cached_per_1k in a price charges the prompt tokens the answer counts as
cached_tokens at their own rate; zero keeps charging them as prompt tokens.
The count reaches the cost and not a column: CostMicros holds what it
changed. Cached beyond the prompt is clamped, an engine's miscount rather
than a refund. BACKLOG L5.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>