Failed to load module script: Expected a JavaScript-or-Wasm module script but the server responded with a MIME type of "text/html"

Find which URL returned HTML

Click the script URL in the console error, or filter the Network tab by JS: the failing request has status 200 and a body that starts with <!doctype html>. From a terminal, the content-type header says the same:

curl -sI https://app.example.com/assets/index-DhsVusns.js | grep -i content-type
content-type: text/html; charset=utf-8

Vite base

Vite's base is the public path the app is served from, / by default. Built with base /app/ and served from /, the HTML asks for /app/assets/..., which does not exist, and the fallback answers with index.html. Vite documents ./ for embedded deployment; in our run it broke on a deep link, because /settings/profile asked for /settings/assets/index-....js. Use / unless the app really lives under a subpath, and then set that subpath.

// vite.config.ts
export default defineConfig({
  base: '/', // or '/app/' when the site is served from /app/
})

Is the rewrite the cause on Vercel or Netlify?

Usually not by itself. Vercel serves real files before it applies a rewrite, and Netlify by default does not let a rewrite shadow a file that exists. So the script came back as HTML because no file exists at that path in the deployment: a wrong base, a different output directory, or a hash from an older build. Keep the single-page-app rewrite for deep links; see the index.html rewrite for Vite apps on Vercel.

The same error in Firefox and Safari

Firefox logs "Loading module from “<url>” was blocked because of a disallowed MIME type (“text/html”)." Safari, through WebKit, rejects with "'text/html' is not a valid JavaScript MIME type for module script '<url>'." Older Chrome versions print the same sentence as today with "JavaScript module script" instead of "JavaScript-or-Wasm module script".

Checklist

How to fix it, then check the fix

1. 01Copy the failing script URL from the console and request it with curl -sI: note the status and content-type.
2. 02Compare its path with the files in your build output, dist/assets for Vite.
3. 03Set Vite's base to the path the site is served from, / for a site at the root of its domain.
4. 04Check the host deploys the build output: the output directory on Vercel or the publish directory on Netlify is dist for Vite.
5. 05Rebuild and redeploy, then load a deep link in a private window and confirm the page renders and the script returns a JavaScript content-type.

Related

Related guides

• VercelWhy does my Vite or React app 404 on refresh on Vercel?Guide
• Failed to fetch dynamically imported moduleGuide
• VercelVercel for AI-built apps: the deploy, env var and function questionsGuide

Sources

Official docs behind this checklist

• Vite, Shared options (base)
• Vite, Building for production (public base path)
• Netlify, Rewrites and proxies (shadowing)
• Chromium source, module_script_fetcher.cc
• Firefox source, security.properties
• WebKit source, ScriptModuleLoader.cpp

FAQ

FAQ for builders before launch