A2A SendMessage, HTTP+JSON binding. Returns a Message, never a Task.

The A2A door onto POST /v1/decision. Admission is identical — free allowance, then x402, and an Enterprise key when one is presented — because it is the same function underneath. There is no A2A variant of the engine.

Send the request as a DataPart. Free text is deliberately not parsed for an address or an amount: those are the two values where a misreading is expensive, and an agent with a payment to make already holds both as data.

The reply carries the decision twice — as a TextPart for clients that read prose, and as a DataPart carrying the exact /v1/decision payload, which is authoritative.

A 402 here is not an A2A error: x402 runs over HTTP, and the handshake is the one documented on /v1/decision.