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

# EffiLink API Authentication: API Keys and OAuth 2.0

> Learn how EffiLink's two authentication methods work — API Keys and OAuth 2.0 — and choose the right approach for your integration.

EffiLink supports two authentication methods: **API Keys** for direct server-to-server integrations, and **OAuth 2.0** for third-party applications that act on behalf of EffiLink users. Every API request must include one of these methods — unauthenticated requests will be rejected.

## Comparison

|                      | API Key                                 | OAuth 2.0                               |
| -------------------- | --------------------------------------- | --------------------------------------- |
| **Best for**         | Server-to-server integrations           | Third-party app integrations            |
| **Request header**   | `ApiKey: YOUR_API_KEY`                  | `OAuth: ACCESS_TOKEN`                   |
| **Credential scope** | Tied to your EffiLink team account      | Tied to an individual user's account    |
| **Setup complexity** | Low — generate a key from the dashboard | Higher — requires an authorization flow |
| **Token expiry**     | Does not expire                         | Permanent by default (`expiresIn: -1`)  |

## When to use each method

**Use API Keys when:**

* Your application is tightly coupled with your own EffiLink account.
* You are building a backend service or script that runs under your team's credentials.
* You want the simplest possible setup with no user-facing login flow.

**Use OAuth 2.0 when:**

* Other people need to grant your application access to *their* EffiLink account.
* You are building a third-party integration or marketplace app.
* You need to act on behalf of different EffiLink users at runtime.

## Detailed guides

<CardGroup cols={2}>
  <Card title="API Keys" icon="key" href="/docs/api-keys">
    Generate an API Key from your dashboard and add it to every request with the `ApiKey` header. Best for server-to-server use.
  </Card>

  <Card title="OAuth 2.0" icon="lock" href="/docs/oauth2">
    Walk through the three-step OAuth 2.0 authorization flow to obtain an access token on behalf of any EffiLink user.
  </Card>
</CardGroup>