Checkout Capability - REST Binding

This document specifies the REST binding for the Checkout Capability.

Protocol Fundamentals

Base URL

All UCP REST endpoints are relative to the business's base URL, which is discovered through the UCP profile at /.well-known/ucp. The endpoint for the checkout capability is defined in the rest.endpoint field of the business profile.

Content Types

• Request: application/json
• Response: application/json

All request and response bodies MUST be valid JSON as specified in RFC 8259.

Transport Security

All REST endpoints MUST be served over HTTPS with minimum TLS version 1.3.

Operations

Operation	Method	Endpoint	Description
Create Checkout	POST	/checkout-sessions	Create a checkout session.
Get Checkout	GET	/checkout-sessions/{id}	Get a checkout session.
Update Checkout	PUT	/checkout-sessions/{id}	Update a checkout session.
Complete Checkout	POST	/checkout-sessions/{id}/complete	Place the order.
Cancel Checkout	POST	/checkout-sessions/{id}/cancel	Cancel a checkout session.

Examples

Create Checkout

RequestResponseError Response

POST /checkout-sessions HTTP/1.1
UCP-Agent: profile="https://platform.example/profile"
Content-Type: application/json

{
  "line_items": [
    {
      "item": {
        "id": "item_123",
        "title": "Red T-Shirt",
        "price": 2500
      },
      "id": "li_1",
      "quantity": 2
    }
  ]
}

HTTP/1.1 201 Created
Content-Type: application/json

{
  "ucp": {
    "version": "draft",
    "capabilities": {
      "dev.ucp.shopping.checkout": [
        {"version": "draft"}
      ]
    },
    "payment_handlers": {
      "com.shopify.shop_pay": [
        {
          "id": "shop_pay_1234",
          "version": "draft",
          "available_instruments": [
            {"type": "shop_pay"}
          ],
          "config": {
            "merchant_id": "shop_merchant_123"
          }
        }
      ]
    }
  },
  "id": "chk_1234567890",
  "status": "incomplete",
  "messages": [
    {
      "type": "error",
      "code": "missing",
      "path": "$.buyer.email",
      "content": "Buyer email is required",
      "severity": "recoverable"
    }
  ],
  "currency": "USD",
  "line_items": [
    {
      "id": "li_1",
      "item": {
        "id": "item_123",
        "title": "Red T-Shirt",
        "price": 2500
      },
      "quantity": 2,
      "totals": [
        {"type": "subtotal", "amount": 5000},
        {"type": "total", "amount": 5000}
      ]
    }
  ],
  "totals": [
    {
      "type": "subtotal",
      "amount": 5000
    },
    {
      "type": "tax",
      "amount": 400
    },
    {
      "type": "total",
      "amount": 5400
    }
  ],
  "links": [
    {
      "type": "terms_of_service",
      "url": "https://business.example.com/terms"
    }
  ],
  "payment": {
    "instruments": [
      {
        "id": "instr_shop_pay_1",
        "handler_id": "shop_pay_1234",
        "type": "shop_pay",
        "selected": true,
        "display": {
          "email": "buyer@example.com"
        }
      }
    ]
  }
}

All items out of stock — no checkout resource is created:

HTTP/1.1 200 OK
Content-Type: application/json

{
  "ucp": { "version": "2026-01-11", "status": "error" },
  "messages": [
    {
      "type": "error",
      "code": "out_of_stock",
      "content": "All requested items are currently out of stock",
      "severity": "unrecoverable"
    }
  ],
  "continue_url": "https://merchant.com/"
}

Update Checkout

Update Buyer Info

All fields in buyer are optional, allowing clients to progressively build the checkout state across multiple calls. Each PUT replaces the entire session, so clients must include all previously set fields they wish to retain.

RequestResponse

PUT /checkout-sessions/{id} HTTP/1.1
UCP-Agent: profile="https://platform.example/profile"
Content-Type: application/json

{
  "id": "chk_123456789", // deprecated: id is provided in URL path
  "buyer": {
    "email": "jane@example.com",
    "first_name": "Jane",
    "last_name": "Doe"
  },
  "line_items": [
    {
      "item": {
        "id": "item_123",
        "title": "Red T-Shirt",
        "price": 2500
      },
      "id": "li_1",
      "quantity": 2
    }
  ]
}

