You Didn’t Get the AI Model You Paid For

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.

And underneath both, at the aggregation layer, OpenRouter warns that some providers serve quantized weights at lower prices, that output can differ from what full-precision weights would have produced, and that your logs will not tell you this happened.

Three products. Three different answers to the question what is a model . Zero cases telling us which one the law will accept.

Model identity is fracturing along three independent axes, and they are not usually distinguished:

The engineering community treats all three as reliability problems. They are also identity problems, and identity is what contracts, warranties, disclosures, and evidence rules are built on.

Start with the oldest question in commercial law: was the description a term of the deal?

If an API call were a sale of goods, this would be near-trivial. UCC §2-313(1)(b) makes any description of the goods that forms part of the basis of the bargain into an express warranty that the goods will conform to it. India’s Sale of Goods Act, 1930, §15 does the same work through the doctrine of sale by description.

But an inference API almost certainly isn’t goods. Courts have generally treated hosted software as a service, which pushes you out of warranty statutes and into common-law contract — where the answer depends entirely on what the documentation said and how specifically the buyer bargained. That is precisely where the ambiguity lives. Enterprise agreements price per-model. Model cards are model-specific. Compliance artifacts name model versions. And then the routing layer treats the name as a hint .