The Agent Offer Protocol gives AI agents and stores one exchange for settling a price. Today an agent reads a product page, guesses the terms and hopes the price holds at checkout; the store has no way to answer the agent directly. With the protocol, the agent sends an OfferRequest (items, quantities, buyer type, currency, delivery window), the store replies with an Offer (line prices, terms, validity and an offer id), and the agent can send a CounterOffer, Accept or Reject. Messages are JSON over HTTPS, and the same operations are available as MCP tools.
A store that takes part publishes /.well-known/agent-offers.json, linked from its llms.txt, naming its offer endpoint and what identity it requires. Agents sign their requests with Web Bot Auth (HTTP Message Signatures, RFC 9421); an unsigned agent gets list prices only. The store's floor and rules are never revealed: an agent learns only offers and their stated reasons, rounds are limited, and every offer states that its pricing is rule-based and carries its expiry.
Accept hands the agent to checkout where the store already sells: an ACP or UCP session, a platform checkout link such as a Shopify draft order, or a single-use discount code that brings the offer's lines to the offered total. The buyer still confirms the order before the agent places it. The protocol sits on top of ACP and UCP rather than competing with them.
The full specification, its agent terms and its conformance rules are on arbling.com/agent-offer-protocol. It is published as an open draft, version 0.1, under CC0.