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A Supabase login that works locally but appears to disappear on Vercel usually points to a difference in deployment configuration or in how the app creates, stores, refreshes, or verifies a session. There is no single root cause established by the symptom alone. Start by finding out whether the session is missing in the browser or only on the server, then check the deployed project settings, callback flow, cookies, and caching in that order.
The fix depends on your architecture: a client-only app can use browser storage, but a server-rendered app needs a session the server can receive, typically through cookies. The checks below distinguish those cases and link to the relevant Supabase and Vercel guidance.
First determine where the session disappears
After a successful sign-in, check the browser and the server separately. Does the browser still show the user as signed in while a server-rendered page treats the request as anonymous? Or does the browser itself lose the session after the redirect or reload? Those are different failures.
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- Server-rendered or hybrid app: use Supabase’s framework-specific browser and server clients with cookie-based session handling. The server needs the browser’s session cookies, and refreshed cookies must be returned to the browser.
Supabase’s server-side rendering guide and SSR client guide describe these distinct patterns. Don’t apply a server-cookie diagnosis to a client-only app unless the failing check actually runs on the server.
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Check Vercel’s deployed Supabase settings
A deployment can use different credentials from local development—or point at a different Supabase project—because Vercel environment variables are scoped separately for Production, Preview, and Development. Compare the values used by the deployed build with the project URL and public key used locally. Also confirm the variable names match your code; in a Next.js client bundle, variables intended for browser use need the NEXT_PUBLIC_ prefix.
- In Vercel, open the project’s Settings → Environment Variables.
- Check the Supabase URL and publishable or anon key in the environment where the failing deployment runs: Production or Preview.
- Compare those values and names with the app’s configuration and the intended Supabase project.
- After changing a variable, create a new deployment. Vercel says environment-variable changes do not apply to previous deployments; they apply to new deployments. See Vercel’s Environment Variables documentation.
If the deployed app is configured for another project, its users, redirect settings, and session state will not match the project you inspected locally. Missing or stale configuration is another possibility; the symptom alone cannot establish which applies.
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Verify the redirect and PKCE callback
For OAuth, magic links, signup confirmation, or password recovery, inspect the actual URL the browser returns to after the provider or email step. The production origin and callback path must be allowed in Supabase’s Redirect URLs. If the flow relies on the Site URL because no redirect is supplied, check that it is the deployed site, not localhost. Supabase’s password-based Auth guide explains that an unspecified redirect uses the configured Site URL and that the default is localhost:3000 until changed.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsFor a server-side PKCE flow, the browser must retain the verifier created when sign-in began, and the callback must exchange the returned code for a session. Supabase documents that the verifier is required for this exchange: the code in the URL and verifier are sent to the Auth server together. The code is single-use, valid for five minutes, and must be exchanged in the same browser and device that started the flow. See the PKCE flow documentation.
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- Confirm the callback URL is permitted and is the URL actually used by the deployed flow.
- Check that the verifier cookie reaches the callback route.
- Confirm that the callback exchanges the code, commonly with
exchangeCodeForSession(code). - If the handler redirects or creates a new response, preserve the relevant
Set-Cookieheaders. Otherwise the provider can return successfully while the app fails to retain the verifier or resulting session.
For SSR, make sure refreshed cookies reach the browser
In a server-rendered app, authentication is not complete merely because a server helper refreshed a token. The response carrying the refreshed cookies must reach the browser. If framework code creates a replacement response without copying the cookies, the next request may arrive without the refreshed session.
For Next.js, Supabase’s current SSR setup uses a Proxy to refresh tokens and make the refreshed cookie available to Server Components. The filename depends on the Next.js version: use proxy.ts in Next.js 16, or middleware.ts in Next.js 15 and earlier. Follow the current Supabase SSR instructions and the API matching the version installed in your app.
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When adapting a handler, return the response that contains the latest cookies. If you must construct another response, copy the cookies and relevant cache headers onto it. Supabase describes @supabase/ssr as beta, with an unstable API, so avoid relying on an older helper pattern without checking the current guide and installed package version. Its server-side Auth page also describes the package status.
Rule out shared caching and cross-request client state
A cached response can cause a session problem that looks intermittent or user-specific. Supabase warns that if a response containing refreshed auth cookies is cached and served to another user, it could expose one user’s refreshed token to someone else. Authenticated responses should not be shared through a CDN or ISR cache in a way that reuses user-specific cookies.
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- For authenticated Next.js pages, Supabase recommends
export const dynamic = 'force-dynamic'. - Where the framework or library setup does not already apply appropriate cache headers, use
Cache-Control: private, no-storefor relevant authenticated responses. - Preserve cache headers provided with refreshed cookies. Supabase says
@supabase/ssrv0.10.0 passes cache headers tosetAllwhen a token refresh occurs.
See the caching details in Supabase’s advanced server-side Auth guide.
Also create the Supabase server client inside the request handler, rather than keeping a user-specific client or state in module scope. Supabase warns that Vercel Fluid Compute can reuse warm server instances; a shared client can therefore cross request boundaries. Keep each request’s auth state isolated.
Use a server-verified identity check for protected data
A cookie-shaped session is not, by itself, proof that a request is authorized. For protected pages and data, Supabase recommends getClaims() to verify claims. Use getUser() when you need a fresh user record confirmed by the Auth server. getSession() can be useful when code needs tokens, but do not rely on its user object alone for authorization when the underlying storage can be influenced by the client. The distinctions are covered in the SSR client guide.
Follow the symptom to the likely layer
| What you observe | First place to investigate |
|---|---|
| Browser code remains signed in, but server-rendered pages see no user | Architecture and session storage: determine whether the server receives cookies, or whether the app relies only on local storage. |
| Sign-in returns to the site, but no session is established | Redirect allow list, callback path, PKCE verifier cookie, and code exchange. |
| Session works until a refresh or later request | SSR refresh Proxy or middleware, and whether its response with updated cookies reaches the browser. |
| Only a deployed environment fails | Vercel Production or Preview variables, their names and values, and whether a new deployment followed any change. |
| Unexpected sign-outs or user-specific behavior across requests | Shared caching of authenticated responses and module-scoped server clients. |
These are diagnostic branches, not a claim that one particular cause explains every Vercel deployment. The framework, auth method, package versions, deployed environment, callback URL, and browser-versus-server symptom determine which branch fits.
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