HTTP/1.1 200 OK
Content-Type: application/json

{
  "ucp": {
    "version": "draft",
    "capabilities": {
      "dev.ucp.shopping.checkout": [
        {"version": "draft"}
      ]
    },
    "payment_handlers": {
      "com.shopify.shop_pay": [
        {
          "id": "shop_pay_1234",
          "version": "draft",
          "available_instruments": [
            {"type": "shop_pay"}
          ],
          "config": {
            "merchant_id": "shop_merchant_123"
          }
        }
      ]
    }
  },
  "id": "chk_1234567890",
  "status": "incomplete",
  "messages": [
    {
      "type": "error",
      "code": "missing",
      "path": "$.fulfillment.method[0].selected_destination_id",
      "content": "Fulfillment address is required",
      "severity": "recoverable"
    }
  ],
  "currency": "USD",
  "line_items": [
    {
      "id": "li_1",
      "item": {
        "id": "item_123",
        "title": "Red T-Shirt",
        "price": 2500
      },
      "quantity": 2,
      "totals": [
        {"type": "subtotal", "amount": 5000},
        {"type": "total", "amount": 5000}
      ]
    }
  ],
  "buyer": {
    "email": "jane@example.com",
    "first_name": "Jane",
    "last_name": "Doe"
  },
  "totals": [
    {
      "type": "subtotal",
      "amount": 5000
    },
    {
      "type": "tax",
      "amount": 400
    },
    {
      "type": "total",
      "amount": 5400
    }
  ],
  "links": [
    {
      "type": "terms_of_service",
      "url": "https://business.example.com/terms"
    }
  ],
  "payment": {
    "instruments": [
      {
        "id": "instr_shop_pay_1",
        "handler_id": "shop_pay_1234",
        "type": "shop_pay",
        "selected": true,
        "display": {
          "email": "buyer@example.com"
        }
      }
    ]
  }
}

Update Fulfillment

Fulfillment is an extension to the checkout capability. Most fields are provided by the business based on buyer inputs, which includes desired fulfillment type & addresses.

RequestResponse

PUT /checkout-sessions/{id} HTTP/1.1
UCP-Agent: profile="https://platform.example/profile"
Content-Type: application/json

{
  "id": "chk_123456789", // deprecated: id is provided in URL path
  "buyer": {
    "email": "jane@example.com",
    "first_name": "Jane",
    "last_name": "Doe"
  },
  "line_items": [
    {
      "item": {
        "id": "item_123",
        "title": "Red T-Shirt",
        "price": 2500
      },
      "id": "li_1",
      "quantity": 2
    }
  ],
  "fulfillment": {
    "methods": [
      {
        "type": "shipping",
        "destinations": [
          {
            "street_address": "123 Main St",
            "address_locality": "Springfield",
            "address_region": "IL",
            "postal_code": "62701",
            "address_country": "US"
          }
        ]
      }
    ]
  }
}

HTTP/1.1 200 OK
Content-Type: application/json

{
  "ucp": {
    "version": "draft",
    "capabilities": {
      "dev.ucp.shopping.checkout": [
        {"version": "draft"}
      ]
    },
    "payment_handlers": {
      "com.google.pay": [
        {
          "id": "gpay_1234",
          "version": "draft",
          "config": {
            "allowed_payment_methods": [
              {
                "type": "CARD",
                "parameters": {
                  "allowed_card_networks": ["VISA", "MASTERCARD", "AMEX"]
                }
              }
            ]
          }
        }
      ]
    }
  },
  "id": "chk_1234567890",
  "status": "incomplete",
  "messages": [
    {
      "type": "error",
      "code": "missing",
      "path": "$.selected_fulfillment_option",
      "content": "Please select a fulfillment option",
      "severity": "recoverable"
    }
  ],
  "currency": "USD",
  "line_items": [
    {
      "id": "li_1",
      "item": {
        "id": "item_123",
        "title": "Red T-Shirt",
        "price": 2500
      },
      "quantity": 2,
      "totals": [
        {"type": "subtotal", "amount": 5000},
        {"type": "total", "amount": 5000}
      ]
    }
  ],
  "buyer": {
    "email": "jane@example.com",
    "first_name": "Jane",
    "last_name": "Doe"
  },
  "totals": [
    {
      "type": "subtotal",
      "amount": 5000
    },
    {
      "type": "tax",
      "amount": 400
    },
    {
      "type": "total",
      "amount": 5400
    }
  ],
  "links": [
    {
      "type": "terms_of_service",
      "url": "https://merchant.com/terms"
    }
  ],
  "fulfillment": {
    "methods": [
      {
        "id": "shipping_1",
        "type": "shipping",
        "line_item_ids": ["item_123"],
        "selected_destination_id": "dest_home",
        "destinations": [
          {
            "id": "dest_home",
            "street_address": "123 Main St",
            "address_locality": "Springfield",
            "address_region": "IL",
            "postal_code": "62701",
            "address_country": "US"
          }
        ],
        "groups": [
          {
            "id": "package_1",
            "line_item_ids": ["item_123"],
            "selected_option_id": "standard",
            "options": [
              {
                "id": "standard",
                "title": "Standard Shipping",
                "description": "Arrives in 5-7 business days",
                "totals": [
                  {
                    "type": "total",
                    "amount": 500
                  }
                ]
              },
              {
                "id": "express",
                "title": "Express Shipping",
                "description": "Arrives in 2-3 business days",
                "totals": [
                  {
                    "type": "total",
                    "amount": 1000
                  }
                ]
              }
            ]
          }
        ]
      }
    ]
  },
  "payment": {
    "instruments": [
      {
        "id": "pi_gpay_5678",
        "handler_id": "gpay_1234",
        "type": "card",
        "selected": true,
        "display": {
          "brand": "mastercard",
          "last_digits": "5678",
          "rich_text_description": "Google Pay •••• 5678"
        }
      }
    ]
  }
}

