Configuration

Active contributors: Rich Harris, Simon H

Compile options

CompileOptions and ModuleCompileOptions are validated by packages/svelte/src/compiler/validate-options.js (~10 KB). The schemas are also documented at https://svelte.dev/docs/svelte/svelte-compiler#CompileOptions. Most-used keys:

Option	Default	Effect
filename	undefined	Used in error messages and source maps.
generate	'client'	'client' or 'server'. Switches transform target.
dev	false	Enables DEV-only checks ($inspect, ownership, HMR markers, etc.).
runes	auto-detect	Force runes vs legacy mode regardless of source.
css	'external'	'external' (default) or 'injected'. Whether to emit a separate CSS file.
cssHash	({ hash, css }) => ...	Function that produces the scope hash for :scope-based encapsulation.
customElement	false	Compile to a Custom Element class.
accessors	false	Generate get/set accessors on the component class.
immutable	false	Treat object props as immutable (legacy mode optimisation).
discloseVersion	true	Emit import 'svelte/internal/disclose-version' so SvelteKit can detect the package version.
hmr	false	Emit HMR markers (used by vite-plugin-svelte).
tracing	false	Enable $inspect.trace runtime tracking.
legacy	{ componentApi: false }	Legacy compatibility toggle for the v4 component class API.
modernAst	false (Svelte 5)	Returns the modern AST shape from parse() instead of the legacy v4 shape.

Runtime flags

These are JS modules under packages/svelte/src/internal/flags/. The compiler imports them only when they're needed, so an app that doesn't use the feature pays no cost:

Flag module	Default	Set when
internal/flags/async.js	false	Component uses async rune features. Compiler emits import 'svelte/internal/flags/async'.
internal/flags/legacy.js	false	Component is in legacy mode.
internal/flags/tracing.js	false	Component uses $inspect.trace(...).

Environment variables (test/dev)

Variable	Effect
SVELTE_NO_ASYNC=true	Disables async-mode reactivity (forces async_mode_flag = false). Used by the TestNoAsync CI job.
UPDATE_SNAPSHOTS=true	Vitest snapshot suites write expected output instead of comparing.
FILTER=<name>	Drop tests not matching <name> from a suite (faster than vitest -t).
CI=true	Set by GitHub Actions; disables interactive prompts in some scripts.
NODE_ENV=production	Used by pnpm bench to mirror production deopts.

Preprocess options

preprocess(source, transformers, options) (packages/svelte/src/compiler/preprocess/index.js):

• transformers — single transformer or array. Each is { name?, markup?, script?, style? }.
• options.filename — for source maps.

Top-level package metadata

• packages/svelte/package.json#engines.node → >= 18. CI matrix tests 18, 20, 22, 24.
• packages/svelte/package.json#packageManager → unset (declared at root: pnpm@10.4.0).

See also

• Data models — the shapes produced by the compiler.
• Compiler — what each phase actually does with these options.