Checkout Capability - MCP Binding¶

This document specifies the Model Context Protocol (MCP) binding for the food Checkout Capability.

Protocol Fundamentals¶

Discovery¶

Businesses advertise MCP transport availability through their UCP profile at /.well-known/ucp.

{
  "ucp": {
    "version": "draft",
    "services": {
      "dev.ucp.food": [
        {
          "version": "draft",
          "spec": "https://ucp.dev/draft/specification/overview",
          "transport": "mcp",
          "schema": "https://ucp.dev/draft/services/food/mcp.openrpc.json",
          "endpoint": "https://food.example.com/ucp/mcp"
        }
      ]
    },
    "capabilities": {
      "dev.ucp.food.checkout": [
        {
          "version": "draft",
          "spec": "https://ucp.dev/draft/specification/food/checkout",
          "schema": "https://ucp.dev/draft/schemas/food/checkout.json"
        }
      ],
      "dev.ucp.food.fulfillment": [
        {
          "version": "draft",
          "spec": "https://ucp.dev/draft/specification/food/extensions/fulfillment",
          "schema": "https://ucp.dev/draft/schemas/food/fulfillment.json",
          "extends": "dev.ucp.food.checkout"
        }
      ],
      "dev.ucp.food.tip": [
        {
          "version": "draft",
          "spec": "https://ucp.dev/draft/specification/food/extensions/tip",
          "schema": "https://ucp.dev/draft/schemas/food/tip.json",
          "extends": "dev.ucp.food.checkout"
        }
      ]
    },
    "payment_handlers": {
      "com.example.processor_tokenizer": [
        {
          "id": "processor_tokenizer_1",
          "version": "draft",
          "spec": "https://example.com/specs/processor-tokenizer",
          "schema": "https://example.com/schemas/processor-tokenizer-config.json",
          "available_instruments": [
            {"type": "card", "constraints": {"properties": {"brand": {"enum": ["visa", "mastercard"]}}}}
          ],
          "config": {...}
        }
      ]
    }
  }
}

Request Metadata¶

MCP clients MUST include a meta object in every request containing protocol metadata:

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/call",
  "params": {
    "name": "create_checkout",
    "arguments": {
      "meta": {
        "ucp-agent": {
          "profile": "https://platform.example/profiles/food-agent.json"
        },
        "idempotency-key": "550e8400-e29b-41d4-a716-446655440000"
      },
      "checkout": {
        "food_establishment": {...},
        "line_items": [...]
      }
    }
  }
}

The meta["ucp-agent"] field is required on all requests to enable capability negotiation. The complete_checkout and cancel_checkout operations also require meta["idempotency-key"] for retry safety — retrying complete_checkout without the same key risks sending the same order to the kitchen twice. Platforms MAY include additional metadata fields.

Tools¶

UCP Capabilities map 1:1 to MCP Tools.

Identifier Pattern¶

MCP tools separate resource identification from payload data:

Requests: For operations on existing checkouts (get, update, complete, cancel), a top-level id parameter identifies the target resource. The checkout object in the request payload MUST NOT contain an id field.
Responses: All responses include checkout.id as part of the full resource state.
Create: The create_checkout operation does not require an id in the request, and the response includes the newly assigned checkout.id.

Tool	Operation	Description
`create_checkout`	Create Checkout	Create a checkout session.
`get_checkout`	Get Checkout	Get a checkout session.
`update_checkout`	Update Checkout	Update a checkout session.
`complete_checkout`	Complete Checkout	Place the order.
`cancel_checkout`	Cancel Checkout	Cancel a checkout session.

The examples below follow the same order used in the REST Binding: two customized pizzas and a salad from Luigi's, delivered ASAP with an 18% tip.

`create_checkout`¶

Maps to the Create Checkout operation. The Platform sends the establishment and the configured dishes; the Business prices them and asks for a fulfillment method.

Input Schema¶

checkout (Checkout): Required. Contains the establishment, dishes, and optional buyer and extension data.
- Extensions (Optional):
  - dev.ucp.food.fulfillment: Fulfillment
  - dev.ucp.food.tip: Tip
  - dev.ucp.common.payment.ap2_mandate: AP2 Mandates

Output Schema¶

Checkout object.

