Hono API on Vercel crashes with ERR_MODULE_NOT_FOUND when importing server files

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vercel • hono • serverless

Summary

I deployed a full-stack application using Hono for the API and Vite/React for the frontend. The deployment succeeds, but I can only the serverless function crashes in the preview page.

From the logs, it appears Vercel cannot resolve a module imported from outside the /api directory.

Project Setup

My project structure looks like this:

honno-react-demo
│
├ api
│   └ index.ts
│
├ server
│   ├ app.ts
│   └ routes
│       └ expenses.route.ts
│
├ frontend
│   ├ src
│   └ dist
│
└ package.json

The API entry file is:

/api/index.ts

import app from "../server/app";
import { handle } from "hono/vercel";

export const GET = handle(app);
export const POST = handle(app);
export const PUT = handle(app);
export const PATCH = handle(app);
export const DELETE = handle(app);
export const OPTIONS = handle(app);

My Hono app is defined in:

/server/app.ts

import { Hono } from "hono";
import { logger } from "hono/logger";
import { expensesRoute } from "./routes/expenses.route";

const app = new Hono();

app.use("*", logger());

const apiRoutes = app
  .basePath("/api")
  .route("/expenses", expensesRoute);

export default app;
export type ApiRoutes = typeof apiRoutes;

Current Behavior

The deployment completes successfully and the frontend loads, but when I call the API endpoint:

/api/expenses/total-spent

the request returns:

500: INTERNAL_SERVER_ERROR
Code: FUNCTION_INVOCATION_FAILED

Serverless Function Logs

Error [ERR_MODULE_NOT_FOUND]: Cannot find module '/var/task/server/app'
imported from /var/task/api/index.js

So it seems the serverless function cannot resolve:

../server/app

What I’ve Tried

  • Verified that the path is correct locally
  • Confirmed the app works locally using vercel dev
  • Removed any Bun-specific imports (hono/bun)
  • Checked that hono is listed in dependencies

Questions

  1. Does Vercel serverless only bundle files inside /api by default?
  2. Should the Hono app be moved into the /api directory?
  3. Is there a recommended structure for deploying a Hono API with Vercel?

Any guidance would be greatly appreciated. Thank you!

Here is the link to my repository: GitHub - lancer751/hono-bun-react--demo · GitHub

2

If you’re having trouble deploying an Express app, this guide can help.

You can also ask v0 for suggestions tailored to your own project setup.

7

It looks like the issue is related to how Vercel bundles and deploys serverless functions, and it seems the app in the /server directory isn’t getting bundled correctly as part of the function in /api.

8

The module resolution system requires explicit file extensions in imports.

Could you try adding file extensions to the imports? Like:

import { something } from './server/file.js'