Systems

Active contributors: Rich Harris, Simon H, Dominic Gannaway

This section covers cross-cutting subsystems that don't map cleanly to a single source directory. Each one ties together pieces of the compiler, the client runtime, and the server runtime.

System	What it covers
Reactivity engine	Sources, deriveds, effects, batches, async support — the heart of Svelte 5.
Compile pipeline	End-to-end view of parse → analyze → transform with cross-references.
Hydration	How server-rendered HTML is paired up with client reactivity at boot.
Preprocess	The pluggable preprocess() API that runs before compile().

For per-feature deep dives (runes, transitions, motion, stores, attachments) see the features section.