## Summary
The run page could show an AI generation cost well above what the
provider actually charged, most visibly for OpenRouter and Vercel AI
Gateway requests where a heavily cache-read prompt was priced at the
full input rate. When the provider reports an exact per-request cost, we
now use that instead of catalog pricing.
## Fix
Gateway and OpenRouter include the exact per-request cost in
`ai.response.providerMetadata` (`openrouter.usage.cost` /
`gateway.cost`). That figure already reflects the cache-read discount
and the real per-provider rate, which the catalog cannot reconstruct:
cache-read counts do not arrive in `gen_ai.usage.*`, and per-model
catalog prices drift from what the provider billed, in either direction.
So provider-reported cost is now preferred, and the catalog is used only
when no provider cost is present.
Fallback routing is covered by the same change: when OpenRouter routes
to a different model, `gen_ai.response.model` already carries the served
model, so the cost follows the served model and the provider's own
figure makes it exact.
`extractProviderCost` now runs on every AI span, so it gets a cheap
`"cost"` substring guard to skip the JSON parse on reasoning-model spans
whose provider metadata carries large reasoning text and no cost field.
Regression tests cover the cache-discount overcharge, fallback
served-model pricing, gateway cost, and the catalog fallback path.
Eric Allam
fe07de4a2c