Feature flag

Also called: feature toggle, feature switch, feature gate

Last updated:

A feature flag is a conditional wrapped around a piece of functionality whose value is decided by remote configuration rather than hard-coded. An SDK evaluates the flag against the current user and the configuration on the server, returning a variation — boolean, string, number, or JSON. Because the value lives in configuration, you change what your application does by editing the flag, not by shipping a release.

How a feature flag works

In code, a flag is a branch: if (client.boolVariation("new-checkout", user, false)). The SDK holds the flag configuration in memory and evaluates locally, so the check is a sub-millisecond lookup with no per-request network hop. When you change the flag in the dashboard, the new configuration is pushed to every connected SDK over a streaming connection and the next evaluation uses it — fleet-wide, with no redeploy.

Why teams use feature flags

Flags decouple deploy from release: you can ship code to production with the feature off, then turn it on when you choose. They make percentage rollouts and gradual exposure a configuration change, give you a kill switch for risky code, and let you serve different variations to beta users or internal staff. For the full treatment of variations, environments, and the evaluation model, see the feature flags concept guide.

Want the full picture? Read the concept guide: What are feature flags? →

Related terms

Feature toggle

Feature toggle is another name for a feature flag — a runtime switch that turns functionality on or off without a code deploy. The terms are interchangeable.

Percentage rollout

A percentage rollout serves a feature to a defined share of users — chosen by a deterministic hash — so the same users stay in the cohort as you ramp the percentage up.

Kill switch

A kill switch is a feature flag used to disable functionality instantly in production — flip it off and every SDK stops running the risky code path within seconds, no redeploy required.

Browse the full feature flag glossary →

Try it in your own app

Free Solo plan covers 10 flags and 2 environments. No credit card, no demo call — sign up and ship.

Get started free See pricing