SvelteKit Production Errors: The Complete Guide for Solo Founders

Why SvelteKit in production is different

SvelteKit's SSR model, routing, and load functions behave differently in production vs dev mode. Errors that never appear locally hit real users. Understanding the SSR boundary is key.

Error in load function

SvelteKit's load() functions run on both server and client. An error here will prevent the route from rendering. Always wrap load functions in try/catch and use SvelteKit's error() helper to send proper HTTP error responses.

Form action errors

SvelteKit form actions that throw unhandled errors return a 500 with no useful context to the user. Always return fail() with a message for form validation errors, and handle unexpected errors with a try/catch.

Adapter-specific issues

SvelteKit has different adapters (Node, Vercel, Cloudflare). Behaviors differ: file system access, environment variables, and edge runtime limitations vary. Test with the same adapter in CI that you use in production.

Environment variable access on client

Public env vars (PUBLIC_*) are accessible client-side. Private env vars are server-only. Accidentally referencing a private env var in client code fails silently — the value is undefined, not an error.

Setting up ProdFix in SvelteKit

Add ProdFix to +layout.svelte (client errors) and hooks.server.js (server errors) to capture the full picture. See the ProdFix docs for the SvelteKit integration guide.