# Authentication

URL: https://docs.sigment.ai/authentication

How agents sign in to the Sigment MCP server, and how you approve and revoke them.

Every request to `https://api.sigment.ai/mcp` needs `Authorization: Bearer <token>`. Two kinds of token are accepted:

| Token | Prefix | How you get it | Approval |
|---|---|---|---|
| OAuth access token | `oat_` | The client (Claude, ChatGPT, Cursor, Claude Code) signs you in through the browser | You approve the agent in **Settings → AI agents** |
| API key | `ak_` | **Settings → AI agents → API keys** | Approved automatically |

Session tokens from the Sigment web app are **not** accepted at `/mcp`. Organization-owned keys aren't accepted either: a key must belong to a user.

## OAuth discovery

Sigment follows the MCP authorization spec, so clients configure themselves. A request without a token gets `401` with a `WWW-Authenticate` header pointing at the protected resource metadata:

```bash
curl https://api.sigment.ai/.well-known/oauth-protected-resource/mcp
```

```json
{
  "resource": "https://api.sigment.ai/mcp",
  "authorization_servers": ["https://clerk.sigment.ai"],
  "bearer_methods_supported": ["header"]
}
```

The authorization server supports dynamic client registration, so there's no client ID to configure.

## Approving and revoking agents

The first call from a new OAuth client registers it in **Sigment → Settings → AI agents**, but its tool calls are refused until you approve it. You can revoke an agent or an API key on the same page at any time. A revoked agent gets a message telling the user to re-approve it, and only the account owner can do that.

## Workspaces

Tools act on your workspaces. Every tool takes an optional `workspace_id`; without it, the tool uses your first workspace. `get_account` lists them.

## Why not the REST API?

The REST API is reserved for the Sigment app. It answers `403 "OAuth access tokens and API keys are accepted only at the /mcp endpoint"` to agent credentials. The MCP tools are bounded, rate-limited and budget-capped, which is what makes it safe to hand them to an agent.