Svelte Core Bestpractices

Comprehensive reference for writing modern Svelte 5 code using runes instead of legacy patterns. Load this when working on any Svelte component to get guidance on reactivity ($state, $derived), proper use of $effect (sparingly), event handling, snippets over slots, and CSS scoping. The advice is opinionated in the right ways: it'll steer you toward $derived instead of effects for computed values, remind you to key your each blocks, and suggest context over shared modules for state. Especially helpful if you're migrating from Svelte 4 or keep reaching for old patterns like export let and $: statements. The $inspect.trace tip alone is worth having loaded when debugging reactive updates.

skills.sh installs: 12.1K
GitHub stars: 337
last updated: Aug 21, 2026

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Install

$ npx -y skills add sveltejs/ai-tools --skill svelte-core-bestpractices --agent claude-code

Installs into .claude/skills of the current project.

Files

`$state`

Only use the $state rune for variables that should be reactive — in other words, variables that cause an $effect, $derived or template expression to update. Everything else can be a normal variable.

Objects and arrays ($state({...}) or $state([...])) are made deeply reactive, meaning mutation will trigger updates. This has a trade-off: in exchange for fine-grained reactivity, the objects must be proxied, which has performance overhead. In cases where you're dealing with large objects that are only ever reassigned (rather than mutated), use $state.raw instead. This is often the case with API responses, for example.

`$derived`

To compute something from state, use $derived rather than $effect:

// do this
let square = $derived(num * num);

// don't do this
let square;

$effect(() => {
	square = num * num;
});

[!NOTE] $derived is given an expression, not a function. If you need to use a function (because the expression is complex, for example) use $derived.by.

Deriveds are writable — you can assign to them, just like $state, except that they will re-evaluate when their expression changes.

SKILL.md on GitHub