Hono app fails in production on Vercel

[DoneWithWork]

Summary

Hi everyone. I currently have turborepo setup with Next Js, and Hono for the backend. I created a package called @repo/db under internal packages so I can reuse the prisma client generation for both frontend and backend under one schema.ts.

Currently Next js deployment has no issue.
But the Hono one deploy successfully on vercel but it outputs this error
Cannot find module '/var/task/apps/backend/node_modules/@repo/db/src/index.ts' imported from /var/task/apps/backend/api/index.js
This causes the app to crash as I rely on @repo/db for the database.
I have no issue running this locally, only in production.
I have attached a link to the github page where my project lives.

Addtionally, right now it's a bit annoying as to test hono with vercel locally I run vercel dev, but this requires me to link to a vercel account and in doing so I have to set the base directory on vercel to apps/backend, but for some reason after setting this I cannot run vercel dev. Any way round this?

Thanks in advance.

Additional information

https://github.com/DoneWithWork/IoT-Project

Example

No response

[GaneshVarma1]

Hi @DoneWithWork

Thanks for sharing your setup!
This is a classic Turborepo + Vercel issue where internal packages are not properly built or included in the deployed output, especially for backend runtimes like Hono running in a separate apps/backend directory.

---

Root Cause

Your Vercel serverless function is trying to import this path:

/var/task/apps/backend/node_modules/@repo/db/src/index.ts

But it doesn’t exist in the deployed bundle. This happens because:

• Vercel doesn’t automatically build or bundle internal packages like @repo/db unless explicitly told to.
• *.ts files are also not included unless compiled.
• Your backend app depends on a workspace package that needs to be bundled, not just linked.

---

✅ Solution Steps

1. Move @repo/db to dependencies in apps/backend/package.json

Even though it's a workspace package, make sure it's listed like this:

{
  "dependencies": {
    "@repo/db": "*"
  }
}

This helps Vercel recognize it as a real dependency and triggers its inclusion.

---

2. Ensure You Are Building All Internal Packages

In apps/backend/vercel.json (or root), define a build script if not already done:

{
  "buildCommand": "pnpm turbo run build --filter=apps/backend..."
}

Make sure your internal @repo/db has a build script:

{
  "name": "@repo/db",
  "scripts": {
    "build": "tsc"
  }
}

And that it outputs to a usable folder (dist/), not raw src/.

---

3. Use tsup or esbuild for Better Bundling (Optional but Recommended)

Instead of relying on raw TypeScript output, use tsup in @repo/db:

pnpm add -D tsup

// @repo/db/package.json
{
  "scripts": {
    "build": "tsup src/index.ts --format esm,cjs --dts --out-dir dist"
  },
  "main": "dist/index.js",
  "types": "dist/index.d.ts"
}

Update Hono backend to import from @repo/db without relying on .ts files.

---

4. Fix Local vercel dev Issues (Optional)

If setting base directory breaks vercel dev, use a vercel.json in the root:

{
  "projects": {
    "apps/backend": {
      "source": "apps/backend",
      "outputDirectory": "dist",
      "buildCommand": "pnpm turbo run build --filter=apps/backend..."
    }
  }
}

Also, use Turborepo's Remote Cache if builds are inconsistent.

---

✅ Summary

• Add @repo/db to dependencies in the backend app
• Make sure @repo/db builds to JS (dist/) and not just .ts
• Use tsup for clean ESM/CJS bundling
• Point your imports to built output, not raw .ts files
• Confirm build commands in vercel.json include required packages

---

Let me know if you’d like me to send a PR i can help you.

[stefan-garofalo]

Hi @DoneWithWork I just merged a PR that might help:

• Feat: Deploy Backend to Vercel with Rspack Bundling stefan-garofalo/foodwise#12

I used rspack to bundle workspace dependencies and serve the bundled dist through Vercel’s entrypoint at /api/index; I had to build it as CommonJS because some internal deps I use aren’t compatible with ESM.

Also, the hono + turborepo example on Vercel is broken as-is (it has no build step and was likely copied from the plain hono example), so it won’t work for a turborepo setup. We should open an issue to get it fixed and improve the docs

[nktnet1]

Had the same issue - it appears that vercel build is not bundling internal/workspace packages correctly.

I ended up using a bundler (tsdown) in combination with vercel primitives and prebuilt deployment.

Solution was inspired by this answer on Stackoverflow.

More details about how I applied it to my specific project here:

• How to deploy the backend (hono) server on vercel? nktnet1/rt-stack#21 (reply in thread)