Anthropic API billing: prepaid credits, invoices and unexpected spend

What's happening

The Anthropic API bills per input and output token. Prepaid credits are consumed as you call the API, and spend rises with prompt length as much as with request volume.

Fix it

1. Open the Console billing page and compare daily spend to spot the day the change began.
2. Break spend down by model — a switch to a larger model multiplies cost per token.
3. Look for long system prompts or full documents resent on every request.
4. Set spend limits and email alerts so a runaway job cannot drain credits overnight.
5. Rotate any API key that may have been committed to a repository.

Still stuck?

Claude Pro on claude.ai is a separate subscription from API credits. Paying for one does not fund the other.