v0 generates Next.js apps and deploys them to Vercel. Most issues after launch aren't bugs in the generated code. They're configuration differences between the v0 preview, Vercel preview deployments and production.
1. Environment variables changed, the site didn't
Vercel's docs say environment variable changes only apply to new deployments, so you have to redeploy after changing a value. NEXT_PUBLIC_ variables are inlined into the browser bundle at build time. If one is missing at build time, the browser gets undefined. Check that each variable is scoped to the right environments: Production, Preview, Development.
2. Supabase is "missing" in one environment
A common report: Supabase works in production but the v0 preview or a Vercel preview deployment says the URL or anon key is missing. Make sure NEXT_PUBLIC_SUPABASE_URL and NEXT_PUBLIC_SUPABASE_ANON_KEY exist for every environment you use, then redeploy. Add your production domain and your preview pattern to Supabase's Redirect URLs.
→ Fix it: Supabase auth broke after deploy
3. Page titles disappear after adding interactivity
In the Next.js App Router, metadata and generateMetadata are only supported in Server Components. If a page file gains "use client" (for a form, a modal or useState), its metadata export no longer works. Keep the page a Server Component and move the interactive part into a child component.
4. Sharing the wrong URL
Vercel preview deployments are meant for review. Share and link your production domain, so search engines and visitors land on the canonical address.
5. A custom domain shows 404 but the vercel.app URL works
Vercel's own guide suggests testing both URLs. If the .vercel.app address works and your custom domain doesn't, the deployment is fine and the problem is DNS or domain configuration.
What FlowCanary's free check looks at
- Homepage status, speed, redirects and HTTPS
- Key pages found: signup, login, pricing, checkout, contact, privacy, terms
- Broken links on your homepage
- Raw HTML vs a real browser, with a "Google may see a blank page" warning
- robots.txt, sitemap,
noindex(meta tag orX-Robots-Tagheader), and a missing title, description, canonical or og:image on your homepage - JavaScript console errors and failed requests on your homepage and key pages
- Forms that appear after load, and pages that overflow on phones
FlowCanary looks for hosting and code fingerprints to tell which builder you used. When it spots v0, every issue comes with a paste-ready prompt for v0. The prompts work when pasted into v0 either way.