# Extensions

Challenge extensions add conditions to an existing challenge. BotStopper uses them to check browser behavior without changing the challenge algorithm.

To enable an extension, add its name to the `extensions` field in your challenge definition:

```yaml
thresholds:
  - name: mild-suspicion
    expression:
      all:
        - weight > 0
        - weight < 10
    action: CHALLENGE
    challenge:
      algorithm: fast
      difficulty: 2
      report_as: 2
      extensions:
        - css-load
```

This example adds the [CSS loading extension](/docs/extensions/css-load.md) to the [fast challenge](/docs/challenges/fast.md).

Extensions add inexpensive browser checks that can help detect scripts which only solve the challenge computation.

## Available extensions

- [CSS loading](/docs/extensions/css-load.md) checks that the client requests a CSS resource from BotStopper.
- [Headless browser detection](/docs/extensions/headless.md) examines the browser for signs of automation and requires JavaScript and WebAssembly.

For policy configuration, refer to [thresholds](/docs/configuration/thresholds.md) and [expressions](/docs/configuration/expressions.md).