Example¶

RequestResponseError Response

href="#__codelineno-2-1">{ "jsonrpc": "2.0", "id": 1, "method": "tools/call", "params": { "name": "create_checkout", "arguments": { "meta": { "ucp-agent": { "profile": "https://platform.example/profiles/food-agent.json" } }, "checkout": { "food_establishment": { "id": "est_luigis_01" }, "line_items": [ { "quantity": 2, "dish": { "id": "dish_margherita_12" }, "modifications": [ {"id": "mod_extra_mozzarella"}, {"id": "mod_fresh_basil"} ], "note": "Well done crust, please" }, { "quantity": 1, "dish": { "id": "dish_caesar" }, "note": "Dressing on the side" } ], "buyer": { "first_name": "Jane", "last_name": "Doe", "email": "jane.doe@example.com", "phone_number": "+14155550199" } } } } }

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "structuredContent": {
      "ucp": {
        "version": "draft",
        "capabilities": {
          "dev.ucp.food.checkout": [
            {"version": "draft"}
          ],
          "dev.ucp.food.fulfillment": [
            {"version": "draft", "extends": "dev.ucp.food.checkout"}
          ],
          "dev.ucp.food.tip": [
            {"version": "draft", "extends": "dev.ucp.food.checkout"}
          ]
        },
        "payment_handlers": {
          "com.example.processor_tokenizer": [
            {"id": "processor_tokenizer_1", "version": "draft", "available_instruments": [{"type": "card"}], "config": {}}
          ]
        }
      },
      "id": "chk_food_7187",
      "status": "incomplete",
      "currency": "USD",
      "food_establishment": {
        "id": "est_luigis_01",
        "name": "Luigi's Artisan Pizzeria",
        "address": {
          "street_address": "210 University Ave",
          "address_locality": "Palo Alto",
          "address_region": "CA",
          "postal_code": "94301",
          "address_country": "US"
        },
        "media": [
          {"type": "image", "url": "https://food.example.com/img/luigis.jpg", "alt_text": "Luigi's storefront"}
        ]
      },
      "buyer": {
        "first_name": "Jane",
        "last_name": "Doe",
        "email": "jane.doe@example.com",
        "phone_number": "+14155550199"
      },
      "line_items": [
        {
          "id": "li_1",
          "quantity": 2,
          "dish": {
            "id": "dish_margherita_12",
            "title": "Margherita Pizza (12\")",
            "price": 1600,
            "media": [
              {"type": "image", "url": "https://food.example.com/img/margherita.jpg"}
            ]
          },
          "modifications": [
            {"id": "mod_extra_mozzarella", "title": "Extra Mozzarella", "price": 200},
            {"id": "mod_fresh_basil", "title": "Fresh Basil", "price": 150}
          ],
          "note": "Well done crust, please",
          "totals": [
            {"type": "subtotal", "amount": 3900},
            {"type": "total", "amount": 3900}
          ]
        },
        {
          "id": "li_2",
          "quantity": 1,
          "dish": {
            "id": "dish_caesar",
            "title": "Caesar Salad",
            "price": 1100
          },
          "note": "Dressing on the side",
          "totals": [
            {"type": "subtotal", "amount": 1100},
            {"type": "total", "amount": 1100}
          ]
        }
      ],
      "fulfillment": {
        "available_methods": [
          {"type": "delivery", "fulfillable_on": "2026-04-10T18:25:00-07:00", "scheduling": "asap_and_scheduled"},
          {"type": "pickup", "fulfillable_on": "2026-04-10T18:15:00-07:00", "scheduling": "asap_and_scheduled"}
        ]
      },
      "tip": {
        "options": [
          {"id": "tip_15", "unit": "percentage", "amount": 15},
          {"id": "tip_18", "unit": "percentage", "amount": 18},
          {"id": "tip_20", "unit": "percentage", "amount": 20}
        ]
      },
      "totals": [
        {"type": "subtotal", "display_text": "Subtotal", "amount": 5000},
        {"type": "tax", "display_text": "Estimated Tax", "amount": 438},
        {"type": "total", "display_text": "Estimated Total", "amount": 5438}
      ],
      "messages": [
        {
          "type": "error",
          "code": "missing",
          "path": "$.fulfillment.methods",
          "content": "Choose delivery or pickup.",
          "severity": "recoverable"
        }
      ],
      "links": [
        {"type": "privacy_policy", "url": "https://food.example.com/privacy"},
        {"type": "terms_of_service", "url": "https://food.example.com/terms"}
      ],
      "continue_url": "https://food.example.com/checkout/chk_food_7187",
      "expires_at": "2026-04-11T00:00:00Z"
    },
    "content": [
      {
        "type": "text",
        "text": "{\"ucp\":{…},…}"
      }
    ]
  }
}

