Most AI APIs still have the same two failure modes:

1. You gate everything behind auth/paywall, so nobody tries it.
2. You give a big free tier, so you get usage but no revenue, and you're afraid to raise prices.

L402 is the first HTTP-native pattern I've used that makes small Lightning payments feel ergonomic. The server just replies with HTTP 402 Payment Required, includes a BOLT11 invoice, and tells the client how to retry.

What the flow looks like (concrete)What the flow looks like (concrete)

• Step 1: allow 1 free call per IP per 24h (so the happy path is dead simple)
• Step 2: after that, return 402 plus a Lightning invoice and a payment_hash (or retry header)
• Step 3: client pays the invoice, then retries the exact same request including the payment_hash

This is normal control flow, like handling 429 with Retry-After.

Example endpoint you can hit right nowExample endpoint you can hit right now

• POST https://maximumsats.com/api/dvm
  • First query free per IP per 24 hours
  • Then 21 sats per call via L402

Curl demoCurl demo

# 1) First call: free (or you'll get a 402 if you've already used it today)
curl -sS https://maximumsats.com/api/dvm \
  -H 'content-type: application/json' \
  -d '{"prompt":"Give me 5 concrete ways to price an LLM feature without subscriptions."}'

# 2) If you got a 402, pay the invoice shown in the JSON.
# Then retry the same request including the payment_hash exactly as instructed by the response.

Why this matters for AI specificallyWhy this matters for AI specifically

• Lots of AI features have tiny marginal costs you can pass through: 5 sats, 21 sats, 100 sats. Subscriptions feel like a mismatch.
• A 1-call free tier is a clean filter for bots and never-pay traffic, without killing try-before-you-buy.
• The retry semantics are easy to automate in clients (no web checkout flows).

What I'd like to see nextWhat I'd like to see next

• SDKs that treat 402 + invoice as a first-class pattern.
• More APIs publishing machine-readable pricing (free tier + costs). For example:
  • https://wot.klabo.world/pricing