Update Fulfillment Selection

Follow-up calls after initial fulfillment data to update selection.

RequestResponse

PUT /checkout-sessions/{id} HTTP/1.1
UCP-Agent: profile="https://platform.example/profile"
Content-Type: application/json

{
  "id": "chk_123456789", // deprecated: id is provided in URL path
  "buyer": {
    "email": "jane@example.com",
    "first_name": "Jane",
    "last_name": "Doe"
  },
  "line_items": [
    {
      "item": {
        "id": "item_123",
        "title": "Red T-Shirt",
        "price": 2500
      },
      "id": "li_1",
      "quantity": 2,
    }
  ],
  "fulfillment": {
    "methods": [
      {
        "id": "shipping_1",
        "type": "shipping",
        "line_item_ids": ["item_123"],
        "selected_destination_id": "dest_home",
        "destinations": [
          {
            "id": "dest_home",
            "street_address": "123 Main St",
            "address_locality": "Springfield",
            "address_region": "IL",
            "postal_code": "62701",
            "address_country": "US"
          }
        ],
        "groups": [
          {
            "id": "package_1",
            "selected_option_id": "express"
          }
        ]
      }
    ]
  }
}

HTTP/1.1 200 OK
Content-Type: application/json

{
  "ucp": {
    "version": "draft",
    "capabilities": {
      "dev.ucp.shopping.checkout": [
        {"version": "draft"}
      ]
    },
    "payment_handlers": {
      "com.shopify.shop_pay": [
        {
          "id": "shop_pay_1234",
          "version": "draft",
          "available_instruments": [
            {"type": "shop_pay"}
          ],
          "config": {
            "merchant_id": "shop_merchant_123"
          }
        }
      ]
    }
  },
  "id": "chk_1234567890",
  "status": "ready_for_complete",
  "currency": "USD",
  "line_items": [
    {
      "id": "li_1",
      "item": {
        "id": "item_123",
        "title": "Red T-Shirt",
        "price": 2500
      },
      "quantity": 2,
      "totals": [
        {"type": "subtotal", "amount": 5000},
        {"type": "total", "amount": 5000}
      ]
    }
  ],
  "buyer": {
    "email": "jane@example.com",
    "first_name": "Jane",
    "last_name": "Doe"
  },
  "totals": [
    {
      "type": "subtotal",
      "amount": 5000
    },
    {
      "type": "tax",
      "amount": 400
    },
    {
      "type": "total",
      "amount": 5400
    }
  ],
  "links": [
    {
      "type": "terms_of_service",
      "url": "https://merchant.com/terms"
    }
  ],
  "fulfillment": {
    "methods": [
      {
        "id": "shipping_1",
        "type": "shipping",
        "line_item_ids": ["item_123"],
        "selected_destination_id": "dest_home",
        "destinations": [
          {
            "id": "dest_home",
            "street_address": "123 Main St",
            "address_locality": "Springfield",
            "address_region": "IL",
            "postal_code": "62701",
            "address_country": "US"
          }
        ],
        "groups": [
          {
            "id": "package_1",
            "line_item_ids": ["item_123"],
            "selected_option_id": "express",
            "options": [
              {
                "id": "standard",
                "title": "Standard Shipping",
                "description": "Arrives in 5-7 business days",
                "totals": [
                  {
                    "type": "total",
                    "amount": 500
                  }
                ]
              },
              {
                "id": "express",
                "title": "Express Shipping",
                "description": "Arrives in 2-3 business days",
                "totals": [
                  {
                    "type": "total",
                    "amount": 1000
                  }
                ]
              }
            ]
          }
        ]
      }
    ]
  },
  "payment": {
    "instruments": [
      {
        "id": "instr_shop_pay_1",
        "handler_id": "shop_pay_1234",
        "type": "shop_pay",
        "selected": true,
        "display": {
          "email": "buyer@example.com"
        }
      }
    ]
  }
}