The kitchen has stopped taking orders — no checkout resource is created:

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "structuredContent": {
      "ucp": {
        "version": "draft",
        "status": "error"
      },
      "messages": [
        {
          "type": "error",
          "code": "location_closed",
          "content": "Luigi's is closed for the night. Ordering reopens at 11:00 AM.",
          "severity": "unrecoverable"
        }
      ],
      "continue_url": "https://food.example.com/"
    },
    "content": [
      {"type": "text", "text": "{\"ucp\":{…},…}"}
    ]
  }
}

`get_checkout`¶

Maps to the Get Checkout operation. Use it to re-read Business-authored state (prices, delivery windows, tip resolution) or to poll while the checkout is complete_in_progress.

Input Schema¶

id (String): Required. The ID of the checkout session.

Output Schema¶

Checkout object.

`update_checkout`¶

Maps to the Update Checkout operation. Update is a full replacement of the writable state: the Platform resends every line item (with its id), the buyer, the fulfillment selection, and the tip — anything omitted is removed.

The Platform MUST NOT start a new update_checkout operation while the Checkout is complete_in_progress. Duplicate requests remain subject to Replay Protection. If the Business receives a new update_checkout request in that state, it MUST leave the Checkout unchanged and return the current Checkout with a recoverable error Message.

Input Schema¶

id (String): Required. The ID of the checkout session to update.
checkout (Checkout): Required. Contains the full writable checkout state.
- Extensions (Optional):
  - dev.ucp.food.fulfillment: Fulfillment
  - dev.ucp.food.tip: Tip
  - dev.ucp.common.payment.ap2_mandate: AP2 Mandates

Output Schema¶

Checkout object.

Example¶

The Platform adds a delivery address and selects the 18% tip preset. The Business returns delivery options, drop-off preferences, fees, and the resolved tip line.

RequestResponse

{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "tools/call",
  "params": {
    "name": "update_checkout",
    "arguments": {
      "meta": {
        "ucp-agent": {
          "profile": "https://platform.example/profiles/food-agent.json"
        }
      },
      "id": "chk_food_7187",
      "checkout": {
        "food_establishment": {
          "id": "est_luigis_01"
        },
        "line_items": [
          {
            "id": "li_1",
            "quantity": 2,
            "dish": {
              "id": "dish_margherita_12"
            },
            "modifications": [
              {"id": "mod_extra_mozzarella"},
              {"id": "mod_fresh_basil"}
            ],
            "note": "Well done crust, please"
          },
          {
            "id": "li_2",
            "quantity": 1,
            "dish": {
              "id": "dish_caesar"
            },
            "note": "Dressing on the side"
          }
        ],
        "buyer": {
          "first_name": "Jane",
          "last_name": "Doe",
          "email": "jane.doe@example.com",
          "phone_number": "+14155550199"
        },
        "fulfillment": {
          "methods": [
            {
              "type": "delivery",
              "destination": {
                "type": "delivery_address",
                "address": {
                  "street_address": "450 Serra Mall",
                  "extended_address": "Apt 3B",
                  "address_locality": "Stanford",
                  "address_region": "CA",
                  "postal_code": "94305",
                  "address_country": "US"
                }
              },
              "notes": "Gate code #4210, second floor"
            }
          ]
        },
        "tip": {
          "selected_option_id": "tip_18"
        }
      }
    }
  }
}

