Commerce protocols

A compatibility check at checkout

Agents buy through checkout protocols such as the Universal Commerce Protocol (UCP) and the Agentic Commerce Protocol (ACP). Neither says whether the items will work with what the customer already owns. Send us the checkout session right before payment; we return a decision per line item, the action to take, and a signed receipt, in a shape that can travel with the checkout as com.tetheredtrust.shopping.compatibility.

Claim	Status
Checkout check for UCP checkouts, ACP CheckoutSessions and plain line items	Implemented tested against the published shapes (UCP line_items[].item.title; ACP ids plus your feed)
`com.tetheredtrust.shopping.compatibility` extension and JSON Schema	Published named `{reverse-domain}.{service}.{capability}`; the schema is served from tetheredtrust.com, the namespace authority
Adopted by UCP or ACP clients, merchants or platforms	Not yet publishing an extension doesn't make anyone use it
Endorsed by the UCP or ACP maintainers	No a third-party extension by Chimes In LLC, not part of either core specification
Part of ChatGPT Instant Checkout	No reading ACP sessions doesn't make us an approved checkout partner

Version 2026-10-14. Each result carries a cart_fingerprint: if the line items, quantities or the owned list change, check again.

1. Call it before payment

POST https://tetheredtrust.com/v1/checkout/check (MCP tool check_cart_before_checkout). Accepts a UCP checkout, an ACP CheckoutSession, or plain line_items. Free during early access; /v1/paid/checkout charges $0.25 per completed decision and nothing for insufficient_evidence.

{
  "checkout": {
    "ucp": {
      "version": "2026-08-25"
    },
    "id": "chk_123",
    "line_items": [
      {
        "id": "li_1",
        "item": {
          "id": "sku_vlt7",
          "title": "Sanus VLT7 tilting TV mount",
          "price": 12999
        },
        "quantity": 1
      },
      {
        "id": "li_2",
        "item": {
          "id": "sku_hdmi",
          "title": "Ultra High Speed HDMI cable 2 m",
          "price": 1999
        },
        "quantity": 1
      }
    ]
  },
  "owned": [
    "TCL 65QM8K"
  ]
}

ACP line items carry only ids, so add a products map from your feed:

{
  "checkout": {
    "id": "cs_123",
    "line_items": [
      {
        "id": "li_1",
        "item": {
          "id": "sku_vlt7",
          "quantity": 1
        }
      }
    ]
  },
  "products": {
    "sku_vlt7": {
      "title": "Sanus VLT7 tilting TV mount"
    }
  },
  "owned": [
    "TCL 65QM8K"
  ]
}

2. Act on `action`

decision	action	What the agent does
`supported`	`proceed`	Complete the checkout.
`conditional`	`resolve_conditions`	Add the named adapter or hub (`required_accessories`) or confirm the `verify` items with the customer, then check again.
`incompatible`	`do_not_checkout`	Remove or replace the item (see `substitutions`) and check again.
`insufficient_evidence`	`ask_customer`	Ask the returned `questions`. Never complete the checkout on a guess.

The whole cart takes its worst line: one incompatible item makes the checkout do_not_checkout.

3. Attach the result to the checkout

The response's extension object is ready to place at the checkout's top level, so the merchant, a payment handler or a later dispute can see what was checked:

{
  "com.tetheredtrust.shopping.compatibility": {
    "version": "2026-10-14",
    "decision": "supported",
    "safe_to_checkout": true,
    "checked_at": "2026-10-14T15:02:11Z",
    "line_items": [
      {
        "line_item_id": "li_1",
        "decision": "supported"
      },
      {
        "line_item_id": "li_2",
        "decision": "supported"
      }
    ],
    "receipt_id": "rcpt_...",
    "valid_until": "2026-11-13T15:02:11Z",
    "verify_url": "https://tetheredtrust.com/v1/receipts/verify",
    "spec": "https://tetheredtrust.com/ucp"
  }
}

Machine-readable definition: /ucp/com.tetheredtrust.shopping.compatibility.json (JSON Schema 2020-12).

4. For merchants: declare it

A business that wants agents to run the check can list the extension among its UCP capabilities:

{
  "capabilities": {
    "com.tetheredtrust.shopping.compatibility": [
      {
        "version": "2026-10-14",
        "spec": "https://tetheredtrust.com/ucp",
        "schema": "https://tetheredtrust.com/ucp/com.tetheredtrust.shopping.compatibility.json",
        "extends": "dev.ucp.shopping.checkout"
      }
    ]
  }
}

Agents that don't know the extension ignore it, as UCP intends for unknown capabilities.

Rules the decision follows

We never return "works" to help a purchase complete. Unknown or unsupported items return insufficient_evidence.
Products read by AI from a manufacturer's page are unreviewed: a "works" on them comes back conditional with a verify condition.
A receipt proves what we said and when (EdDSA JWS, public keys); it isn't a guarantee. See Trust.

1. Call it before payment

2. Act on action

3. Attach the result to the checkout

4. For merchants: declare it

Rules the decision follows

2. Act on `action`