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Next.js fixes

Next.js errors that survive the deploy: hydration, env vars, and server routes.

13 fixes, most common first.

  1. Rank 3. Environment variables are undefined in production

    Works locally but env vars are undefined once deployed? Add them to your host, use the VITE_ or NEXT_PUBLIC_ prefix for browser code, then redeploy.

  2. Rank 6. Webhook signature verification failed. No signatures found matching the expected signature for payload

    Stripe webhook fails signature check? Usually the wrong whsec_ secret or your framework changed the raw request body. Here's the fix.

  3. Rank 7. Stripe webhook failing: 404, 307/308 redirect, timeout or 500

    Stripe shows webhook deliveries Failed with 3xx, 4xx, 5xx or timed out? Point it at the exact final URL and respond 200 fast.

  4. Rank 12. Module not found: Can't resolve (build works locally, fails on Vercel)

    Vercel build fails with "Module not found: Can't resolve" but works on your Mac/PC? Usually filename casing. How to find and fix it.

  5. Rank 13. Changed an environment variable on Vercel but the live site still uses the old value

    Updated a key in Vercel env vars but nothing changed? Env var changes only apply to new deployments. Redeploy and check the right environment.

  6. Rank 21. Vercel FUNCTION_INVOCATION_TIMEOUT (504) on API routes or AI calls

    Vercel API route times out with 504 FUNCTION_INVOCATION_TIMEOUT? Common with slow AI calls. Raise maxDuration, stream, or move work to background.

  7. Rank 22. 500: INTERNAL_SERVER_ERROR / FUNCTION_INVOCATION_FAILED on Vercel

    Live site shows 500 FUNCTION_INVOCATION_FAILED but works locally? Your server code crashed. Read the runtime logs, then check env vars.

  8. Rank 23. too many connections / remaining connection slots are reserved (Supabase + serverless)

    Database errors like "too many connections" after deploying to Vercel? Use Supabase's transaction pooler (port 6543) for serverless functions.

  9. Rank 24. Hydration failed because the server rendered HTML didn't match the client

    Next.js "Hydration failed" error? The server and browser drew different HTML. Fix nesting, window/localStorage use and dates.

  10. Rank 33. ReferenceError: window is not defined

    Next.js crashes with window is not defined? The page is rendered on the server, where window does not exist. Read it after the component mounts.

  11. Rank 34. Supabase error: JWT expired

    API calls fail with JWT expired? The Supabase access token is meant to be short-lived. Refresh the session once, and don't refresh it twice at the same time.

  12. Rank 35. Clerk sign-in keeps redirecting in a loop

    The app bounces between the page and Clerk sign-in forever? The redirect target is protected too, or the force-redirect URL points back at the gate.

  13. Rank 39. Prisma on Vercel: query engine not found

    Vercel runs the app and Prisma can't find its query engine? The Rust engine file was left out of the serverless bundle, or Prisma Client was generated for the wrong machine.

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