{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "structuredContent": {
      "ucp": {
        "version": "draft",
        "capabilities": {
          "dev.ucp.food.checkout": [
            {"version": "draft"}
          ],
          "dev.ucp.food.fulfillment": [
            {"version": "draft", "extends": "dev.ucp.food.checkout"}
          ],
          "dev.ucp.food.tip": [
            {"version": "draft", "extends": "dev.ucp.food.checkout"}
          ]
        },
        "payment_handlers": {
          "com.example.processor_tokenizer": [
            {"id": "processor_tokenizer_1", "version": "draft", "available_instruments": [{"type": "card"}], "config": {}}
          ]
        }
      },
      "id": "chk_food_7187",
      "status": "ready_for_complete",
      "currency": "USD",
      "food_establishment": {...},
      "buyer": {...},
      "line_items": [...],
      "fulfillment": {
        "available_methods": [
          {
            "type": "delivery",
            "fulfillable_on": "2026-04-10T18:25:00-07:00",
            "scheduling": "asap_and_scheduled"
          },
          {
            "type": "pickup",
            "fulfillable_on": "2026-04-10T18:15:00-07:00",
            "scheduling": "asap_and_scheduled"
          }
        ],
        "methods": [
          {
            "id": "method_delivery",
            "type": "delivery",
            "destination": {
              "id": "dest_home",
              "type": "delivery_address",
              "address": {
                "street_address": "450 Serra Mall",
                "extended_address": "Apt 3B",
                "address_locality": "Stanford",
                "address_region": "CA",
                "postal_code": "94305",
                "address_country": "US"
              }
            },
            "preference_options": [
              {"id": "pref_leave_at_door", "label": "Leave at my door"},
              {"id": "pref_hand_to_me", "label": "Hand it to me"}
            ],
            "selected_preference_option_ids": ["pref_leave_at_door"],
            "notes": "Gate code #4210, second floor",
            "selected_option_id": "opt_standard",
            "options": [
              {
                "id": "opt_priority",
                "title": "Priority Delivery",
                "description": "Sent directly to you with no other stops",
                "earliest_fulfillment_time": "2026-04-10T18:25:00-07:00",
                "latest_fulfillment_time": "2026-04-10T18:35:00-07:00",
                "totals": [{"type": "total", "amount": 599}]
              },
              {
                "id": "opt_standard",
                "title": "Standard Delivery",
                "earliest_fulfillment_time": "2026-04-10T18:35:00-07:00",
                "latest_fulfillment_time": "2026-04-10T18:50:00-07:00",
                "totals": [{"type": "total", "amount": 299}]
              }
            ]
          }
        ]
      },
      "tip": {
        "selected_option_id": "tip_18",
        "options": [
          {"id": "tip_15", "unit": "percentage", "amount": 15},
          {"id": "tip_18", "unit": "percentage", "amount": 18},
          {"id": "tip_20", "unit": "percentage", "amount": 20}
        ]
      },
      "totals": [
        {"type": "subtotal", "display_text": "Subtotal", "amount": 5000},
        {"type": "fulfillment", "display_text": "Standard Delivery", "amount": 299},
        {
          "type": "fee",
          "display_text": "Service & Regulatory Fees",
          "amount": 550,
          "lines": [
            {"display_text": "Service Fee", "amount": 350},
            {"display_text": "City Courier Regulatory Fee", "amount": 200}
          ]
        },
        {"type": "tax", "display_text": "Tax (8.75%)", "amount": 438},
        {"type": "tip", "display_text": "Courier Tip (18%)", "amount": 900},
        {"type": "total", "display_text": "Total", "amount": 7187}
      ],
      "links": [...],
      "continue_url": "https://food.example.com/checkout/chk_food_7187",
      "expires_at": "2026-04-11T00:00:00Z"
    },
    "content": [
      {
        "type": "text",
        "text": "{\"ucp\":{…},…}"
      }
    ]
  }
}

`complete_checkout`¶

Maps to the Complete Checkout operation. The Platform submits payment; the Business places the order with the kitchen and returns the order confirmation.

Input Schema¶

id (String): Required. The ID of the checkout session.
checkout (Checkout): Required. Contains payment and optional signals.
- Extensions (Optional):
  - dev.ucp.common.payment.ap2_mandate: AP2 Mandates

Output Schema¶

Checkout object.

Note: Response MUST include an order object (id, tracking_url, and optionally a pickup label) if completion succeeds. See Order Confirmation & Tracking.

Example¶

RequestResponse

{
  "jsonrpc": "2.0",
  "id": 3,
  "method": "tools/call",
  "params": {
    "name": "complete_checkout",
    "arguments": {
      "meta": {
        "ucp-agent": {
          "profile": "https://platform.example/profiles/food-agent.json"
        },
        "idempotency-key": "772d0622-941d-4e86-9a38-668877662222"
      },
      "id": "chk_food_7187",
      "checkout": {
        "payment": {
          "instruments": [
            {
              "id": "instr_1",
              "handler_id": "processor_tokenizer_1",
              "type": "card",
              "selected": true,
              "brand": "visa",
              "last_digits": "4242",
              "credential": {
                "type": "token",
                "token": "tok_visa_4242"
              }
            }
          ]
        },
        "signals": {
          "dev.ucp.user_agent": "Mozilla/5.0 ..."
        }
      }
    }
  }
}

