What is acp-checkout-gateway?

ACP Checkout Gateway is a reference implementation for merchants who want to accept purchases made by AI agents, meaning software that can shop and buy on behalf of users. The Agentic Commerce Protocol, or ACP, is a standard that defines how agents communicate with merchant checkout systems. This repository gives merchants a ready made HTTP service that speaks the protocol, so they do not have to build the integration from scratch. When an AI agent wants to buy something, it sends requests to the merchant's gateway. This service receives those requests, validates them, interacts with the merchant's own order system, and returns a confirmed cart state on every response. It handles security concerns such as signature verification, idempotency, which means making sure the same order is not processed twice even if the request is sent more than once, authentication, retries, and input validation. It also emits webhooks, which are automated notifications, when order state changes, so the merchant's other systems stay in sync. The server is built with Express.js, a web framework for JavaScript and TypeScript, and is designed to run as a serverless function on AWS Lambda, meaning it scales automatically and only costs money while it is handling real requests. It can also be run locally for development. Deployment is done through Terraform or AWS SAM. Merchant specific logic such as pricing, inventory, tax calculation, and fulfillment options is connected through a defined adapter interface with two required methods: one that fetches pricing and fulfillment data for a checkout session, and another that handles order creation once payment completes. The README includes a full example of creating a checkout session, showing line items, discounts, tax, and totals in the response. It also documents the API endpoints for creating, retrieving, updating, completing, and cancelling checkout sessions.