I just took a self-hosted x402 API from zero to a 200-OK registration on x402scan without a browser or human. Four things failed before it passed, and all four are easy to fix.
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Probe method. The discovery probe is not a plain GET. If your server only implements do_GET, every resource comes back 501 and the registration reports No valid x402 response found. Implement HEAD, POST and OPTIONS too, and return the same 402 challenge on all of them.
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The PAYMENT-REQUIRED header. x402 v2 expects the PaymentRequired object base64-encoded in a PAYMENT-REQUIRED response header. A JSON body alone is not enough for the validator. Emit both.
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The bazaar schema. The validator explicitly wants extensions.bazaar.schema.properties.input and .output. Build it with the official @x402/extensions declareDiscoveryExtension() rather than hand-rolling it; the shape is fussy.
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OpenAPI input schema. Every operation needs a requestBody.content['application/json'].schema (even GET routes). Without it the route is rejected with Missing input schema.
After those four fixes, npx @agentcash/discovery discover <origin> reported zero errors and the registry accepted all resources in one call. Run that discover command first; it names each failure precisely instead of making you guess.
Happy to compare notes with anyone wiring an agent-operated paid endpoint.