Svelte Production Troubleshooting - Debug Like a Pro

SvelteKit 5.0 broke half the internet. OK, maybe just half my apps, but it felt like the whole damn internet. That innocent upgrade command turned into a three-day debugging marathon that nearly made me switch back to React.

The problem? SvelteKit 5.0 introduced stricter hydration validation. All those subtle bugs your v4 app was silently ignoring suddenly became fatal errors. Your console fills up with "Hydration failed because the initial UI does not match what was rendered on the server" and you start questioning your life choices.

The Timestamp Death Trap

Here's the exact code that took down our user dashboard for 6 hours:

<script>
  import { browser } from '$app/environment';
  
  let currentTime = new Date().toISOString();
  
  if (browser) {
    currentTime = new Date().toISOString(); // Different timestamp!
  }
</script>

<p>Last updated: {currentTime}</p>

Server renders timestamp A. Client hydrates with timestamp B. SvelteKit 5.0 throws a fit. User sees a broken dashboard. Boss asks uncomfortable questions.

The LocalStorage State Bomb

This innocent user preferences code murdered our mobile performance:

<script>
  import { browser } from '$app/environment';
  
  let userPrefs = null;
  
  onMount(() => {
    userPrefs = JSON.parse(localStorage.getItem('preferences'));
  });
</script>

{#if userPrefs}
  <UserDashboard {userPrefs} />
{:else}
  <DefaultDashboard />
{/if}

Server always renders DefaultDashboard. Client switches to UserDashboard. Hydration mismatch. Mobile users bounce because the page jerks around like it's having a seizure.

The Fix That Actually Works

After debugging this crap for three straight days, I built a hydration-safe store that hasn't failed once:

// HydrationSafeStore.js - Copy this, it works
import { writable } from 'svelte/store';
import { browser } from '$app/environment';

export function createSafeStore(key, defaultValue) {
  const store = writable(defaultValue);
  
  if (browser) {
    const stored = localStorage.getItem(key);
    if (stored) {
      try {
        store.set(JSON.parse(stored));
      } catch (e) {
        console.warn(`Corrupted data in ${key}:`, e);
        localStorage.removeItem(key);
      }
    }
    
    store.subscribe(value => {
      localStorage.setItem(key, JSON.stringify(value));
    });
  }
  
  return store;
}

This pattern works because server and client start with identical state. The localStorage sync happens after hydration completes. No more mismatches, no more broken deployments.

Memory Leak Hell in Production

Two months after fixing hydration, our memory usage started climbing. Users reported browser tabs freezing on mobile. The culprit? Svelte 5's new runes system has some nasty memory leak patterns that only show up under load.

The problem code looked harmless:

<script>
  import { $state } from 'svelte';
  
  let data = $state([]);
  
  async function loadMore() {
    const newItems = await fetch('/api/items');
    data = [...data, ...newItems]; // Memory leak!
  }
</script>

Every loadMore() call created new array references that Svelte's garbage collector couldn't clean up properly. After an hour of scrolling, mobile browsers would crash with out-of-memory errors.

The fix was annoyingly simple once I found it:

async function loadMore() {
  const newItems = await fetch('/api/items');
  data.push(...newItems); // Mutate in place
}

The Production Debugging Workflow That Saved My Sanity

When hydration breaks at 2am and your monitoring is screaming, you need a systematic approach. Here's the exact process I use:

1. Enable Debug Mode: Add __SVELTEKIT_DEBUG__: true to your Vite config
2. Isolate the Problem: Comment out half your components until hydration works
3. Check the Basics: Different timestamps? localStorage access? API calls in wrong places?
4. Nuclear Option: Wrap problematic components in client-only boundaries

The nuclear option looks like this:

<script>
  import { browser } from '$app/environment';
  import { onMount } from 'svelte';
  
  let mounted = false;
  
  onMount(() => {
    mounted = true;
  });
</script>

{#if browser && mounted}
  <ProblematicComponent />
{:else}
  <div style=\"height: 200px;\">Loading...</div>
{/if}

Bundle Size Exploded After SvelteKit 5.0

Our bundle jumped from like 80-something KB to over 200KB after the upgrade. The new reactivity system ships more runtime code by default, and the official migration guide barely mentions this.

Took us a while to figure out what was causing the bloat:

1. Tree-shaking audit: Had to remove a bunch of unused store subscriptions we'd forgotten about
2. Code splitting: Started lazy loading components that weren't critical on first load
3. Bundle analysis: Used vite-bundle-analyzer to find what was eating space

Got it back down to around 90-95KB - still chunkier than v4 but workable.

SSR Performance Tanks Under Load

Our Time to First Byte went from around 200ms to like 700-800ms after upgrading. Wasn't even our code - SvelteKit 5.0's SSR pipeline just eats more CPU than v4. Under load, our Node servers started choking.

Had to rethink our approach. Switched to static generation where we could get away with it:

// +page.js
export const prerender = true; // Static generation
export const ssr = false;      // Client-only rendering

For the dynamic stuff, we threw some aggressive caching at it using ISR patterns. Got TTFB back down to somewhere around 250ms - not perfect but better.

The bottom line: SvelteKit 5.0 is solid now, but the upgrade path is brutal. Every production app will hit at least three of these issues. Plan for a week of debugging, not a day.

Our staging environment was perfect. Zero errors, great performance, team was ready to ship. Then production deployment happened and everything caught fire. If this sounds familiar, welcome to the club of developers who learned the hard way that SvelteKit production deployments have their own special flavor of pain.

