Feature Flagging

Feature flags let you ship code without enabling features for users. Use them for gradual rollouts, kill switches, and A/B tests. AI agents either (a) skip them entirely, or (b) flag every line of code — both wrong.

The opinion

Flipper for Rails feature flags. Flag launches (gradual rollouts, kill switches), not every line. Set a deprecation date when you add a flag — flags that live forever become technical debt. Read flags once per request via a wrapper that's mockable in tests. Admin UI for ops, but flag flips should ideally be in code (committed) for audit trail.

Counter-positions:

• LaunchDarkly / Statsig / GrowthBook — managed services with deeper analytics. Pay-per-flag-evaluation gets expensive. Flipper covers 80% for small/medium apps.
• rollout gem — older, less actively maintained. Flipper won.
• Conditional based on Rails.env.production? — anti-pattern. That's not a flag, that's a fork.

Core patterns

Pattern 1: Flipper setup