Svelte 5: `$:` is not allowed in runes mode, use `$derived` or `$effect` instead
As soon as a component uses any rune ($state, $props, $derived), Svelte compiles the whole file in runes mode, and the old $: reactive statements stop being valid. So adding one $state to a legacy component breaks every $: in it, and setting runes: true in the config breaks them everywhere. Values worked out from other values become $derived; statements with side effects, such as logging or fetching, become $effect. Components can stay in legacy mode until you convert each one whole.
Quick fix
Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.
# Components that use runes but still contain $: statements
grep -rlE '\$(state|props|derived)\(' src | xargs grep -n '^\s*\$:'
# Is runes mode forced for every file?
grep -n "runes" svelte.config.js 2>/dev/null
# Let the official migration rewrite the components
npx sv migrate svelte-5
# By hand:
# $: doubled = count * 2; -> let doubled = $derived(count * 2);
# $: total = sum(items); -> let total = $derived(sum(items));
# $: console.log(count); -> $effect(() => { console.log(count); });
# Prefer $derived wherever a value is being computed: an $effect that
# writes state is slower and easy to turn into an update loopHow to diagnose Svelte errors
Svelte errors mostly arise at the compiler's boundaries: the $ store prefix only works on real stores in component context, and SvelteKit's load functions run in two different environments with different capabilities. A 500 from a load function usually means server-only code (a database client, a secret, fs) reached a universal load that also runs in the browser.
If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.
- Check whether the file is a universal load (
+page.js) or a server-only load (+page.server.js). Anything touching secrets or the filesystem belongs in the latter. - Read the full server-side stack trace in the terminal, not just the browser's 500 page: SvelteKit hides details in production for security.
- For
$prefix errors, confirm the value is an actual store with asubscribemethod and that you are inside a component, not a plain module. - Use
$env/static/privateand$env/dynamic/privatefor secrets so the compiler enforces the server-only boundary for you. - Reproduce with
vite build && vite preview; dev-mode behaviour differs meaningfully from the production build.
Tools worth reaching for
vite build && vite previewSvelteKit terminal logssvelte-check$env/static/private
Authoritative references
Primary documentation for this error, worth reading before applying any fix in production.
Related Svelte errors
- Svelte 5: `$effect` can only be used inside an effectEffects have to be created while a component is initialising, so calling `$effect` from a…
- Svelte 5: `$props()` can only be used at the top level of componentsRunes are compiled away rather than called, so `$props()` has to appear exactly once as the…
- Svelte 5: `export let` is not allowed in runes mode, use `$props()` insteadA component is in runes mode, where props arrive through $props() rather than exported let…
- Svelte 5: $state / $derived can only be used inside a .svelte or .svelte.js fileRunes are compiler features, not runtime functions. They only work in files the Svelte…
- Svelte 5: Cannot export state from a module if it is reassignedIn a .svelte.js or .svelte.ts module, $state works because the compiler rewrites every read…
- Svelte 5: effect_update_depth_exceeded (Maximum update depth exceeded)An effect wrote to state that it also reads, so running it schedules it again and Svelte…
- Svelte 5: State referenced in its own scope will never updateReading a `$state` variable into a plain `const`, or passing it to a function, copies the…
- Svelte 5: Updating state inside a derived is forbiddenA `$derived` expression re-runs whenever its inputs change, so writing state from inside one…
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Something missing or wrong?
This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.