The Adapter Configuration Disaster

SvelteKit adapters are supposed to make deployment simple. In reality, they're configuration minefields waiting to explode. I spent 8 hours debugging a 500 error that turned out to be one misplaced line in svelte.config.js.

Our original config looked fine:

import adapter from '@sveltejs/adapter-node';

export default {
  kit: {
    adapter: adapter()
  }
};

But production kept throwing "Cannot find module" errors. The fix required platform-specific configuration:

import adapter from '@sveltejs/adapter-node';

export default {
  kit: {
    adapter: adapter({
      out: 'build',
      precompress: false,
      envPrefix: 'SVELTEKIT_'
    }),
    files: {
      assets: 'static'
    }
  }
};

The envPrefix setting was the killer. Without it, environment variables weren't loading properly in our Docker containers.

Database Connection Pooling Nightmare

This one nearly ended my career. Our PostgreSQL connections were leaking like crazy in production, but only under high load. Local testing was fine. Staging was fine. Production would max out connections within 30 minutes.

The problem? SvelteKit's SSR runs on the server, but I was treating it like client-side code:

// DON'T DO THIS - Creates new connection per request
import { createPool } from '@vercel/postgres';

export async function load() {
  const pool = createPool(); // New pool every time!
  const result = await pool.query('SELECT * FROM users');
  return { users: result.rows };
}

The fix required a singleton pattern:

// database.js - One pool for the entire app
let pool;

export function getPool() {
  if (!pool) {
    pool = createPool({
      connectionString: process.env.DATABASE_URL,
      max: 20,
      idleTimeoutMillis: 30000
    });
  }
  return pool;
}

// +page.server.js - Reuse the same pool
export async function load() {
  const pool = getPool();
  const result = await pool.query('SELECT * FROM users');
  return { users: result.rows };
}

The CORS Nightmare That Won't Die

CORS errors are the herpes of web development - they always come back when you least expect them. SvelteKit's development server masks CORS issues, so they only surface in production when your boss is breathing down your neck.

Our API calls worked perfectly locally:

// This works in dev, breaks in prod
const response = await fetch('/api/users', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify(userData)
});

Production threw CORS errors because our API was on a different subdomain. The fix required proper hooks configuration:

// hooks.server.js
export async function handle({ event, resolve }) {
  if (event.request.method === 'OPTIONS') {
    return new Response(null, {
      status: 200,
      headers: {
        'Access-Control-Allow-Origin': '*',
        'Access-Control-Allow-Methods': 'GET, POST, PUT, DELETE, PATCH, OPTIONS',
        'Access-Control-Allow-Headers': 'Content-Type, Authorization'
      }
    });
  }

  const response = await resolve(event);
  response.headers.append('Access-Control-Allow-Origin', '*');
  return response;
}

Docker Container Memory Limits

Our Docker deployment kept getting killed by the OOM killer. Local development used 200MB RAM. Production containers were hitting 2GB and getting terminated.

The culprit? SvelteKit's SSR pre-rendering was loading our entire product catalog into memory during build. 50,000 products * detailed JSON = memory explosion.

## Before - OOM killer bait
FROM node:18-alpine
RUN npm run build

## After - Memory limits and optimizations
FROM node:18-alpine
ENV NODE_OPTIONS="--max-old-space-size=1024"
RUN npm run build --verbose

We also had to disable pre-rendering for dynamic content:

// +page.js
export const prerender = false; // Don't load everything at build time
export const ssr = true;        // Render on demand instead

SSL Certificate Hell in Reverse Proxy Setups

Our nginx reverse proxy configuration worked perfectly for everything except SvelteKit's WebSocket connections. Hot reload worked in development but broke in our staging environment behind nginx.

The issue was WebSocket upgrade headers:

## nginx.conf - WebSocket support for SvelteKit dev mode
server {
  location / {
    proxy_pass http://sveltekit:5173;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
    proxy_set_header Host $host;
  }
}

Without the upgrade headers, SvelteKit's dev server couldn't establish WebSocket connections, breaking hot reload and dev tools integration.

Environment Variable Leakage

This security issue almost got me fired. We were accidentally leaking server-side environment variables to the client bundle. Database passwords, API keys, the works - all visible in the browser's source code.

The problem was subtle naming:

// BAD - These leak to client side
const PUBLIC_API_URL = import.meta.env.PUBLIC_API_URL;
const DATABASE_URL = import.meta.env.DATABASE_URL; // Leaked!

// GOOD - Only PUBLIC_ variables go to client
const PUBLIC_API_URL = import.meta.env.PUBLIC_API_URL;
const DATABASE_URL = import.meta.env.PRIVATE_DATABASE_URL; // Server only

SvelteKit only exposes PUBLIC_ prefixed variables to the client. Everything else stays server-side. But if you mess up the naming convention, sensitive data ends up in your JavaScript bundle for everyone to see.

The Performance Cliff After 10K Users

Everything worked great until we hit 10,000 concurrent users. Then performance fell off a cliff. Response times went from 200ms to 8 seconds. The problem wasn't our code - it was SvelteKit's default configuration.

The fix required three changes:

1. Enable compression: adapter-node doesn't compress by default
2. Add response caching: Static assets weren't being cached properly
3. Database connection limits: Default pool size couldn't handle the load

// Production-optimized adapter config
adapter: adapter({
  out: 'build',
  precompress: true, // Enable gzip compression
  envPrefix: 'SVELTEKIT_'
}),
prerender: {
  handleHttpError: 'warn',
  crawl: false // Don't auto-discover pages
}

The lesson? SvelteKit's defaults are optimized for development convenience, not production scale. Every production deployment needs custom configuration based on your specific infrastructure and load patterns.