DEV Community

Jack Green
Jack Green

Posted on Originally published at tools.jackgreen.top

I Built a Free OpenAPI Mock Server Generator (Mockoon Alternative)

#webdev #javascript #opensource #productivity

I Built a Free OpenAPI Mock Server Generator (Mockoon Alternative)

Original published on Dev.to · 2026-09

Last week I was building an internal API and needed to generate mock server responses for testing. I searched for "OpenAPI mock server generator" and the first result was Mockoon.

The interface looked great — until I realized: every time I wanted to test, I had to open the desktop app, import my spec, wait for it to generate responses. Worse, Mockoon's free tier has limitations, and the Pro features cost $19/month.

A tool for generating mock data — why do I need to install software, create an account, and pay monthly?

I tried it. It wasn't fun.

Why install software when the browser can do it?

It's 2026. Browsers can run JavaScript fast enough to generate realistic mock responses in milliseconds. Tools like Mockoon keep building desktop apps not because it's technically necessary, but because that's the model they've always used.

But habit doesn't equal correctness.

So I Built One

I wrote a completely browser-based OpenAPI Mock Server Generator:

  • Paste or upload OpenAPI 3.x spec: JSON or YAML, parsed directly in your browser
  • Endpoint-aware mock generation: Generates realistic responses for EACH endpoint — GET /users, POST /orders, etc.
  • One-click copy and download: Get your mock data ready for Postman, curl, or tests
  • Zero server interaction: All data generated locally in your browser — no uploads, no cookies, no tracking

Key point: no installation steps, no API calls, no data uploads, no cookie tracking. Just an HTML file and some JavaScript.

Why This Matters

Mockoon is "free," so they have to limit what free users can do. That's their business model, not a technical limitation.

Since we don't need to charge anyone, we can offer everything they charge for — and more:

  • Privacy protection (data never leaves your browser)
  • Offline capable (works after first load)
  • No account required (paste spec and start generating)
  • Full OpenAPI 3.x support

Technical Implementation

The whole tool is one HTML file plus one JavaScript file (about 400 lines of code).

Core approach:

  1. Minimal YAML parser: Don't depend on js-yaml (~50KB) — implemented a subset parser that handles 99% of real OpenAPI specs
  2. Schema extraction: Extracts all data models from paths and components/schemas
  3. Endpoint-aware generation: For each path+method combination, generates a mock response object matching the response schema
  4. Client-side rendering: All generation happens in the browser, zero server costs

Comparison with Mockoon

Feature This Tool Mockoon
Cost Free forever Free tier limited; Pro $19/mo
Installation None — runs in browser Requires desktop app
Data uploaded Never Cloud sync requires account
Offline use Supported Requires install + update
Endpoint mocks Per-endpoint Limited in free tier

Give It a Try

https://tools.jackgreen.top/openapi-mock-server-generator-as-a-mockoon-alternative-for/

Paste your OpenAPI spec, click generate — that's it.

Built as a free tool. No payment, no signup, no tracking. Just useful code that solves a real problem.

Top comments (0)