acp-discount-extension

name	acp-discount-extension
description	Implement the ACP discount extension — discount codes, applied discounts, rejected codes, and line-item allocations. Use when adding coupon/promo code support to checkout flows.
allowed-tools	Read, Write, Edit, Bash, Grep, Glob, WebSearch, WebFetch

ACP Discount Extension

Before writing code

Fetch live docs:

1. Web-search site:github.com agentic-commerce-protocol rfcs discount_extension for the discount extension RFC
2. Web-search site:github.com agentic-commerce-protocol examples discount-extension for discount examples
3. Fetch https://developers.openai.com/commerce/specs/checkout/ for how discounts interact with checkout
4. Web-search site:github.com agentic-commerce-protocol spec json-schema discount for the discount JSON schema

Conceptual Architecture

What the Discount Extension Does

The discount extension adds coupon/promo code support to ACP checkout sessions. It's a built-in extension that follows the extensions framework with independent versioning.

Extension Identity

• Name: discount
• Version: Calendar versioned (e.g., discount@2026-01-27)
• JSONPath target: $.CheckoutSessionCreateRequest.discounts and related locations

Three Components

1. Submitted Codes — Agent sends discounts.codes[] with discount codes the buyer wants to apply
2. Applied Discounts — Merchant responds with successfully applied discounts, including coupon details and monetary allocations to line items
3. Rejected Codes — Merchant responds with codes that couldn't be applied, including error reasons

Discount Error Codes

When a code is rejected, the merchant returns a reason:

• discount_code_expired — Code has expired
• discount_code_invalid — Code doesn't exist
• discount_code_combination_disallowed — Can't combine with other active discounts
• discount_code_minimum_not_met — Cart total below minimum threshold
• discount_code_usage_limit_reached — Code has been used too many times
• discount_code_already_applied — Code is already active on this session
• discount_code_user_not_logged_in — Discount requires an authenticated user
• discount_code_user_ineligible — User does not qualify for this discount

Discount Allocations

When a discount is applied, the merchant shows how the discount is distributed:

• Allocations map discount amounts to specific line items using a JSONPath path field (e.g., $.line_items[0])
• Each allocation specifies the JSONPath path to the target line item and the discount amount
• This allows the agent to explain exactly how the discount was applied

Interaction with Totals

Applied discounts affect the totals[] array:

• items_discount — Total discount on items
• discount — Order-level discounts
• These reduce the total accordingly

Extension Negotiation

The discount extension follows capability negotiation:

1. Agent includes discount in capabilities.extensions[]
2. Merchant confirms support in the response
3. If not negotiated, discount fields are ignored

Best Practices

• Validate codes against your promotion system before applying
• Return clear error codes with human-readable messages
• Support multiple concurrent discount codes if your business allows
• Include allocations so the agent can explain the discount to the buyer
• Handle edge cases: expired during session, minimum not met after item removal
• Test code combination rules thoroughly

Fetch the discount extension RFC and JSON schema for exact field names, error code enumeration, and allocation structure before implementing.