{
  "jsonrpc": "2.0",
  "id": 3,
  "result": {
    "structuredContent": {
      "ucp": {
        "version": "draft",
        "capabilities": {
          "dev.ucp.food.checkout": [
            {"version": "draft"}
          ],
          "dev.ucp.food.fulfillment": [
            {"version": "draft", "extends": "dev.ucp.food.checkout"}
          ],
          "dev.ucp.food.tip": [
            {"version": "draft", "extends": "dev.ucp.food.checkout"}
          ]
        },
        "payment_handlers": {
          "com.example.processor_tokenizer": [
            {"id": "processor_tokenizer_1", "version": "draft", "available_instruments": [{"type": "card"}], "config": {}}
          ]
        }
      },
      "id": "chk_food_7187",
      "status": "completed",
      "currency": "USD",
      "food_establishment": {...},
      "buyer": {...},
      "line_items": [...],
      "fulfillment": {...},
      "tip": {...},
      "totals": [...],
      "payment": {
        "instruments": [
          {
            "id": "instr_1",
            "handler_id": "processor_tokenizer_1",
            "type": "card",
            "selected": true,
            "brand": "visa",
            "last_digits": "4242"
          }
        ]
      },
      "order": {
        "id": "ord_food_9876543210",
        "label": "#42",
        "tracking_url": "https://food.example.com/orders/ord_food_9876543210",
        "earliest_fulfillment_time": "2026-04-10T18:35:00-07:00",
        "latest_fulfillment_time": "2026-04-10T18:50:00-07:00"
      },
      "links": [...]
    },
    "content": [
      {
        "type": "text",
        "text": "{\"ucp\":{…},…}"
      }
    ]
  }
}

`cancel_checkout`¶

Maps to the Cancel Checkout operation. Cancels an in-progress checkout before an order is placed; it does not cancel an order the kitchen has already accepted.

Input Schema¶

id (String): Required. The ID of the checkout session.

Output Schema¶

Checkout object.

Note: Response MUST include "status": "canceled" if cancellation succeeds.

Error Handling¶

UCP distinguishes between protocol errors and business outcomes. See the Core Specification for the complete error code registry and Error Handling for the food-specific codes.

Protocol errors: Transport-level failures (authentication, rate limiting, unavailability) that prevent request processing. Returned as JSON-RPC error with code -32000 (or -32001 for discovery errors).
Business outcomes: Application-level results from successful request processing, returned as JSON-RPC result with UCP envelope and messages.

Business Outcomes¶

Business outcomes — a sold-out dish, a closed kitchen, an address outside the delivery zone — are returned as JSON-RPC result with structuredContent containing the UCP envelope and messages:

{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "structuredContent": {
      "ucp": {
        "version": "draft",
        "capabilities": {
          "dev.ucp.food.checkout": [
            {"version": "draft"}
          ]
        },
        "payment_handlers": {
          "com.example.processor_tokenizer": [
            {"id": "processor_tokenizer_1", "version": "draft", "available_instruments": [{"type": "card"}], "config": {}}
          ]
        }
      },
      "id": "chk_food_7187",
      "status": "incomplete",
      "currency": "USD",
      "food_establishment": {
        "id": "est_luigis_01",
        "name": "Luigi's Artisan Pizzeria"
      },
      "line_items": [
        {
          "id": "li_1",
          "quantity": 1,
          "dish": {
            "id": "dish_truffle_arancini",
            "title": "Truffle Arancini",
            "price": 1200
          },
          "totals": [
            {"type": "subtotal", "amount": 1200},
            {"type": "total", "amount": 1200}
          ]
        }
      ],
      "totals": [
        {"type": "subtotal", "amount": 1200},
        {"type": "total", "amount": 1200}
      ],
      "links": [],
      "messages": [
        {
          "type": "error",
          "code": "out_of_stock",
          "path": "$.line_items[0].dish",
          "content": "Truffle Arancini is sold out for tonight's dinner service.",
          "severity": "recoverable"
        }
      ]
    },
    "content": [
      {"type": "text", "text": "{\"ucp\":{…},…}"}
    ]
  }
}

For create_checkout, when no checkout session can be created, the Business returns JSON-RPC result with structuredContent containing the UCP envelope and messages (see the create_checkout Error Response example):

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "structuredContent": {
      "ucp": { "version": "draft", "status": "error" },
      "messages": [
        {
          "type": "error",
          "code": "location_closed",
          "content": "This location is no longer taking orders.",
          "severity": "unrecoverable"
        }
      ],
      "continue_url": "https://food.example.com/"
    },
    "content": [
      {"type": "text", "text": "{\"ucp\":{…},…}"}
    ]
  }
}

Message Signing¶

Platforms SHOULD authenticate agents when using MCP transport. When using HTTP Message Signatures, all checkout operations follow the Message Signatures specification.

