Every tool-using API request carries a system prompt Anthropic adds for you. It shrank between Opus 4.7 and Opus 4.8. Here is what that is worth.
Overhead per request, before your own prompt is counted.
| Input tokens saved per day | 3,850,000 |
| Saved per day | $19.25 |
| Saved per 30 days | $577.50 |
Where these numbers come from. Anthropic publishes the tool-use system prompt size per model in its pricing documentation: Claude Opus 4.7 uses 675 tokens with a tool choice of auto or none and 804 with any or tool; Claude Opus 4.8 uses 290 and 410. Both models list at $5 per million input tokens. This counts that fixed overhead only — not your own prompt, not tool definitions, not output, and not any effect of prompt caching. It is a floor on the difference, not a total bill.