Imported from jpoindexter/theft-kit (debugging-vercel-builds/SKILL.md). Install upstream with npx skills add jpoindexter/theft-kit --skill debugging-vercel-builds. Copyright stays with the author.

Debugging Vercel Builds

Use this skill when a Vercel build or deployment fails. The goal is to identify the root cause before changing code or asking the user for secrets.

Step 1: Inspect the Build Error

Start with the Vercel build output:

• vercel-list-deployments to find the failed deployment ID, target, branch, and timestamp
• vercel-get-deployment-events with the deployment ID to read the build log
• Look for the first error, not the last — cascading failures start from one root cause

If the project ID is unknown, use vercel-list-projects first.

Step 2: Classify the Failure

Read the build log and match it to one of these categories before acting:

Missing environment variables

Symptoms: process.env.X is undefined, Missing required environment variable, NEXT_PUBLIC_* is empty at build time, Stripe/Supabase client initialization crashes.

Triage:

1. Run vercel-get-project-env-vars for the deployment target that failed (production, preview, or development). Check names, targets, and branch scope — never secret values.
2. Compare the env var names against what the app code expects.
3. Identify exactly which keys are missing and which deployment target they are missing from.

Response rules:

• Name the missing keys explicitly and explain why the app needs each one (e.g., "NEXT_PUBLIC_SUPABASE_URL is required at build time for the Supabase client to initialize; without it the client constructor throws").
• Never ask the user to paste secrets into chat. Secrets must flow through secure tooling, not conversation text.
• For Stripe credentials (STRIPE_SECRET_KEY, NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY, STRIPE_WEBHOOK_SECRET, or any STRIPE_PRICE_ID_* / STRIPE_PRODUCT_ID_* var):
  • Ask the user to connect Stripe using cofounder run connect_stripe instead of pasting keys.
  • Distinguish test mode vs live mode: test keys come from a Stripe Sandbox or test-mode dashboard; live keys come from the live-mode dashboard.
  • If you have access to Stripe tools, run cofounder run get_stripe_connection_status first to check whether Stripe is already connected and which credential slots are populated before asking the user to reconnect.
  • Once connected, use cofounder run sync_stripe_env only when env.sync.synced is false or env.sync.missing_keys is non-empty.
• For database/Supabase secrets (NEXT_PUBLIC_SUPABASE_URL, NEXT_PUBLIC_SUPABASE_ANON_KEY, SUPABASE_SERVICE_ROLE_KEY, DATABASE_URL, DIRECT_URL):
  • If you have access to add-secret / get-secret, use that secure tool flow to provision them.
  • If you do not have add-secret, tell the user which keys are missing and why, then ask them to add the values through the platform's secret management or escalate to Engineer.
  • Explain which target (production, preview, development) needs the value.
• For any other secret (API keys, OAuth secrets, signing keys):
  • If you have access to add-secret / get-secret, use that secure tool flow.
  • If you do not have add-secret, name the missing keys and explain what they are for, then ask the user to add them through the platform's secret management or escalate to the Engineer Agent.
  • Never ask the user to paste secret values into chat.

Wrong env var mode or scope

Symptoms: build succeeds but runtime uses wrong values; preview deployment reads production keys or vice versa; staging has no env vars at all.

Triage:

1. Check vercel-get-project-env-vars and compare the target array on each var against the deployment that failed.
2. Verify branch-scoped env vars if the project uses Vercel's branch-based preview scoping.
3. Confirm NEXT_PUBLIC_* vars are set — these must be present at build time, not only at runtime.

Missing Stripe test or live keys

Symptoms: Stripe client throws No API key provided, checkout redirects fail, webhook signature verification fails.

Triage:

1. Determine whether the failing deployment is production (needs live keys) or preview/staging (needs test keys).
2. Check vercel-get-project-env-vars for STRIPE_SECRET_KEY, NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY, and STRIPE_WEBHOOK_SECRET.
3. Verify mode alignment: test keys (starting with sk_test_ / pk_test_) must not be on production, and live keys (sk_live_ / pk_live_) must not be on preview unless intentionally configured.
4. If keys are missing, direct the user to connect Stripe via cofounder run connect_stripe — never accept pasted keys.

Missing database or Supabase URL/keys

Symptoms: connection refused, ECONNREFUSED, Invalid URL, supabaseUrl is required, Prisma/Drizzle migration fails during build.

