react-spring is a spring-physics-first animation library for React, providing a cross-platform animation engine that targets both the DOM (via @react-spring/web) and React Three Fiber (via @react-spring/three). The library is built on a layered architecture: a custom requestAnimationFrame scheduler (rafz), a shared utilities layer (FluidValue observables, color/string interpolation, easing functions), an animated value system (Animated, AnimatedValue, AnimatedString), a core animation engine (Controller, SpringValue, Interpolation), and target-specific adapters that inject renderer behavior via Globals.assign and createHost. The public API consists of declarative hooks (useSpring, useSprings, useTrail, useTransition, usePresence, useChain, useScroll, useResize, useInView), imperative controls (SpringRef), and low-level components (Spring, Trail, Transition). The library is published as a monorepo with 12 npm packages, all version-locked and released via changesets. The repository also includes a comprehensive documentation site built with React Router 7, MDX, and Vanilla Extract, plus a demo app with 30+ interactive sandboxes. The codebase is extensively tested with unit tests (Vitest browser mode), type-level tests, and E2E tests, using a custom mock-raf for deterministic animation testing. The architecture emphasizes performance on the hot path, target-agnostic core logic, and a strict bottom-up dependency graph (rafz → shared → animated → core → targets).