svelte 5 throws state_unsafe_mutation when i set a count inside $derived Svelte

Replacement is: stop having two sources of truth, derive both.

let items = $state([]);
let visible = $derived(items.filter(i => i.active));
let visibleCount = $derived(visible.length);

That's the whole thing. Deriveds are memoised, so deriving from a derived doesn't re-run the filter.

Why it's forbidden: deriveds in Svelte 5 are pull-based. They run when something reads them, which might be twice, might be never if the component isn't rendering. If a derived writes state, the value of that state depends on whether anything happened to read the derived. That's how you get a bug that only reproduces when a panel is collapsed. Svelte 4's $: had exactly that class of bug, it just never told you about it.

If the real computation is heavier than a filter and you really do want one pass producing two values, derive the object:

let result = $derived.by(() => {
  const out = [];
  let skipped = 0;
  for (const i of items) i.active ? out.push(i) : skipped++;
  return { out, skipped };
});

Then read result.out and result.skipped. One pass, no state writes. This is the pattern for anything expensive enough that you care.

Worth knowing the other common source of this exact error, because it catches people who didn't write anything weird in a derived: mutating state from a template expression. A function called inside {#each} that pushes to an array will do it. Same rule, much less obvious. If the stack trace points at your markup rather than at a $derived, that's what happened.

you can wrap the write in untrack() and it'll stop complaining

Migration note since you came from 4: the whole family of $: total = a + b maps to $derived, and $: doSomething(a) maps to $effect. Most of the pain in migrating is discovering how many of your $: lines were quietly both at once.

Not your bug, but confirm items is actually $state and not something you reassigned out of a prop. Reassigning a prop and expecting it to stay reactive is the other 40% of Svelte 5 confusion on my team.