Free · seller checklist

The x402 seller pre-flight checklist

Ten checks before you list — and after every deploy. Built from probing all 1,225 Bazaar sellers and running 17 paid services of our own. One in four sellers fail at least one of these, and almost none of them know.

1. 1

   Your listed URL answers a 402 from outside your network

   Not a 200, not a 404, not a redirect. Test from a machine that isn’t yours — internal requests often bypass the proxy that agents actually hit. In our census, 206 sellers 404 and 40 serve their paid product free without knowing it.

2. 2

   The payment-required header decodes and validates

   Base64-decode it: you need JSON with a numeric x402Version and a non-empty accepts[] array. This header is how an agent constructs the payment — a 402 without it is a paywall that names no price a machine can read.

3. 3

   Every accepts[] entry is complete

   scheme, network (eip155:8453 for Base), asset (token contract), payTo (your wallet — checksum it), amount (atomic units as a string). Strict clients reject the entire offer over one missing field.

4. 4

   GET gets the challenge too, not just POST

   If your routes are POST-only, a bare 405 on GET reads as "not a paid resource" to a discovering agent. Answer the same 402 + envelope on GET, even if only POST does the real work.

5. 5

   Your description is under 500 characters

   The cliff is real and undocumented: one byte over and your listing becomes silently unpurchasable in the Bazaar. We measured it at exactly 500. Count bytes, not vibes.

6. 6

   Your listing is actually in the catalog

   Check your resources appear in the Bazaar after registration and after every manifest change. Catalog purges happen without announcement — 43% of the catalog vanished overnight in July.

7. 7

   The listed URL and your live route agree exactly

   Scheme, host (www vs apex), path, trailing slash, version prefix. Most dead listings aren’t dead services — they’re services that moved while the listing didn’t.

8. 8

   You never charge for your own failures

   Upstream broke? Refund or don’t settle. An agent that pays for an error message doesn’t retry you — and its operator remembers.

9. 9

   Consider MPP dual-stack

   A WWW-Authenticate: Payment challenge alongside your x402 envelope makes the same endpoint payable by MPP-speaking agents — same Base USDC settlement. 15% of sellers already answer both; they sell into two buyer pools with one endpoint.

10. 10

    Re-verify after every deploy

    The most common failure we see isn’t a bad launch — it’s a silent regression: a middleware reorder or env change that never got re-tested from outside. Automate the check or make it a deploy ritual.