Your auth works fine locally. Deploy to prod and users get randomly logged out for no fucking reason. I spent 3 weeks on this. Login works, cookie gets set, redirect happens - then the dashboard acts like the user doesn't exist. Refresh the page and sometimes it magically fixes itself. Sometimes it doesn't.
Shane Chang documented this exact problem in his debugging deep-dive - same symptoms, same frustration. Every SvelteKit developer hits this eventually. The GitHub discussions are full of people losing their minds over auth that "should work but doesn't."
It's not your login flow. It's a timing nightmare between SSR rendering and client hydration that the official docs barely mention. Progressive enhancement my ass - this breaks the one thing it's supposed to fix.
Here's What's Actually Happening
User logs in, gets redirected to dashboard. Server tries to render the page but the session cookie hasn't shown up yet - timing is fucked.
- ~0ms: SvelteKit starts rendering
/dashboardon the server - ~1ms: Server hook looks for session cookie - it's not fucking there yet
- ~2ms: Server renders "you're not logged in" HTML
- ~5ms: Browser gets the HTML showing login screen
- ~10ms: Login response finally completes, session cookie gets set
- ~15ms: Client hydrates with server's "not authenticated" data
- ~20ms: Dashboard components load with
user = null
Login worked fine. Cookie exists. But server rendering happened before the cookie arrived. It's like showing up at a restaurant before your reservation gets confirmed - the hostess has no idea you're supposed to be there.
I Wasted Three Weeks on the Stupidest "Fix"
First thing I tried: client-side "wait for auth to be ready" bullshit everywhere.
// Don't fucking do this, I'm embarrassed I wrote it
let authReady = false;
let resolveAuth;
const authPromise = new Promise(resolve => resolveAuth = resolve);
onMount(async () => {
await authPromise; // Wait for auth gods to smile upon us
loadUserData(); // This will never happen
});
Seemed smart at the time. Actually fighting SvelteKit's server-first approach like an idiot. Made everything depend on JavaScript loading, which breaks accessibility and defeats the whole point of using SvelteKit.
The Fix That Actually Works
Stop thinking imperatively ("do this when auth is ready") and start thinking reactively ("do this whenever auth state changes").
// The reactive approach - embraces SvelteKit's design
let sessionToken = $state($page.data.user?.token || null);
$effect(() => {
if (sessionToken) {
// This runs whenever sessionToken changes, not just once
loadUserData();
} else {
clearUserData();
}
});
But this only works if your server hooks are bulletproof. Which brings us to the next disaster that breaks session persistence in production...
Why Your Server Hook is Failing Silently
Most SvelteKit auth tutorials show you this pattern:
// hooks.server.js - The fragile version everyone copies
export const handle = async ({ event, resolve }) => {
const sessionId = event.cookies.get('session');
if (sessionId) {
const user = await validateSession(sessionId);
if (user) {
event.locals.user = user;
} else {
// This line destroys sessions on any validation hiccup
event.cookies.delete('session');
event.locals.user = null;
}
}
return resolve(event);
};
See the problem? Every network timeout, database hiccup, or backend 500 error results in session deletion. Your users get randomly logged out because your database was slow for 200ms. This violates OWASP session management guidelines and creates terrible UX.
I found this bug after our auth started failing when we moved from local SQLite to managed PostgreSQL. The additional network latency occasionally caused session validation timeouts, which triggered session deletion. Users would get logged out mid-session for no apparent reason.