Triage:

1. Check vercel-get-project-env-vars for NEXT_PUBLIC_SUPABASE_URL, NEXT_PUBLIC_SUPABASE_ANON_KEY, SUPABASE_SERVICE_ROLE_KEY, DATABASE_URL, and DIRECT_URL.
2. Confirm the values point to the correct Supabase project (not localhost or a deleted project).
3. Use the secure add-secret / get-secret flow to provision missing keys.

Failed install or build command

Symptoms: npm ERR!, bun install failed, pip install errors, exit code 1 during the install phase.

Triage:

1. Check whether installCommand or buildCommand in vercel.json or project settings matches the actual package manager.
2. Verify the lockfile matches the package manager (package-lock.json for npm, bun.lockb / bun.lock for Bun, pnpm-lock.yaml for pnpm, yarn.lock for Yarn).
3. Check for private registry or scoped package auth issues.

Lockfile or package manager mismatch

Symptoms: Lockfile was generated with a different package manager, multiple lockfiles detected, Vercel auto-detects the wrong package manager.

Triage:

1. Check which lockfiles exist in the repo root and in the framework root directory.
2. Remove extra lockfiles or set the correct installCommand in Vercel project settings or vercel.json.
3. If the project uses Bun, confirm Vercel is configured for Bun, not npm.

TypeScript, lint, or build errors

Symptoms: tsc errors, ESLint errors, next build type failures, Type error: in the build log.

Triage:

1. Read the specific error messages — these are app code issues, not configuration problems.
2. Do not "fix" code if the root cause is a missing env var that causes a type error (e.g., process.env.X being undefined when the code expects string). Fix the env var first.
3. If the error is genuine app code, fix it with direct repository coding tools using the exact error message, file, and line number.

Missing framework config or wrong root directory

Symptoms: No framework detected, Could not find a Next.js config, build runs from the wrong directory.

Triage:

1. Check vercel.json or Vercel project settings for framework and rootDirectory.
2. Verify the root directory contains the expected config file (next.config.js, next.config.ts, vite.config.ts, etc.).
3. For monorepos, confirm the root directory is set to the app package, not the repo root.

Vercel project or team mismatch

Symptoms: deployment targets the wrong project, env vars belong to a different project, vercel link points to a stale project.

Triage:

1. Check .vercel/project.json in the repo for projectId and orgId.
2. Compare against the Vercel dashboard project.
3. Re-link if the IDs do not match.

Domain or env mismatch for webhook URLs

Symptoms: webhooks work in preview but fail in production, or vice versa; APP_URL or NEXT_PUBLIC_APP_URL points to the wrong domain.

Triage:

1. Check vercel-get-project-env-vars for APP_URL, NEXT_PUBLIC_APP_URL, NEXTAUTH_URL, or similar.
2. Confirm the value matches the actual domain serving each deployment target.
3. For Stripe webhooks, verify the webhook endpoint URL matches the production domain, not a preview URL.

Stripe Payments Notes

When this skill is activated for Engineer-owned Stripe or payments work:

• If Stripe credentials are missing, run cofounder run get_stripe_connection_status --include_payments_implementation true first to check existing connection state before asking the user to reconnect.
• Distinguish test mode vs live mode credentials. Test keys must come from a Stripe Sandbox or test-mode dashboard. Live keys come from the live-mode dashboard.
• Use cofounder run connect_stripe --mode test or cofounder run connect_stripe --mode live when credentials need to be connected.
• Only run cofounder run sync_stripe_env when env.sync.synced is false or env.sync.missing_keys is non-empty.
• Do not assume missing Stripe env vars mean Stripe is not connected — the connection may exist but env sync may be pending.
• For non-Stripe secrets (database URLs, Supabase keys, other API keys), use the secure secret tool flow when available. Otherwise name the missing keys and explain why they are needed without asking the user to paste values into chat.

Engineer Agent Notes

When this skill is activated by the Engineer Agent:

• Identify whether the problem is app code vs configuration before changing code. Most Vercel build failures are caused by missing env vars, wrong project settings, or lockfile issues — not by app code bugs.
• Do not "fix" code when the root cause is missing configuration. A type error caused by process.env.X being undefined is an env var problem, not a TypeScript problem.
• If the error is genuinely in app code, fix it with a narrow brief: the exact error, file, line, and what the fix should achieve.
• If env vars need to be added, use the secure tool flow — do not hardcode fallback values, add placeholder strings, or commit secrets.

Output Shape

Report:

• the failure category from the list above
• the specific error message or log lines that identify the cause
• exactly which keys, files, or settings are wrong or missing
• the recommended fix with secure tool references for any credentials
• verification steps: what to check after the fix (redeploy, check build log, confirm env var presence)