GrowthBook Constants API
**Beta** — these endpoints are new and may change in backwards-incompatible ways. Reusable named values referenced from feature flag values as `@const:key` and resolved into the SDK payload at build time. String constants are interpolated via `{{ @const:key }}`; JSON (object) constants are composed via an `$extends` array. A constant's own keys **replace** what its `$extends` bases provide, wholesale — constants are atomic building blocks. (Config and feature values compose as deep, targeted patches instead.)