Clarification on well-known endpoint and possible combinatorial explosion of versions

[matthutchinson]

As we work to implement UCP versioning on the well-known endpoint, should the merchant always list ALL supported versions of the UCP version's available capabilities and extensions in their business profile.

As an example, would the well-known endpoint for UCP draft version include, both draft and 2026-01-23 (and any/all other prior) versions of the checkout capability (and same for capability extensions)?

	"ucp": {
		"version": "draft",
		"supported_versions": {
			"draft": "http://retro-rewind-shop.myshopify.com/.well-known/ucp/draft",
			"2026-01-23": "http://retro-rewind-shop.myshopify.com/.well-known/ucp/2026-01-23"
		},
		"services": { ... },
		"capabilities": {
			"dev.ucp.shopping.checkout": [
				{
					"version": "draft",
					"spec": "https://ucp.dev/latest/specification/checkout",
					"schema": "https://ucp.dev/latest/schemas/shopping/checkout.json"
				},
                                 {
					"version": "2026-01-23",
					"spec": "https://ucp.dev/2026-01-23/specification/checkout",
					"schema": "https://ucp.dev/2026-01-23/schemas/shopping/checkout.json"
				}
			],
                         "dev.ucp.shopping.fulfillment": [
                                  {
					"version": "draft",
					"spec": "https://ucp.dev/latest/specification/fulfillment",
					"schema": "https://ucp.dev/latest/schemas/shopping/fulfillment.json",
					"extends": "dev.ucp.shopping.checkout",
					"config": { ... }
				},
				{
					"version": "2026-01-23",
					"spec": "https://ucp.dev/2026-01-23/specification/fulfillment",
					"schema": "https://ucp.dev/2026-01-23/schemas/shopping/fulfillment.json",
					"extends": "dev.ucp.shopping.checkout",
					"config": { ... }
				}
			],

Since the well-known endpoint does not accept an agent profile (HTTP GET), I assume this is the case. But as versions grow, this list could become quite verbose, and also leads to a combinatorial explosion of versions to reason about, (at 3 levels);

• UCP version
• Capability version
• Capability extension version

(but I assume this is intentional, and part of the spec's flexibility)

-- cc @richmolj / @igrigorik