The Resilient Hook Pattern
// The version that survives production reality
export const handle = async ({ event, resolve }) => {
const sessionId = event.cookies.get('session');
if (sessionId) {
try {
const user = await validateSession(sessionId);
if (user) {
event.locals.user = user;
event.locals.isAuthenticated = true;
} else {
// Session is invalid - mark as unauthenticated but keep cookie
event.locals.user = null;
event.locals.isAuthenticated = false;
// Only delete cookie on explicit logout, not validation failure
}
} catch (error) {
// Network/database error - assume unauthenticated but keep session
console.warn('Session validation failed:', error.message);
event.locals.user = null;
event.locals.isAuthenticated = false;
// Cookie survives temporary failures
}
} else {
event.locals.user = null;
event.locals.isAuthenticated = false;
}
return resolve(event);
};
This pattern treats validation failures as temporary. Your users stay logged in through database restarts, network blips, and backend deployments. It follows graceful degradation principles instead of failing catastrophically on any error.
The Production Cookie Nightmare
Working locally with http://localhost:3000? Your cookies work fine. Deploy to production with HTTPS and suddenly nobody can log in. The issue is cookie settings that work in development but fail in production.
// Development-only settings that break in production
event.cookies.set('session', sessionId, {
path: '/',
maxAge: 60 * 60 * 24 * 7, // 7 days
httpOnly: true,
// Missing: secure, sameSite settings for production
});
Production needs different cookie settings:
// Production-ready cookie settings
const isProduction = process.env.NODE_ENV === 'production';
event.cookies.set('session', sessionId, {
path: '/',
maxAge: 60 * 60 * 24 * 7,
httpOnly: true,
secure: isProduction, // HTTPS only in production
sameSite: isProduction ? 'strict' : 'lax',
// Correct domain for your deployment
domain: isProduction ? '.yourdomain.com' : undefined
});
I spent an entire weekend debugging why auth worked on our staging environment but failed on production. Both used HTTPS, both had identical code. The difference? Staging used a single domain, production used a subdomain. Cookie domain settings matter.
The Hydration Timing Problem
Even with resilient server hooks, you'll hit hydration timing issues. The server renders with one auth state, the client hydrates with different auth state, and your UI flickers between logged in and logged out. This hydration mismatch issue is documented but not well understood.
The fix is embracing SvelteKit's universal load functions instead of fighting them:
// +layout.server.js - Always provide auth state to client
export async function load({ locals }) {
return {
user: locals.user,
isAuthenticated: locals.isAuthenticated
};
}
// +layout.svelte - Single source of truth for auth state
<script>
import { page } from '$app/stores';
// This data comes from the server, no client-side race conditions
$: user = $page.data.user;
$: isAuthenticated = $page.data.isAuthenticated;
</script>
{#if isAuthenticated}
<AuthenticatedLayout {user}>
<slot />
</AuthenticatedLayout>
{:else}
<PublicLayout>
<slot />
</PublicLayout>
{/if}
This pattern eliminates hydration mismatches because client and server use identical data. No more flickering auth states or race conditions.
The Memory Leak That Kills Production Apps
Got your auth working? Great. Now your SvelteKit app mysteriously crashes every few days with out-of-memory errors. Welcome to the session storage memory leak nobody talks about.
The problem: most session examples use in-memory storage for development. Push to production and your server's memory usage grows until it dies. I found this out when our auth started working great but the app would randomly crash after 2-3 days of uptime.
// This kills production servers - don't use MemoryStore
import session from 'express-session';
const store = new session.MemoryStore(); // Memory leak waiting to happen
Had to switch to Redis session storage, but that meant setting up Redis infrastructure and dealing with connection pooling strategies. What should have been simple session management became distributed systems architecture.
For database sessions, you need cleanup jobs or your session table grows forever:
// Cleanup job that actually runs
setInterval(async () => {
await db.session.deleteMany({
where: { expiresAt: { lt: new Date() } }
});
}, 1000 * 60 * 60); // Every hour, delete expired sessions
Production memory leaks are the worst debugging experience. Everything works fine until it doesn't, and when it fails, it takes the entire app with it.
These are the main disasters you'll hit building auth in SvelteKit. But there are dozens of smaller gotchas that'll waste hours of your time. Let me save you some pain and cover the questions I get asked most often about SvelteKit authentication...