
Two weeks later, Cursor shipped Router: a classifier trained on 600,000-plus live requests that reads each query’s context, complexity, and domain and dispatches it to whichever model it judges best with three early-access accounts reporting 30–50% savings against routing everything to Opus 4.8. Cursor published its routing rules but did not name a specific model per task type.
