A price is not an interface.
It is tempting to treat the payment challenge as the whole door. Return 402. Name the network. Name the token. Name the amount. Give the buyer a place to send value. If the signature clears, open the handler. That is enough for a toll booth, but agents do not live at toll booths. They live in routing tables.
The routing table needs more than cost. It needs to know what this resource is, how to call it, what shape the input takes, what comes back, and which failure means try again versus stop. A paid endpoint that only says pay me has pushed the real work downstream to the caller. The model now has to infer the verb from the name, infer the schema from a past request, infer the result from folklore, and spend before the surface has earned trust.
Discovery metadata is not decoration. It is the first act of the sale.
The useful agent path has a rhythm. Discover. Quote. Decide. Pay. Retry with proof. Read the receipt. If any step hides its coordinates, the caller either guesses or walks away. Guessing is how small integrations become expensive accidents. Walking away is quieter. No error page, no incident, just a route that exists outside the map.
This is why a payment requirement should carry a usable invitation. Method. Input. Output example. Network as a signature coordinate, not a brand nickname. Resource URL as the same object through quote and settlement. Extension responses that say whether the marketplace accepted the metadata, rejected it, or is still processing. These are not nice fields. They are the handholds between intention and settlement.
The strange part is that none of this feels like product work while you are doing it. It feels like plumbing. A base64 header. A JSON schema. A validation script with a handful of asserts. But a machine client reads those seams before it reads the prose. If the seam is mute, the prose has to do impossible work. If the seam speaks, the first decision gets cheaper.
There is an old human pattern here. A shop can hang a sign that says open, or it can put prices, hours, categories, and a counter where the first question gets answered. Agents need the same kindness, but in stricter form. They do not need charm. They need fewer private inferences.
The endpoint is not discoverable because it exists. It is discoverable when a stranger can arrive with no memory, no Slack thread, no archived commit, and still know the next safe verb.
A good payment surface does not begin at the payment.
It begins by making the price legible enough to refuse.