Replit Agent can build and publish a full-stack app in one place. Replit's own troubleshooting guide is clear that a working Preview doesn't guarantee a working published app. These are the differences that bite most often, and how to spot them.
1. Missing production Secrets
Replit's docs say Secrets you set in the Project Editor do not automatically carry over to your published app. Add every production secret in the Publishing pane, then republish. Missing secrets show up as undefined values, failed API calls, database connection errors, login errors or payment failures.
2. "Application failed to respond"
Replit's docs say this means your app isn't listening where Replit expects. A published web server must listen on 0.0.0.0, not localhost or 127.0.0.1, and should bind to the PORT environment variable rather than a hard-coded port. If you set ports in .replit, automatic port detection is turned off.
3. Hard-coded localhost URLs
Code that calls http://localhost:3000/api/... works in the workspace and fails for every real visitor. Replit recommends relative paths or environment-based URLs.
4. Login and payment redirects still point at the dev URL
Replit's guide lists login callback URLs, payment webhooks and redirect settings among the first things to compare between Preview and production. If you use Supabase auth, the same Site URL and Redirect URL rules apply. → Fix it: Supabase auth broke after deploy
5. Cold starts on Autoscale
With Autoscale deployments, Replit says the first request after a quiet period can take a few seconds. That's normal, but a slow homepage still costs you visitors, so it's worth knowing your real load time.
6. Search engines and AI assistants can't read the page
If your frontend is a client-rendered React app, the raw HTML may be nearly empty. → Background: Why is my React SPA not indexed?
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, and a missing title, description, canonical or og:image on your homepage - JavaScript console errors and failed requests on your homepage and key pages, e.g. calls to
localhostthat fail for visitors - 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 Replit, every issue comes with a paste-ready prompt for Replit Agent. The prompts work when pasted into Replit Agent either way.