Cart Capability - REST Binding

Cart Capability - REST Binding

This document specifies the REST binding for the Cart Capability.

Protocol Fundamentals

Discovery

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

{
  "ucp": {
    "version": "2026-01-11",
    "services": {
      "dev.ucp.shopping": {
        "version": "2026-01-11",
        "spec": "https://ucp.dev/2026-01-11/specification/overview",
        "rest": {
          "schema": "https://ucp.dev/2026-01-11/services/shopping/rest.openapi.json",
          "endpoint": "https://business.example.com/ucp/v1"
        }
      }
    },
    "capabilities": [
      {
        "name": "dev.ucp.shopping.checkout",
        "version": "2026-01-11"
      },
      {
        "name": "dev.ucp.shopping.cart",
        "version": "2026-01-11"
      }
    ]
  }
}

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 cart 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 Cart	POST	/carts	Create a cart session.
Get Cart	GET	/carts/{id}	Get a cart session.
Update Cart	PUT	/carts/{id}	Update a cart session.
Cancel Cart	POST	/carts/{id}/cancel	Cancel a cart session.

Create Cart

Example

=== “Request”

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

{
  "line_items": [
    {
      "item": {
        "id": "item_123"
      },
      "quantity": 2
    }
  ],
  "context": {
    "address_country": "US",
    "address_region": "CA",
    "postal_code": "94105"
  }
}
```

=== “Response”

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

{
  "ucp": {
    "version": "2026-01-11",
    "capabilities": [
      {"name": "dev.ucp.shopping.checkout", "version": "2026-01-11"},
      {"name": "dev.ucp.shopping.cart", "version": "2026-01-11"}
    ]
  },
  "id": "cart_abc123",
  "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}
      ]
    }
  ],
  "currency": "USD",
  "totals": [
    {"type": "subtotal", "amount": 5000},
    {"type": "total", "amount": 5000, "display_text": "Estimated total (taxes calculated at checkout)"}
  ],
  "continue_url": "https://business.example.com/checkout?cart=cart_abc123",
  "expires_at": "2026-01-16T12:00:00Z"
}
```

=== “Error Response”

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

```json
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/"
}
```

Get Cart

Example

=== “Request”

```json
GET /carts/{id} HTTP/1.1
UCP-Agent: profile="https://platform.example/profile"
```

=== “Not Found”

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

{
  "ucp": {
    "version": "2026-01-11",
    "status": "error",
    "capabilities": [
      {"name": "dev.ucp.shopping.cart", "version": "2026-01-11"}
    ]
  },
  "messages": [
    {
      "type": "error",
      "code": "not_found",
      "content": "Cart not found or has expired",
      "severity": "unrecoverable"
    }
  ],
  "continue_url": "https://merchant.com/"
}
```

Update Cart

Example

=== “Request”

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

{
  "id": "cart_abc123",
  "line_items": [
    {
      "item": {"id": "item_123"},
      "id": "li_1",
      "quantity": 3
    },
    {
      "item": {"id": "item_456"},
      "id": "li_2",
      "quantity": 1
    }
  ],
  "context": {
    "address_country": "US",
    "address_region": "CA",
    "postal_code": "94105"
  }
}
```

Cancel Cart

Example

=== “Request”

```json
POST /carts/{id}/cancel HTTP/1.1
UCP-Agent: profile="https://platform.example/profile"
Content-Type: application/json

{}
```

HTTP Headers

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

Status Code	Description
200 OK	The request was successful.
201 Created	The cart 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.
500 Internal Server Error	An unexpected condition was encountered on the server.
503 Service Unavailable	Temporary unavailability.

Error Responses

• 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.

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.