# Speech-to-text usage tracking

STT usage is duration-billed and enters the normal `TokenUsage` cost spine. The catalog offering
owns `usage_basis`; `duration_to_stt_input_units()` currently supports:

- `audio_second_input`: `floor(billed_seconds × 100)` synthetic input units.
- `audio_hour_input`: `floor(billed_seconds / 3600 × 1,000,000)` synthetic input units.

Unsupported bases raise before persistence. Usage records carry the catalog model name, provider
model id, vendor, offering id, raw duration, billed duration, and operation.