Complete Checkout

If businesses have specific logic to enforce field existence in buyer and addresses (i.e. fulfillment_address, billing_address), this is the right place to set these expectations via messages.

RequestResponse

POST /checkout-sessions/{id}/complete
UCP-Agent: profile="https://platform.example/profile"
Content-Type: application/json

{
  "payment": {
    "instruments": [
      {
        "id": "pi_gpay_5678",
        "handler_id": "gpay_1234",
        "type": "card",
        "selected": true,
        "display": {
          "brand": "mastercard",
          "last_digits": "5678",
          "card_art": "https://cart-art-1.html",
          "description": "Google Pay •••• 5678"
        },
        "billing_address": {
          "street_address": "123 Main St",
          "address_locality": "Anytown",
          "address_region": "CA",
          "address_country": "US",
          "postal_code": "12345"
        },
        "credential": {
          "type": "PAYMENT_GATEWAY",
          "token": "examplePaymentMethodToken"
        }
      }
    ]
  },
  "risk_signals": {
    //... risk signal related data (device fingerprint / risk token)
  }
}

HTTP/1.1 200 OK
Content-Type: application/json

{
  "ucp": {
    "version": "draft",
    "capabilities": {
      "dev.ucp.shopping.checkout": [
        {"version": "draft"}
      ]
    },
    "payment_handlers": {
      "com.google.pay": [
        {
          "id": "gpay_1234",
          "version": "draft",
          "config": {
            "allowed_payment_methods": [
              {
                "type": "CARD",
                "parameters": {
                  "allowed_card_networks": ["VISA", "MASTERCARD", "AMEX"]
                }
              }
            ]
          }
        }
      ]
    }
  },
  "id": "chk_123456789",
  "status": "completed",
  "currency": "USD",
  "order": {
    "id": "ord_99887766",
    "permalink_url": "https://merchant.com/orders/ord_99887766"
  },
  "line_items": [
    {
      "id": "li_1",
      "item": {
        "id": "item_123",
        "title": "Red T-Shirt",
        "price": 2500
      },
      "quantity": 2,
      "totals": [
        {"type": "subtotal", "amount": 5000},
        {"type": "total", "amount": 5000}
      ]
    }
  ],
  "buyer": {
    "email": "jane@example.com",
    "first_name": "Jane",
    "last_name": "Doe"
  },
  "totals": [
    {
      "type": "subtotal",
      "amount": 5000
    },
    {
      "type": "tax",
      "amount": 400
    },
    {
      "type": "total",
      "amount": 5400
    }
  ],
  "links": [
    {
      "type": "terms_of_service",
      "url": "https://merchant.com/terms"
    }
  ],
  "fulfillment": {
    "methods": [
      {
        "id": "shipping_1",
        "type": "shipping",
        "line_item_ids": ["item_123"],
        "selected_destination_id": "dest_home",
        "destinations": [
          {
            "id": "dest_home",
            "street_address": "123 Main St",
            "address_locality": "Springfield",
            "address_region": "IL",
            "postal_code": "62701",
            "address_country": "US"
          }
        ],
        "groups": [
          {
            "id": "package_1",
            "line_item_ids": ["item_123"],
            "selected_option_id": "express",
            "options": [
              {
                "id": "standard",
                "title": "Standard Shipping",
                "description": "Arrives in 5-7 business days",
                "totals": [
                  {
                    "type": "total",
                    "amount": 500
                  }
                ]
              },
              {
                "id": "express",
                "title": "Express Shipping",
                "description": "Arrives in 2-3 business days",
                "totals": [
                  {
                    "type": "total",
                    "amount": 1000
                  }
                ]
              }
            ]
          }
        ]
      }
    ]
  },
  "payment": {
    "instruments": [
      {
        "id": "pi_gpay_5678",
        "handler_id": "gpay_1234",
        "type": "card",
        "selected": true,
        "display": {
          "brand": "mastercard",
          "last_digits": "5678",
          "rich_text_description": "Google Pay •••• 5678"
        }
      }
    ]
  }
}

