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Grok Bot+18 agentsVercel + Node/Express Deploy Doctor
Diagnose and fix Node.js/Express deployment errors, routing conflicts, and filesystem issues on Vercel.
Free
Vercel + Node/Express Deploy Doctor
Example session with this skill installed
My Express app works fine on my laptop, but when I deploy to Vercel, my /api/users route gives a 404, and my file uploads to the 'uploads' folder aren't appearing. Here is my server.js.
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- Compiled the vercel + node
- Generated the UI component
The 404 on your /api/users route and the disappearing uploads are both symptoms of the transition from a persistent server to Vercel's serverless architecture. In your server.js, you are using app.listen(), which Vercel ignores, and you are attempting to write to the local filesystem, which is read-only and non-persistent in a serverless function.
Vercel Deployment Diagnosis
| Issue | Root Cause | Evidence | Required Fix |
|---|---|---|---|
| Routing 404 | Entry Point Mismatch | server.js uses app.listen() instead of exporting the Express instance. | Export the app via module.exports = app; and add a vercel.json rewrite. |
| Missing Uploads | Ephemeral Filesystem | Writing to /uploads folder. Serverless storage is non-persistent. | Use Vercel Blobs, AWS S3, or another external object storage provider. |
| Function Lifecycle | Serverless Model | App expects a long-running process to handle multiple requests. | Restructure to handle stateless, atomic requests per function invocation. |
Implementation Details
Add a vercel.json to your root directory to route all traffic to your Express entry point:
{
"rewrites": [{ "source": "/(.*)", "destination": "/server.js" }]
}
Update the bottom of your server.js:
// Remove app.listen(3000...);
module.exports = app;
Next steps
- Move any file upload logic to use an external SDK (like
@vercel/blob). - Update
server.jsto export theappobject instead of calling.listen(). - Deploy via
vercel --prodto apply the newvercel.jsonrouting rules.
vercel-node-express-deploy-doctor.tsx
TSX · React component
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About this skill
Deploying Node.js or Express applications to Vercel often leads to failures that are impossible to replicate locally. This skill identifies the disconnect between a traditional long-running server and Vercel's serverless execution model, pinpointing why builds fail, routes 404, or files disappear. It eliminates the guesswork of debugging production-only errors by focusing on the architectural constraints of the Vercel runtime.
What it does
- Diagnoses build failures by analyzing Vercel dashboard logs for case-sensitivity issues, Node version mismatches, and missing build steps.
- Fixes routing errors where Express apps work locally but return 404 or 500 on Vercel due to incorrect
vercel.jsonrewrites or entry point exports. - Resolves filesystem issues by identifying illegal local writes to read-only serverless environments and suggesting external storage alternatives.
- Optimizes timeouts by identifying operations like LLM calls or slow DB queries that exceed Vercel's plan-specific function limits.
- Audits environment variables to ensure proper scoping across Production, Preview, and Development environments.
How it works
- Analyze logs and config to compare local behavior against Vercel's serverless runtime constraints.
- Identify the failure category from build logs, routing mismatches, filesystem writes, or execution timeouts.
- Apply the triage protocol to determine if the issue is architectural, such as
app.listen()usage or non-persistent disk writes. - Provide a concrete fix including code changes,
vercel.jsonupdates, or environment variable adjustments.
Frameworks & tools
Node.js, Express, Vercel CLI, Vercel Serverless Functions, and vercel.json configuration.
Why this beats prompting it yourself
Generic prompts often miss the nuances of the serverless lifecycle, such as why a file write "succeeds" but the data vanishes instantly. This skill uses a targeted triage protocol specifically for the Vercel runtime, saving hours of chasing phantom bugs that don't exist in your local environment.
Use cases
- Migrating a legacy Express server to a serverless Vercel deployment.
- Troubleshooting a route that works in
npm run devbut 404s in production. - Fixing "missing" files or database uploads that aren't persisting between requests.
- Debugging intermittent 504 errors on long-running API calls.
Known limitations
Does not debug general JavaScript logic errors unrelated to the Vercel environment. Limited to Node.js and Express; does not support non-Vercel hosting providers.
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