> For the complete documentation index, see [llms.txt](https://docs.millimetric.ai/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.millimetric.ai/api-reference/identify.md).

# POST /v1/identify

POST /v1/identify — link an anonymous\_id to a user\_id.

Bind a known `user_id` to the visitor's `anonymous_id`. Persists as a special `$identify` event so future analysis can stitch pre- and post-login behavior together.

## Auth

|                |                 |
| -------------- | --------------- |
| Required scope | `ingest`        |
| Key kinds      | `pk_*` · `sk_*` |

## Request

```http
POST /v1/identify
Authorization: Bearer {key}
Content-Type: application/json
```

```json
{
  "anonymous_id": "u_abc",
  "user_id": "user_42",
  "traits": { "email": "matt@example.com", "plan": "pro" }
}
```

| Field          | Required | Notes                                                                                |
| -------------- | -------- | ------------------------------------------------------------------------------------ |
| `anonymous_id` | **yes**  | The pseudonymous id the visitor's browser/server has been using.                     |
| `user_id`      | **yes**  | Your stable internal user id.                                                        |
| `traits`       | no       | Free-form attributes about the user, stored on the `$identify` event's `properties`. |

## Response

```json
{ "ok": true }
```

## How it integrates

After `/v1/identify`:

1. Subsequent events should include `"user_id"` on each `track` call. The SDK does this automatically once you call `identify()`.
2. Historical events for the same `anonymous_id` remain — they're not retroactively rewritten. Joins for user-level analysis use the `$identify` event as a stitch point.
3. The `sessions_v` view sees `user_id` on the `$identify` event itself; running totals per user are easy.

## Example query: time-to-conversion per user

```sql
SELECT
  user_id,
  MIN(if(event_name = '$identify', timestamp, NULL)) AS signed_up_at,
  MIN(if(event_name = 'purchase', timestamp, NULL)) AS first_purchase
FROM events
WHERE project_id = '...'
  AND timestamp > now() - INTERVAL 30 DAY
  AND user_id IS NOT NULL
GROUP BY user_id;
```


---

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