Next.js Deployment Troubleshooting: A Step-by-Step Guide

When a Next.js deploy breaks, guessing wastes time. This guide gives you a repeatable flow.

1. Confirm the Failure Context

1. Note the platform (Vercel, Netlify, Docker, custom VPS).
2. Copy the failing build logs locally (do not rely on scrolling UIs).
3. Identify: build phase vs runtime error.

2. Reproduce Locally in Production Mode

npm ci
npm run build
npm start
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Look for:

• Usage of window / document / navigator during build (SSR mismatch).
• Dynamic imports that assume client APIs.

3. Environment Variables Audit

Run:

npx envinfo --system --binaries --browsers
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Then compare required vars against .env.example or README.

Checklist:

• Are secrets available in build env (NEXTPUBLIC* vs server-only)?
• Any trailing spaces or quoted values? (They matter in some shells.)

4. Dependency & Lock Integrity

• Ensure only one lockfile (package-lock.json OR pnpm-lock.yaml).
• Remove accidental file: or Git deps unless intentional.
• Clear caches if native modules present: npm rebuild.

5. Common Build Error Patterns

Symptom	Likely Cause	Fix
ReferenceError: window is not defined	Client API at module top-level	Guard with typeof window !== 'undefined' or lazy import
Cannot find module	Missing optional peer / path typo	Install peer or fix import path
Memory OOM	Huge images / large JSON in repo	Optimize assets, code split, stream large data
404 after deploy	Wrong basePath or routes not pre-rendered	Re-run next build; verify dynamic paths

6. Analyze Bundle & Output

Use:

NEXT_PRIVATE_DEBUG=1 npm run build
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Or add @next/bundle-analyzer to inspect bloat.

7. Edge vs Node Runtime Mismatch

If using experimental edge functions, confirm all imports are edge-compatible (no fs, crypto.randomBytes fallback, etc.).

8. Fallback Strategy

If blocked:

1. Roll back to last good deploy.
2. Create a branch to isolate suspected change.
3. Bisect with git bisect if history is long.

9. Add Preventative Automation

• CI build + lint on PRs.
• PR template asking: new env vars? new large assets?

10. Debug Template

Copy this for future incidents:

Context:
Platform:
Error excerpt:
Recent changes:
Hypotheses:
Tests run:
Resolution:
Preventative action:
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Ship calmer. Debug faster.