Targeting & rollout

Control who sees what — from fully off to 100% of users, with stable per-user results.

Per-environment toggle

Every flag has an independent on/off toggle for each environment on its detail page. You can keep a flag enabled in Staging while it's completely off in Production — perfect for QA testing before a launch.

Gradual rollout

Use a Gradual flag type to release to a percentage of your users. Set the rollout to 10% and only 1 in 10 users will see the feature. When you're confident, slide it up to 100%.

// The SDK uses a stable hash of userId
// so the same user always gets the same result
const inRollout = await ff.evaluate<boolean>('new-dashboard', {
  userId: 'user_123',
  environment: 'production',
})
// → true for ~10% of users when rollout = 10

Stable hashing

The SDK hashes your userId to determine which bucket a user falls into. This means the same user always gets the same value — they won't randomly flip between the old and new experience on different page loads.

Default values

Every flag has a configured default value per environment. If the SDK can't reach the Vexil API (network error, timeout), it returns this default value so your app keeps working normally.