> ## Documentation Index
> Fetch the complete documentation index at: https://documentation.qonversion.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Identities

> Link Qonversion users to your own authentication system

An **Identity** binds a Qonversion User ID to your own external user ID (e.g., your auth system's primary key). This enables consistent user tracking across devices and platforms. Identities are the preferred way to reconcile anonymous SDK users with authenticated users after login.

## Key concepts

* **Identity ID** — The external ID you control (e.g., `user_42`). Up to 255 characters, alphanumeric + `.`, `_`, `-`.
* **Qonversion User ID** — The `QON_…` ID the identity is attached to.
* **Append-only** — Identities cannot be re-bound via this API. POSTing the same `identity_id` twice (even with a different `user_id`) returns `409 Identity already exists`.
* **Create shape** — v3 used `POST /identities/{identity_id}`; v4 replaces this with `POST /identities` where the identity ID is sent in the body for cleaner REST semantics.

## Available endpoints

| Method | Endpoint                    | Description                    | Status                              |
| ------ | --------------------------- | ------------------------------ | ----------------------------------- |
| POST   | `/identities`               | Create an identity link        | New                                 |
| GET    | `/identities/{identity_id}` | Get an identity by external ID | Updated — [v3](/reference/identity) |

## Authentication

All v4 endpoints require a **Secret Key**. See [Authentication](/reference/v4/authentication).