Get Checkout

RequestResponse

GET /checkout-sessions/{id}
UCP-Agent: profile="https://platform.example/profile"
Content-Type: application/json

{}

HTTP/1.1 200 OK
Content-Type: application/json

{
  "ucp": {
    "version": "draft",
    "capabilities": {
      "dev.ucp.shopping.checkout": [
        {"version": "draft"}
      ]
    },
    "payment_handlers": {
      "com.shopify.shop_pay": [
        {
          "id": "shop_pay_1234",
          "version": "draft",
          "available_instruments": [
            {"type": "shop_pay"}
          ],
          "config": {
            "merchant_id": "shop_merchant_123"
          }
        }
      ]
    }
  },
  "id": "chk_123456789",
  "status": "completed",
  "currency": "USD",
  "order": {
    "id": "ord_99887766",
    "permalink_url": "https://merchant.com/orders/ord_99887766"
  },
  "line_items": [
    {
      "id": "li_1",
      "item": {
        "id": "item_123",
        "title": "Red T-Shirt",
        "price": 2500
      },
      "quantity": 2,
      "totals": [
        {"type": "subtotal", "amount": 5000},
        {"type": "total", "amount": 5000}
      ]
    }
  ],
  "buyer": {
    "email": "jane@example.com",
    "first_name": "Jane",
    "last_name": "Doe"
  },
  "totals": [
    {
      "type": "subtotal",
      "amount": 5000
    },
    {
      "type": "tax",
      "amount": 400
    },
    {
      "type": "total",
      "amount": 5400
    }
  ],
  "links": [
    {
      "type": "terms_of_service",
      "url": "https://merchant.com/terms"
    }
  ],
  "fulfillment": {
    "methods": [
      {
        "id": "shipping_1",
        "type": "shipping",
        "line_item_ids": ["item_123"],
        "selected_destination_id": "dest_home",
        "destinations": [
          {
            "id": "dest_home",
            "street_address": "123 Main St",
            "address_locality": "Springfield",
            "address_region": "IL",
            "postal_code": "62701",
            "address_country": "US"
          }
        ],
        "groups": [
          {
            "id": "package_1",
            "line_item_ids": ["item_123"],
            "selected_option_id": "express",
            "options": [
              {
                "id": "standard",
                "title": "Standard Shipping",
                "description": "Arrives in 5-7 business days",
                "totals": [
                  {
                    "type": "total",
                    "amount": 500
                  }
                ]
              },
              {
                "id": "express",
                "title": "Express Shipping",
                "description": "Arrives in 2-3 business days",
                "totals": [
                  {
                    "type": "total",
                    "amount": 1000
                  }
                ]
              }
            ]
          }
        ]
      }
    ]
  },
  "payment": {
    "instruments": [
      {
        "id": "instr_shop_pay_1",
        "handler_id": "shop_pay_1234",
        "type": "shop_pay",
        "selected": true,
        "display": {
          "email": "buyer@example.com"
        }
      }
    ]
  }
}

Cancel Checkout

RequestResponse

POST /checkout-sessions/{id}/cancel
UCP-Agent: profile="https://platform.example/profile"
Content-Type: application/json

