> This page is for Cash App Pay Partner API.

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# Reference IDs

The Cash App Pay API provides a `reference_id` field on all create, update, and upsert endpoints which can be used by an API client to set its own identifier for the resource. This allows API clients to establish a "link" between their systems and the Cash App Pay API.

All resources support querying by `reference_id` using "list" endpoints. If you lose the Cash App Pay-provided ID of a resource but have the `reference_id` from your systems, you can use this functionality to look up the Cash App Pay IDs of each resource.

## Requirements

Reference IDs must:

* Not contain any PII (personally identifiable information)
* Have between 1 and 1024 characters.

Reference IDs are required for brands and merchants. This prevents duplicate brands and merchants from being onboarded to Cash App Pay.