acp-extensions-authoring

name	acp-extensions-authoring
description	Author custom ACP extensions — composable protocol add-ons with JSONPath targeting, schema composition, and independent versioning. Use when building proprietary or domain-specific extensions beyond the built-in ones.
allowed-tools	Read, Write, Edit, Bash, Grep, Glob, WebSearch, WebFetch

ACP Custom Extension Authoring

Before writing code

Fetch live docs:

1. Web-search site:github.com agentic-commerce-protocol rfcs extensions for the extensions framework RFC
2. Web-search site:github.com agentic-commerce-protocol examples for extension examples
3. Fetch https://developers.openai.com/commerce/specs/checkout/ for how extensions integrate with checkout
4. Study the built-in discount extension as a reference: web-search site:github.com agentic-commerce-protocol rfcs discount_extension

Conceptual Architecture

What Extensions Are

Extensions are composable, optional add-ons that augment ACP's core capabilities. They allow the protocol to grow without bloating the core spec — new features are added as extensions with independent lifecycles.

Extension Properties

Every extension defines:

• Name — Core extensions use simple names (discount); third-party use reverse-domain (com.example.loyalty)
• Version — Calendar versioned (YYYY-MM-DD)
• Parent capability — extends field linking to the parent (e.g., extends checkout)
• JSONPath targets — Where in the core schema this extension adds fields
• Schema — JSON Schema for the extension's data

JSONPath Targeting

Extensions use JSONPath (RFC 9535) to inject fields into core schema locations:

• $.CheckoutSessionCreateRequest.discounts — Add fields to the create request
• $.CheckoutSession.discounts — Add fields to the response
• Multiple targets per extension are allowed

Extension Lifecycle

draft → experimental → stable → deprecated → retired

• draft — In development, not for production
• experimental — Available for testing, may change
• stable — Production-ready, backwards-compatible changes only
• deprecated — Still functional, but replacement available
• retired — No longer supported

Schema Composition Rules

• Extension schemas are composed alongside (not inside) core schemas
• Extensions must not conflict with core fields
• Extensions must not introduce authentication bypasses
• Extension data follows the same PCI/PII rules as core fields
• Extensions can reference other extensions (dependency chain)

Naming Conventions

• Core extensions: Simple lowercase names — discount, intent_traces, affiliate_attribution
• Third-party: Reverse-domain notation — com.mycompany.loyalty, io.myplatform.subscription

Built-in Extensions as Reference

Study these before authoring custom extensions:

1. Discount — Discount codes, applied/rejected discounts, allocations
2. Intent Traces — Cart abandonment signals with reason codes
3. Affiliate Attribution — Privacy-preserving attribution tracking

Custom Extension Ideas

• Loyalty program integration
• Subscription management
• Gift wrapping options
• Delivery instructions
• Tax exemption certificates
• Custom buyer preferences

Best Practices

• Start in draft lifecycle, graduate through experimental to stable
• Use reverse-domain naming for all non-core extensions
• Define clear JSON Schemas with validation rules
• Document the extension's purpose, fields, and interaction with core checkout
• Test that your extension survives capability negotiation pruning
• Version independently from the core protocol

Fetch the extensions framework RFC for the exact extension manifest structure, JSONPath targeting syntax, and schema composition rules before implementing.