{}

HTTP/1.1 200 OK
Content-Type: application/json

{
  "ucp": {
    "version": "draft",
    "capabilities": {
      "dev.ucp.shopping.checkout": [
        {"version": "draft"}
      ]
    },
    "payment_handlers": {
      "com.google.pay": [
        {
          "id": "gpay_1234",
          "version": "draft",
          "config": {
            "allowed_payment_methods": [
              {
                "type": "CARD",
                "parameters": {
                  "allowed_card_networks": ["VISA", "MASTERCARD", "AMEX"]
                }
              }
            ]
          }
        }
      ]
    }
  },
  "id": "chk_123456789",
  "status": "canceled", // Status is updated to canceled.
  "currency": "USD",
  "line_items": [
    {
      "id": "li_1",
      "item": {
        "id": "item_123",
        "title": "Red T-Shirt",
        "price": 2500
      },
      "quantity": 2,
      "totals": [
        {"type": "subtotal", "amount": 5000},
        {"type": "total", "amount": 5000}
      ]
    }
  ],
  "buyer": {
    "email": "jane@example.com",
    "first_name": "Jane",
    "last_name": "Doe"
  },
  "totals": [
    {
      "type": "subtotal",
      "amount": 5000
    },
    {
      "type": "tax",
      "amount": 400
    },
    {
      "type": "total",
      "amount": 5400
    }
  ],
  "links": [
    {
      "type": "terms_of_service",
      "url": "https://merchant.com/terms"
    }
  ],
  "fulfillment": {
    "methods": [
      {
        "id": "shipping_1",
        "type": "shipping",
        "line_item_ids": ["item_123"],
        "selected_destination_id": "dest_home",
        "destinations": [
          {
            "id": "dest_home",
            "street_address": "123 Main St",
            "address_locality": "Springfield",
            "address_region": "IL",
            "postal_code": "62701",
            "address_country": "US"
          }
        ],
        "groups": [
          {
            "id": "package_1",
            "line_item_ids": ["item_123"],
            "selected_option_id": "express",
            "options": [
              {
                "id": "standard",
                "title": "Standard Shipping",
                "description": "Arrives in 5-7 business days",
                "totals": [
                  {
                    "type": "total",
                    "amount": 500
                  }
                ]
              },
              {
                "id": "express",
                "title": "Express Shipping",
                "description": "Arrives in 2-3 business days",
                "totals": [
                  {
                    "type": "total",
                    "amount": 1000
                  }
                ]
              }
            ]
          }
        ]
      }
    ]
  },
  "payment": {
    "instruments": [
      {
        "id": "pi_gpay_5678",
        "handler_id": "gpay_1234",
        "type": "card",
        "selected": true,
        "display": {
          "brand": "mastercard",
          "last_digits": "5678",
          "rich_text_description": "Google Pay •••• 5678"
        }
      }
    ]
  }
}

HTTP Headers

The following headers are defined for the HTTP binding and apply to all operations unless otherwise noted.

Request Headers

Header	Required	Description
Authorization	No	Should contain oauth token representing the following 2 schemes: 1. Platform self authenticating (client_credentials). 2. Platform authenticating on behalf of end user (authorization_code).
X-API-Key	No	Authenticates the platform with a reusable api key allocated to the platform by the business.
Signature	No	RFC 9421 HTTP Message Signature. Required when using HTTP Message Signatures for authentication. Format: sig1=:<base64-signature>:.
Signature-Input	No	RFC 9421 Signature-Input header. Required when using HTTP Message Signatures for authentication. Format: sig1=("@method" "@path" ...);created=<timestamp>;keyid="<key-id>".
Content-Digest	No	Body digest per RFC 9530. Required for requests/responses with a body. Format: sha-256=:<base64-digest>:.
Idempotency-Key	Yes	Ensures duplicate operations don't happen during retries.
Request-Id	Yes	For tracing the requests across network layers and components.
User-Agent	No	Identifies the user agent string making the call.
UCP-Agent	Yes	Identifies the UCP agent making the call. All requests MUST include the UCP-Agent header containing the signer's profile URI using RFC 8941 Dictionary syntax. The URL MUST point to /.well-known/ucp. Format: profile="https://example.com/.well-known/ucp".
Content-Type	No	Representation Metadata. Tells the receiver what the data in the message body actually is.
Accept	No	Content Negotiation. The client tells the server what data formats it is capable of understanding.
Accept-Language	No	Localization. Tells the receiver the user's preferred natural languages, often with "weights" or priorities.
Accept-Encoding	No	Compression. The client tells the server which content-codings it supports, usually for compression

Specific Header Requirements

• UCP-Agent: All requests MUST include the UCP-Agent header containing the platform profile URI using Dictionary Structured Field syntax (RFC 8941). Format: profile="https://platform.example/profile".
• Idempotency-Key: Operations that modify state SHOULD support idempotency. When provided, the server MUST:
  1. Store the key with the operation result for at least 24 hours.
  2. Return the cached result for duplicate keys.
  3. Return 409 Conflict if the key is reused with different parameters.

