Feature Flagging

A senior engineer's playbook for using feature flags well, not just frequently.

Feature flags are infrastructure. Treated as such, they enable kill switches, gradual rollouts, A/B experiments, permission gates, and operational toggles without redeploys. Treated casually, they become the largest accumulating technical debt in your codebase: thousands of dead flags, conflicting evaluation logic, brittle targeting, and a permission surface no one fully understands.

This skill is the discipline that prevents the second outcome. It assumes you have a feature flag platform (LaunchDarkly, Flagsmith, Split.io, VWO FME, GrowthBook, Statsig, PostHog, Optimizely; the platform does not matter for the principles). It assumes your engineering team can implement targeting rules and SDK integration. The hard part is the operational discipline, and that is what is here.

When to use this skill: any time you are about to introduce a flag, modify a flag, audit existing flags, or design a flag-related governance policy.

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What this skill is for

The skill spans the operational lifecycle of a flag from creation through retirement. Flag types and the discipline of not mixing them. Naming conventions that survive code review. Lifecycle expectations baked in at creation, not bolted on later. Targeting rules that compose without fragility. Rollout strategies that match the risk profile of the launch. Stale flag management on a quarterly cadence. Governance and permissions that balance access with audit. Performance considerations so flags do not become a latency tax. Testing patterns that cover both branches. Rollback discipline. Observability across rollouts.

The skill does not cover experiment design; for hypothesis writing, sample size, MDE, and the discipline of arriving at a defensible decision, see the experiment-design skill. It does not cover statistical analysis or variance reduction; those live in the experimentation-analytics skill. It does not cover platform-specific tooling; for MCP commands, auth, and platform-specific configuration, consult the chosen platform's documentation. This skill produces the operational shape; the platform implements it.

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