Svelte 5: Updating state inside a derived is forbidden

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.

<script>
  let items = $state([]);
  let selected = $state(null);

  // Throws: the derived mutates state as a side effect
  const total = $derived.by(() => {
    selected = items[0];
    return items.length;
  });

  // Derivations stay pure
  const total2 = $derived(items.length);

  // Side effects belong in an effect, which is allowed to write
  $effect(() => {
    if (!items.includes(selected)) selected = items[0] ?? null;
  });
</script>

How 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.

1. 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.
2. Read the full server-side stack trace in the terminal, not just the browser's 500 page: SvelteKit hides details in production for security.
3. For $ prefix errors, confirm the value is an actual store with a subscribe method and that you are inside a component, not a plain module.
4. Use $env/static/private and $env/dynamic/private for secrets so the compiler enforces the server-only boundary for you.
5. Reproduce with vite build && vite preview; dev-mode behaviour differs meaningfully from the production build.

Tools worth reaching for

• vite build && vite preview
• SvelteKit terminal logs
• svelte-check
• $env/static/private

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

📖 svelte.dev

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 / $derived can only be used inside a .svelte or .svelte.js fileRunes are compiler features, not runtime functions. They only work in files the Svelte…
• Svelte 5: State referenced in its own scope will never updateReading a `$state` variable into a plain `const`, or passing it to a function, copies the…
• Svelte: '$' prefix can only be used with storesThe $ auto-subscription was used on a value that is not a store (no subscribe method), or a…
• SvelteKit: 500 error in load functionA load function threw, returned a non-serialisable value, or accessed browser-only APIs…
• SvelteKit: Cannot prerender pages with actionsPrerendering happens at build time and produces a static file, while a form action needs a…

See all 10 Svelte errors →

Browse other categories

• HTTP 494xx client errors, 5xx server errors, redirects, headers and protocol problems.
• JavaScript 42npm resolution, async pitfalls, hydration, memory limits and runtime type…
• Database 41Connections, deadlocks, constraints, replication and memory limits.
• AI 35Rate limits, context windows, GPU memory and model-serving failures.
• Network 35Refused connections, timeouts, resets, MTU problems and port exhaustion.
• Python 35Imports, virtual environments, encoding, concurrency and dependency conflicts.
• Kubernetes 34CrashLoopBackOff, ImagePullBackOff, OOMKilled, RBAC, scheduling and storage.
• Docker 27Daemon connectivity, disk space, image pulls, ports and architecture mismatches.
• System 26Disk space, systemd units, file descriptors, OOM killer and scheduled jobs.
• Cloud 25IAM permissions, quotas, service limits and credential failures.
• Security 25JWT validation, CSRF, OAuth grants, SELinux, SSH host keys and CSP.
• TLS 24Untrusted authorities, expiry, hostname mismatch, chains and cipher negotiation.

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.