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

# Overview

The three.dev APIs are available at `https://api.three.dev`. Authenticate every request with a three.dev API key:

```
Authorization: Bearer <THREE_DEV_API_KEY>
```

Create and manage API keys at [Settings > API Keys](https://app.three.dev/settings/api-keys).

## Endpoints

* [Report Metric](/api-reference/report-metric.md) — report a binary quality metric outcome for a session.
* [Assign Session](/api-reference/assign-session.md) — get the variant assignment for a session in a running live experiment.
* [Render Prompt](/api-reference/render-prompt.md) — render a prompt by substituting variables into the active published version.

## Error responses

API errors return JSON with a `message` field:

```json
{
  "message": "Description of what went wrong"
}
```

Common HTTP status codes:

| Code | Meaning                                                                          |
| ---- | -------------------------------------------------------------------------------- |
| 400  | Bad request — invalid or missing parameters.                                     |
| 401  | Unauthorized — missing or invalid API key.                                       |
| 404  | Not found — the resource does not exist or does not belong to your organization. |
| 409  | Conflict — for example, attempting to create a duplicate resource.               |
| 422  | Unprocessable entity — the request is well-formed but cannot be processed.       |
| 500  | Internal server error.                                                           |


---

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