> ## Documentation Index > Fetch the complete documentation index at: https://docs.knotapi.com/llms.txt > Use this file to discover all available pages before exploring further. # Sync Cart > Add one or more products to a user's merchant cart. ## OpenAPI ````yaml POST /cart openapi: 3.1.0 info: title: Knot API description: An API to interact with the Knot merchant connectivity platform. version: 1.0.0 servers: - url: https://development.knotapi.com description: Development server security: - basicAuth: [] paths: /cart: post: description: Add one or more products to a user's merchant cart. operationId: cart_sync requestBody: description: >- The input parameters required for adding products to a user's merchant cart. content: application/json: schema: type: object properties: external_user_id: type: string description: Your unique identifier for the user. example: abc123 merchant_id: type: integer description: Unique identifier for the merchant. example: 44 products: type: array description: List of products to add to the cart. items: type: object properties: external_id: type: string description: >- Merchant's identifier for the product. Format varies by merchant. example: '3324893784' fulfillment: type: object description: '' properties: id: type: string description: >- Unique identifier for the fulfillment option sent in [`SYNC_CART_SUCCEEDED`](https://docs.knotapi.com/shopping/webhook-events#sync-cart-succeeded). example: >- 5cfbe095d0cb9e597e316170d6ee9a6c667bddcde4a999222d2cc2b5bbfef7cf required: - id required: - external_id delivery_location: type: object description: '' properties: address: type: object description: '' properties: line1: type: string description: First line of the delivery address. example: 123 Main St maxLength: 46 line2: type: string description: Second line of the delivery address. example: Floor 4 maxLength: 46 city: type: string description: City portion of the delivery address. example: Los Angeles region: type: string description: >- Region portion of the delivery address, usually a state abbreviation. Must be an ISO 3166-2 sub-division code. example: CA postal_code: type: string description: Postal code of the delivery address. example: '94105' minLength: 5 maxLength: 10 country: type: string description: >- Country portion of the delivery address. Must be an ISO 3166-1 alpha-2 code. example: US required: - line1 - city - region - postal_code - country first_name: type: string maxLength: 255 description: First name for the delivery address. example: Ada last_name: type: string maxLength: 255 description: Last name for the delivery address. example: Lovelace phone_number: type: string description: Phone number for the delivery address in E.164 format. example: '+11234567890' set_as_default: type: boolean description: >- Whether the delivery address should be saved as the user's default in the merchant account. example: true default: false required: - address - first_name - last_name - phone_number - set_as_default simulate: description: Simulate a failure in the development environment. oneOf: - type: string enum: - failed description: The sync cart attempt fails. example: failed required: - external_user_id - merchant_id - products examples: Success: summary: Success value: external_user_id: abc123 merchant_id: 44 products: - external_id: '3324893784' Failure: summary: Failure value: external_user_id: abc123 merchant_id: 44 products: - external_id: '3324893784' simulate: failed responses: '202': description: Successful request. content: application/json: schema: type: object properties: message: type: string description: Success message. example: Success '400': description: Bad request. content: application/json: schema: $ref: '#/components/schemas/Error' examples: UserNotFound: summary: User was not found value: error_type: USER_ERROR error_code: USER_NOT_FOUND error_message: >- The user was not found. Please check the external_user_id provided. MerchantAccountNotFound: summary: Merchant account was not found value: error_type: MERCHANT_ACCOUNT_ERROR error_code: MERCHANT_ACCOUNT_NOT_FOUND error_message: >- The merchant account was not found. Please check the merchant_id provided. MerchantAccountDisconnected: summary: Merchant account is disconnected value: error_type: MERCHANT_ACCOUNT_ERROR error_code: MERCHANT_ACCOUNT_DISCONNECTED error_message: >- The merchant account is disconnected. Please check the merchant_id provided. MerchantNotFound: summary: Merchant was not found value: error_type: MERCHANT_ERROR error_code: MERCHANT_NOT_FOUND error_message: >- The merchant was not found. Please check the merchant_id provided. MerchantUnavailable: summary: Merchant is not available value: error_type: MERCHANT_ERROR error_code: MERCHANT_UNAVAILABLE error_message: >- The merchant is unavailable. Please check the merchant_id provided. FulfillmentNotFound: summary: Fulfillment was not found value: error_type: CART_ERROR error_code: FULFILLMENT_NOT_FOUND error_message: >- The fulfillment option was not found. Please check the fulfillment.id provided. MerchantIdRequired: summary: merchant_id field is required value: error_type: INVALID_REQUEST error_code: INVALID_FIELD error_message: The merchant_id field is required. ExternalUserIdRequired: summary: external_user_id is required value: error_type: INVALID_REQUEST error_code: INVALID_FIELD error_message: The external_user_id field is required. ProductExternalIdRequired: summary: product.external_id is required value: error_type: INVALID_REQUEST error_code: INVALID_FIELD error_message: The product.external_id field is required. DeliveryLocationFieldRequired: summary: Delivery location field required value: error_type: INVALID_REQUEST error_code: INVALID_FIELD error_message: The delivery_location.phone_number field is required. InvalidDeliveryLocationField: summary: Invalid delivery_location field value: error_type: INVALID_REQUEST error_code: INVALID_FIELD error_message: >- The delivery_location.first_name must not be greater than 255 characters. OngoingOperation: summary: There is an existing operation value: error_type: MERCHANT_ACCOUNT_ERROR error_code: ONGOING_OPERATION error_message: An existing operation is in progress. '401': description: Unauthorized request. content: application/json: schema: $ref: '#/components/schemas/Error' examples: AuthFailed: summary: Auth failed value: error_type: INVALID_INPUT error_code: INVALID_API_KEYS error_message: Invalid client_id or secret provided. '403': description: Forbidden request. content: application/json: schema: $ref: '#/components/schemas/Error' examples: NoAccess: summary: No access value: error_type: INVALID_REQUEST error_code: NO_ACCESS error_message: Please contact Knot for access to this endpoint. '500': description: Internal server error. content: application/json: schema: $ref: '#/components/schemas/Error' examples: InternalServerError: summary: Unexpected server error value: message: Server Error components: schemas: Error: type: object properties: error_type: type: string description: Type of error. enum: - INVALID_INPUT - INVALID_REQUEST - USER_ERROR - SESSION_ERROR - MERCHANT_ACCOUNT_ERROR - MERCHANT_ERROR - SUBSCRIPTION_ERROR - TRANSACTION_ERROR - CART_ERROR example: INVALID_REQUEST error_code: type: string description: Error code. enum: - INVALID_API_KEYS - INVALID_FIELD - INVALID_JWE - INVALID_CURSOR_FORMAT - USER_NOT_FOUND - MERCHANT_ACCOUNT_NOT_FOUND - MERCHANT_ACCOUNT_DISCONNECTED - SESSION_NOT_FOUND - EXTEND_NOT_SUPPORTED - MERCHANT_UNAVAILABLE - NO_ACCESS - TRANSACTION_NOT_FOUND - NO_TRANSACTIONS - SUBSCRIPTION_NOT_FOUND - ONGOING_OPERATION - CART_NOT_FOUND - FULFILLMENT_NOT_FOUND example: INVALID_FIELD error_message: type: string description: Detailed error message. example: The limit may not be greater than 10. securitySchemes: basicAuth: type: http scheme: basic description: >- Basic authentication header of the form `Basic `, where `` is the base64-encoded string `username:password`. Use your `client_id` as the `username` and your `secret` as the `password` value. ````