# Update custom attributes on a subscription contract

Updates or replaces custom key-value attributes on a subscription contract. These attributes are stored with the subscription and can be used to track custom data, preferences, or metadata that's important for your business processes.

Custom Attributes Overview:
Custom attributes are key-value pairs that allow you to store additional information on subscriptions. They are:
- Visible in the Shopify admin and accessible via API
- Included in order data when subscription orders are created
- Preserved across subscription lifecycle events
- Useful for integrations and custom workflows

Update Modes:
- Merge Mode (overwriteExistingAttributes=false): Adds new attributes and updates existing ones with matching keys. Other attributes remain unchanged.
- Replace Mode (overwriteExistingAttributes=true): Completely replaces all existing attributes with the provided list.

Common Use Cases:
- Store gift messages or special instructions
- Track referral sources or marketing campaigns
- Add internal reference numbers or tracking codes
- Store customer preferences or customization options
- Integration data for third-party systems

Important Notes:
- Attribute keys should not conflict with Shopify's reserved attributes
- Both keys and values are stored as strings
- Changes are logged in the activity history
- Invalid discount codes may be automatically removed during update

Authentication: Requires valid X-API-Key header

Endpoint: POST /subscriptions/cp/api/update-custom-note-attributes
Version: 0.0.1

## Request fields (application/json):

  - `subscriptionContractId` (integer, required)
    The ID of the subscription contract to update
    Example: 123456789

  - `customAttributesList` (array, required)
    List of custom attributes to set on the subscription contract. Each attribute is a key-value pair that will be stored with the subscription.

  - `customAttributesList.key` (string, required)
    The attribute key. Should be a unique identifier for this attribute. Use descriptive keys like 'gift_message', 'delivery_instructions', etc.
    Example: "gift_message"

  - `customAttributesList.value` (string)
    The attribute value. Can be any string value including JSON-encoded data. Set to empty string to clear an attribute.
    Example: "Happy Birthday! Enjoy your subscription!"


## Response 200 fields

## Response 400 fields

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## Response 403 fields