Issue 256 - In Case You Missed Editions: WebMCP, AGENTS.md for Themes, and Checkout Kit v4
In case you missed it: the Fall '26 Editions announcements for developers
Shopify's Fall '26 Edition came out at the end of October, and the developer section is a long one. If you were heads-down that week, start here. You've already seen some of it in recent issues, like Next Gen Events, the security check, theme app add-ons and Polaris 2.0, since those shipped in the changelog first. The rest of this issue covers what's actually new.
By Shopify
Add shopping and Shop Pay checkout to any agent with Shop MCP
Shop MCP is one MCP connection that lets an agent or app search Shopify Catalog, build a cart and check out with Shop Pay. It's live at mcp.shop.com, covers more than a billion products, and search doesn't need an API key. If you already use the Catalog API, nothing changes for you.
By Shopify
WebMCP tools are live on every Liquid storefront
Every Liquid storefront now registers WebMCP tools, and so do Hydrogen 2026-10 storefronts that render ShopifyScripts. That means a shopper's in-browser agent can search products, change the cart and move to checkout through real tool calls instead of guessing at buttons. The cart tools go through the same Shopify.actions that apps use, so if your theme opens a cart drawer when something is added, it'll open for agents too. For now it only works with agents in Chromium-based browsers.
By Shopify
Cart and checkout permalinks move to a UCP /buy route
Cart permalinks have a new home at /buy. One link can add items, prefill the buyer's email, set country and language, apply a discount code and send the shopper to checkout, and it works the same way on any store that supports UCP. Your old /cart links keep working, but new features will only land on /buy. If you rely on selling plans, line item properties, cart notes or cart attributes, stay on /cart for now, since /buy doesn't support them yet.
By Shopify
Ask MCP lets agents answer questions about a store's policies
Ask MCP gives agents an ask_business tool for buyer questions about a store, like its return policy or shipping options. Answers come from the store's own policies and knowledge base. It's only in UCP's draft version right now, so your agent profile has to opt into draft to use it. It also handles one question at a time, with no back-and-forth conversation yet.
By Shopify
Catalog API beta can now return products from brands that don't sell on Shopify
The Catalog API beta can now return products from brands that aren't on Shopify. You opt in with a new sources parameter on search_catalog, and if you leave it out, your existing calls behave exactly as before. External products can't use Shopify's checkout, so they come back with a link to the brand's own product page instead, and your app needs to handle that. During the beta they only show up for shoppers in the US.
By Shopify
Promoted Placements beta now pays merchant-funded commission on Shop Campaigns
Developers in the Promoted Placements beta can now earn more per sale. When a search_catalog result comes from a merchant running Shop Campaigns, you get that merchant's commission on top of Shopify's base rate. Since the rate now varies by product, read placement.commission.percentage.value if your app displays it, rather than hardcoding a number. It only applies to search_catalog, and clicks land on shop.com.
By Shopify
Themes can now ship AGENTS.md and DESIGN.md for Sidekick and coding agents
Themes can now include an AGENTS.md file that explains how the theme is built, and a DESIGN.md file that captures its design intent. Sidekick reads both before it edits a theme in Canvas, and your local coding agent can read the same files, so everyone works from the same notes. They sit at the theme root, the names are case-sensitive, and each can be up to 50 KB. Shopify CLI 4.8.4 and up includes them when you pull and push. Note that this is separate from the agents.md.liquid template that shopping agents read on your storefront.
By Shopify
Checkout Kit v4 replaces Checkout Sheet Kit, with breaking changes
If your mobile app uses Checkout Sheet Kit, plan some time for this one. It's now Checkout Kit v4, with iOS, Android and React Native in a single repo, and it adds live updates while the buyer is still in checkout. A few things go away, though. Web pixel callbacks are gone because pixels now fire inside checkout, buyer details are hidden from events unless the merchant grants a new access scope, and React Native now requires the New Architecture.
By Shopify
Shopify Tax managed mode makes some Admin API writes fail validation
Shopify Tax has a new managed mode for US tax, and it takes over some settings that apps used to write directly. If your app configures markets, publishes products, creates custom line items or previews checkout branding, some of those writes can now fail validation when they conflict with managed mode. Requests that leave those settings out keep working, so check that your app only sends the settings it actually needs.
By Shopify
