@crumbjs/core

CrumbJS is a lightweight API framework for Bun focused on backend development. It layers configuration, validation and documentation on top of Bun's built-in router while keeping a familiar Express-like developer experience. Validation is powered by Zod and every route can be automatically documented through OpenAPI.

Features

• Built for Bun.serve and only targets backend APIs
• Zod-based validation for bodies, params, queries and headers
• Automatic OpenAPI 3.1 document generation and UI (Swagger or Scalar)
• Simple middleware system and optional global middlewares
• Zero lock-in: use as little or as much as you need

Installation

You can scaffold a new project with the official template:

bun create crumbjs myapp

This runs the create-crumbjs script which copies a template, installs dependencies with Bun and prints common commands.

Alternatively, add the framework to an existing Bun project:

bun add @crumbjs/core

Quick start

import { App, spec } from '@crumbjs/core';
import { z } from 'zod';

const app = new App('api').get('/hello/:name', ({ params: { name } }) => ({ name }), {
    params: z.object({ name: z.string() }),
    responses: [spec.response(200, z.object({ name: z.string() }))],
});

app.serve();

Run your server with Bun:

bun run src/index.ts

OpenAPI documentation is served automatically at http://localhost:8080/openapi by default.

Environment variables

Configuration can be supplied via environment variables or programmatically. The following variables are supported:

Example .env:

PORT=3000
OPENAPI=false

Programmatic configuration

You can also override settings in code using the exported config helper:

import { config } from '@crumbjs/core';

config.merge({ port: 3000, withOpenapi: false });

Philosophy

CrumbJS is inspired by modern frameworks like Hono and Elysia but has a distinct goal: a clean, Bun-only backend layer with first-class validation and automatic documentation. It does not implement an HTTP router—instead it relies on Bun's own routing and adds typed validation, middleware chaining and OpenAPI generation on top.

License

MIT