The part I keep coming back to isn't the payment rail, it's what happens before the payment. At $0.005 a search the unit price looks trivial, but the agents I've run don't search once — they re-search the same question with slightly different phrasing when the first results don't quite fit, and each retry is a fresh paid call. Removing the API key also removes the one place I used to see that happening, the monthly bill.
So the missing piece for me isn't 402 itself, it's a budget envelope around it: a per-task cap, client-side dedup/caching of near-identical queries, and a receipt ledger I can audit after the run. Without that, making payment frictionless mostly makes runaway spend frictionless too. Does Brave's proxy do any dedup or idempotency for repeated identical queries, or is that entirely left to the harness?