Short answer
Open the browser’s Network tab, reload, and look at the failed request. If its address contains undefined, the Supabase URL never reached the published build. If the address is right, check whether the project is paused, whether an Edge Function is failing its CORS check, and whether a browser extension is blocking the call.
What the failed request tells you, and the fix
01
The Supabase URL is missing from the published build
“Variables prefixed with VITE_ will be exposed in client-side source code after Vite bundling.” Vite, Env variables and modes
In a Vite app, only variables whose names start with VITE_ reach the code in the browser, and their values are written into the build when it runs. If the variable is missing where the published app is built, or has another name, the client starts with undefined. We also see this as a request to an address like https://undefined/ in the Network tab. Set the variable where the app is built, then build and publish again.
02
The project is paused
“Supabase pauses Free Plan projects that show low activity over a 7-day period to save server resources.” Supabase, Project pausing
A paused Free Plan project doesn’t answer requests, so every call from the app fails. Restore the project from the Supabase dashboard.
03
An Edge Function fails its CORS check
When the failing call goes to /functions/v1/, the browser may be blocking the function’s answer because it lacks CORS headers. In our experience the missing headers are often on an error response or the OPTIONS preflight. The console then shows a CORS message next to Failed to fetch. The Edge Function CORS page below covers the fix.
04
A browser extension blocks the request
“Although Failed to Fetch can occur for a variety of reasons a common one in the dashboard or when using update code is related to a browser extension.” Supabase, Failed to Fetch: browser extension
Supabase names browser extensions as a common cause, in the dashboard or when using update code. Try the published app in a private window with extensions turned off. If it works there, the app is fine and the extension is the problem.
Checked against the vendor’s docs on : Vite docs: Env variables and modes; Supabase docs: Project pausing; Supabase docs: CORS support for Edge Functions; Supabase troubleshooting: Failed to Fetch and browser extensions.
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Related: Lovable app blank after publishing · Environment variables undefined in production · Supabase Edge Function CORS error · Supabase project paused