I assumed SvelteKit 5 would be a drop-in upgrade. Update the package.json, run npm install , maybe fix a few type errors. That's how it worked with Svelte 3 to 4. This time, the code compiled fine on the first try — but my brain didn't. The migration took three days longer than expected, not because of broken builds, but because I kept trying to think in Svelte 4 patterns while the framework wanted me to think in runes. Here's what actually caught me off guard. The Migration Script Lied (Kind Of) Running npx sv migrate svelte-5 worked surprisingly well. It converted my let declarations to $state() , my $: statements to $derived() and $effect() . The diff was thousands of lines, mostly mechanical changes. I committed everything and ran the dev server expecting green checkmarks. But here's the part that tripped me up: the migration script can't migrate your understanding.…