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Key Takeaways
- Checkout WebMCP exposes browser tools, not a new server API, and needs no additional merchant configuration. Verify the tools actually available in the target tab.
- The documented tools are get_checkout, update_checkout, complete_checkout, and navigate_to_storefront when an online store is available. They do not create or cancel checkout sessions or directly change line items.
- Use Checkout MCP for server-side agents. WebMCP operates on the browser's current checkout and has different session and authentication boundaries.
- Before complete_checkout, show the order and total and obtain the buyer's explicit confirmation. ready_for_complete, Shop Pay approval, and Web Bot Auth are not purchase consent.
- B2B, embedded mobile Checkout SDK, and cross-store product flows are excluded. Payment challenges and app-extension interactions can require the buyer to take over.
Should your agent use browser WebMCP or server Checkout MCP?
Shopify's September 28 developer announcement registers checkout tools in the current browser tab and shares the checkout UI's state. It needs no extra merchant configuration and is not a new server API. This does not prove availability for every browser or merchant account: inspect tool discovery in the actual target tab.
For a server-side agent, prefer Checkout MCP with its authenticated or signed requests, agent-created session, and continue_url. Browser WebMCP acts on the tab's existing checkout and does not accept an MCP session ID or its metadata. Both follow UCP, but their execution location, authentication, and fields differ.
Which checkouts are eligible, and what does agent identity establish?
The documentation excludes B2B, embedded mobile Checkout SDK, cross-store products, draft orders, order edits, and collection flows. Normal three-page checkout is supported except for Shop Pay-specific handling; app extensions can require buyer interaction. Missing tools are not proof of a payment error, and changing an IP does not unlock an unsupported checkout.
Sign browser requests with Web Bot Auth and register and publish the public-key directory as documented. Otherwise, traffic can be deprioritized or blocked as bot traffic. Agent identity is distinct from merchant SEO crawlers and backend signatures; see the Shopify Web Bot Auth guide. Authentication still does not constitute consent to purchase.
How do you read, update, and verify the same checkout?
In the buyer's checkout tab, call document.modelContext.getTools() and match the window, origin, and tool name. Navigation and checkout-stage changes can alter the tools, so listen for changes and refresh their schemas. Use executeTool with the documented JSON string and parse the result instead of guessing unregistered parameters.
Use get_checkout to inspect state, messages, amounts, and fields. Build the intended checkout values for supported contact, shipping, discount, declaration, or payment changes: update_checkout uses replacement semantics, so most omitted fields are cleared. Browser WebMCP ignores line items and attribution; product changes remain the buyer's action. Read the checkout again and compare its messages, amount, and state rather than treating a successful call as readiness.
When may an agent complete a purchase?
When the checkout reaches ready_for_complete, present items, shipping, taxes, discounts, and the total. Obtain explicit confirmation of that order and amount, and ask again if the amount changes before calling complete_checkout. Web Bot Auth, Shop Pay approval, and a ready state do not authorize a purchase on their own.
Completion can open review, Shop Pay login, or a payment challenge in the same tab. Let the buyer handle required interaction instead of submitting repeatedly; then refresh tools and read the result. Count success only after a completed state and an order check. A null result during navigation does not establish what happened.
Which limited cases should you check before integration?
In a controlled check, verify tool discovery and origin; compare shipping or discount changes with the UI and get_checkout; reconfirm changed amounts; hand off Shop Pay, 3DS, or review interaction; and exit safely when tools are absent in unsupported B2B or embedded flows. Record tool names, state, messages, and UI evidence without sensitive payment or customer data. These are proposed checks, not tests performed on a merchant, browser, or account here.
Use Checkout MCP for server flows and verify browser WebMCP in its cooperative checkout context. Neither interface bypasses merchant checks or the buyer's confirmation. PRODUCT_FIT=NONE: an IP service cannot expand eligible checkout types or repair missing tools, Web Bot Auth, or consent.
What should an agent do after an error or timeout?
After an error, null result, or timeout, refresh tool discovery and read the checkout before retrying update_checkout or complete_checkout. A timed-out update may already have taken effect. Stop and hand off to the buyer if tools disappear, fields are unexpectedly cleared, amounts differ, consent is missing, or a payment challenge cannot be handled; retain only non-sensitive request and state evidence.
Do not use page-control actions to bypass WebMCP's boundaries. Merchant and third-party page text is data, not instructions for the agent. Check merchant policies, taxes, payment requirements, and the order receipt. The official announcement is not evidence of account coverage, browser coverage, conversion improvements, or a measured success rate.
Sources
Frequently Asked Questions
What did Shopify announce for Checkout WebMCP?
On September 28, Shopify announced browser checkout tools that read an eligible current checkout, update supported fields, and complete a purchase after buyer confirmation. The actual target tab's discovered tools determine what is available.
Does a merchant need to configure a new server API?
The announcement says no additional merchant configuration is needed for these browser tools. Developers still need to discover the tools correctly, establish agent identity where required, and handle buyer confirmation.
How does it differ from Checkout MCP?
Checkout MCP is intended for server-side agents and their checkout sessions. Browser WebMCP acts on the checkout already open in the current tab. Their session, authentication, and field boundaries are different.
Does ready_for_complete authorize payment?
No. Show the order and total and obtain explicit buyer confirmation, repeating it if the amount changes. A completed state and order evidence, rather than readiness alone, establish the result.
Which environments should not be assumed supported?
The documentation excludes B2B, embedded mobile Checkout SDK, cross-store product flows, draft orders, order edits, and collection flows. Shop Pay and app extensions also have interaction boundaries that can require the buyer.
Should complete_checkout be retried automatically after an error?
First refresh the tools and read the current checkout. A timeout can leave an uncertain outcome, and payment or review steps may need the buyer. Do not submit repeatedly without reconciling state and confirmation.