Protocol Mechanics

Status Codes

UCP uses standard HTTP status codes to indicate the success or failure of an API request.

Status Code	Description
200 OK	The request was successful.
201 Created	The resource was successfully created.
400 Bad Request	The request was invalid or cannot be served.
401 Unauthorized	Authentication is required and has failed or has not been provided.
403 Forbidden	The request is authenticated but the user does not have the necessary permissions.
409 Conflict	The request could not be completed due to a conflict (e.g., idempotent key reuse).
422 Unprocessable Entity	The profile content is malformed (discovery failure).
424 Failed Dependency	The profile URL is valid but fetch failed (discovery failure).
429 Too Many Requests	Rate limit exceeded.
503 Service Unavailable	Temporary unavailability.
500 Internal Server Error	An unexpected condition was encountered on the server.

Error Responses

See the Core Specification for the complete error code registry and transport binding examples.

• Protocol errors: Return appropriate HTTP status code (401, 403, 409, 429, 503) with JSON body containing code and content.
• Business outcomes: Return HTTP 200 with UCP envelope and messages array.

Business Outcomes

Business outcomes (including errors like unavailable merchandise) are returned with HTTP 200 and the UCP envelope containing messages:

{
  "ucp": {
    "version": "draft",
    "capabilities": {
      "dev.ucp.shopping.checkout": [{"version": "draft"}]
    }
  },
  "id": "checkout_abc123",
  "status": "incomplete",
  "line_items": [
    {
      "id": "item_456",
      "quantity": 100,
      "available_quantity": 12
    }
  ],
  "messages": [
    {
      "type": "warning",
      "code": "quantity_adjusted",
      "content": "Quantity adjusted, requested 100 units but only 12 available",
      "path": "$.line_items[0].quantity"
    }
  ],
  "continue_url": "https://merchant.com/checkout/checkout_abc123"
}

For create_checkout, when all items unavailable and no checkout can be created, returns HTTP 200 and the UCP envelope containing messages

{
  "ucp": { "version": "2026-01-11", "status": "error" },
  "messages": [
    {
      "type": "error",
      "code": "item_unavailable",
      "content": "All items are not available for purchase",
      "severity": "unrecoverable"
    }
  ],
  "continue_url": "https://merchant.com/"
}

Message Signing

Platforms MAY choose among authentication mechanisms (API keys, OAuth, mTLS, HTTP Message Signatures). When using HTTP Message Signatures, checkout operations follow the Message Signatures specification.

Request Signing

When HTTP Message Signatures are used, requests MUST include valid Signature-Input and Signature headers (and Content-Digest when a body is present) per RFC 9421:

Header	Required	Description
Signature-Input	Yes	Describes signed components
Signature	Yes	Contains the signature value
Content-Digest	Cond.*	SHA-256 hash of request body

* Required for requests with a body (POST, PUT)

Example Signed Request:

POST /checkout-sessions HTTP/1.1
Host: merchant.example.com
Content-Type: application/json
UCP-Agent: profile="https://platform.example/.well-known/ucp"
Idempotency-Key: 550e8400-e29b-41d4-a716-446655440000
Content-Digest: sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:
Signature-Input: sig1=("@method" "@authority" "@path" "idempotency-key" "content-digest" "content-type");keyid="platform-2025"
Signature: sig1=:MEUCIQDTxNq8h7LGHpvVZQp1iHkFp9+3N8Mxk2zH1wK4YuVN8w...:

{"line_items":[{"item":{"id":"item_123"},"quantity":2}]}

See Message Signatures - REST Request Signing for the complete signing algorithm.

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="merchant-2025"
Signature: sig1=:MFQCIH7kL9nM2oP5qR8sT1uV4wX6yZaB3cD...:

{"id":"chk_123","status":"completed","order":{"id":"ord_456"}}

See Message Signatures - REST Response Signing for the complete signing algorithm.

Security Considerations

Authentication

Authentication is optional and depends on business requirements. When authentication is required, the REST transport MAY use:

1. Open API: No authentication required for public operations.
2. API Keys: Via X-API-Key header.
3. OAuth 2.0: Via Authorization: Bearer {token} header, following RFC 6749.
4. Mutual TLS: For high-security environments.
5. HTTP Message Signatures: Per RFC 9421 (see Message Signing above).

Businesses MAY require authentication for some operations while leaving others open (e.g., public checkout without authentication).