Request Signing¶

UCP's MCP transport uses streamable HTTP, allowing the same RFC 9421 signature mechanism as REST. The signature is applied at the HTTP layer:

Header	Required	Description
`Signature-Input`	Yes	Describes signed components
`Signature`	Yes	Contains the signature value
`Content-Digest`	Yes	SHA-256 hash of request body
`UCP-Agent`	Yes	Signer identity (profile URL)
`Idempotency-Key`	Cond.*	Unique key for replay protection

* Required for complete_checkout and cancel_checkout

Example Signed Request:

POST /mcp HTTP/1.1
Host: food.example.com
Content-Type: application/json
UCP-Agent: profile="https://platform.example/.well-known/ucp"
Idempotency-Key: 772d0622-941d-4e86-9a38-668877662222
Content-Digest: sha-256=:RK/0qy18MlBSVnWgjwz6lZEWjP/lF5HF9bvEF8FabDg=:
Signature-Input: sig1=("@method" "@authority" "@path" "content-digest" "content-type" "ucp-agent" "idempotency-key");keyid="platform-2026"
Signature: sig1=:MEUCIQDXyK9N3p5Rt...:

{"jsonrpc":"2.0","id":3,"method":"tools/call","params":{"name":"complete_checkout","arguments":{"id":"chk_food_7187","checkout":{"payment":{...}}}}}

The Content-Digest binds the JSON-RPC body to the signature. No JSON canonicalization is required.

See Message Signatures - MCP Transport for details.

Response Signing¶

Response signatures are RECOMMENDED for:

complete_checkout responses (order confirmation)

Response signatures are OPTIONAL for:

create_checkout, get_checkout, update_checkout, cancel_checkout

Example Signed Response:

HTTP/1.1 200 OK
Content-Type: application/json
Content-Digest: sha-256=:Y5fK8nLmPqRsT3vWxYzAbCdEfGhIjKlMnO...:
Signature-Input: sig1=("@status" "content-digest" "content-type");keyid="business-2026"
Signature: sig1=:MFQCIH7kL9nM2oP5qR8sT1uV4wX6yZaB3cD...:

{"jsonrpc":"2.0","id":3,"result":{"content":[{"type":"text","text":"..."}],"structuredContent":{"id":"chk_food_7187","status":"completed","order":{"id":"ord_food_9876543210","label":"#42",...}}}}

See Message Signatures - REST Response Signing for the signing algorithm (identical for MCP over HTTP).

Conformance¶

A conforming MCP transport implementation MUST:

Implement JSON-RPC 2.0 protocol correctly.
Provide all core checkout tools defined in this specification.
Return errors per the Core Specification.
Return business outcomes as JSON-RPC result with UCP envelope and messages array.
Validate tool inputs against UCP schemas.
Support HTTP transport with streaming.

A conforming implementation SHOULD:

Authenticate agents using one of the supported mechanisms (API keys, OAuth, mTLS, or HTTP Message Signatures per Message Signatures).
Verify authentication on incoming requests before processing.

Implementation¶

UCP operations are defined using OpenRPC (JSON-RPC schema format). The MCP specification requires all tool invocations to use a tools/call method with the operation name and arguments wrapped in params. Implementers MUST apply this transformation:

OpenRPC	MCP
`method`	`params.name`
`params`	`params.arguments`

Param conventions:

meta contains request metadata
id identifies the target resource (path parameter equivalent)
checkout contains the domain payload (body equivalent)

Checkout Capability - MCP Binding¶

Protocol Fundamentals¶

Discovery¶

Request Metadata¶

Tools¶

Identifier Pattern¶

create_checkout¶

Input Schema¶

Output Schema¶

Example¶

get_checkout¶

Input Schema¶

Output Schema¶

update_checkout¶

Input Schema¶

Output Schema¶

Example¶

complete_checkout¶

Input Schema¶

Output Schema¶

Example¶

cancel_checkout¶

Input Schema¶

Output Schema¶

Error Handling¶

Business Outcomes¶

Message Signing¶

Request Signing¶

Response Signing¶

Conformance¶

Implementation¶

`create_checkout`¶

`get_checkout`¶

`update_checkout`¶

`complete_checkout`¶

`cancel_checkout`¶