Most people assume an AI agent buying something on your site pays the way you do. It can't.
A typical online card fee is about 2.9% plus $0.30. Below roughly $0.31, the fee costs more than the purcha...
The arithmetic is the whole story. Once the fee floor is above the price of the thing being bought, you're not optimizing a payment rail, you need a different one.
What we found running this in practice: 402 as a status code is the easy part — every HTTP stack already speaks it. The hard part is the agent side holding a budget and settling without a human in the loop. Lightning via NWC gets you there because the invoice is per-request and the fee is sub-sat at those amounts, so a $0.002 API call actually clears.
Two things that bit us: quoting has to be cheap (don't make the agent pay to find out the price — return the price in the 402 body), and you need idempotency on the paid retry or you'll double-charge on flaky connections.
What we found running this in practice: 402 as a status code is the easy part — every HTTP stack already speaks it. The hard part is the agent side holding a budget and settling without a human in the loop. Lightning via NWC gets you there because the invoice is per-request and the fee is sub-sat at those amounts, so a $0.002 API call actually clears.
Two things that bit us: quoting has to be cheap (don't make the agent pay to find out the price — return the price in the 402 body), and you need idempotency on the paid retry or you'll double-charge on flaky connections.
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