Open Source Postman for MCP

The missing devtools for Model Context Protocol servers.

Test and debug MCP servers with a desktop GUI that actually works with stdio transport — because let's be honest, most MCP servers aren't HTTP.

The Problem

MCP is amazing, but testing servers is painful:

• No visual tools — You're stuck with JSON-RPC logs in a terminal
• No schema inspector — "What args does this tool need?" Good luck figuring it out
• No request history — Replaying requests means re-typing everything
• HTTP-only tools — 90% of MCP servers use stdio, which most API clients can't handle

Open Source Postman for MCP solves this. Connect to any MCP server (stdio, HTTP, or SSE), explore its tools with auto-generated forms, execute requests with one click, and save everything to history.

Features

• Multi-transport support — stdio, HTTP, and SSE (finally, a tool that handles stdio properly)
• Schema inspector — View full JSON schemas, auto-generated input forms
• AI auto-select — Describe what you want in plain English, let Claude pick the right tool + args
• Request history — Every request saved to SQLite with one-click replay
• Dark mode UI — Postman/Insomnia-style interface that doesn't hurt your eyes

Quick Start

git clone https://github.com/baristaGeek/open-source-postman-for-mcp
cd open-source-postman-for-mcp
npm install
npm run dev

Usage

Connect to a stdio server

Most MCP servers use stdio transport (like the official filesystem server):

1. Select STDIO transport
2. Enter command: npx -y @modelcontextprotocol/server-filesystem /tmp
3. Click Connect
4. Browse tools, fill in parameters, execute

Connect to an HTTP server

For HTTP-based MCP servers:

1. Select HTTP transport
2. Enter URL: http://localhost:3001
3. (Optional) Add authentication under "Authentication" dropdown
4. Click Connect

Configuration

Copy .env.example to .env:

cp .env.example .env

Required environment variables:

# Required for AI features (auto-select, summarization)
ANTHROPIC_API_KEY=sk-ant-...

# Required for credential encryption (generate with: openssl rand -hex 32)
MCP_SECRET_KEY=your-32-byte-hex-key

# Database location (default: ./dev.db)
DATABASE_URL=file:./dev.db

Generate a secret key:

openssl rand -hex 32

Architecture

┌─────────────────────────────────────────────┐
│              Next.js 15 App                  │
├──────────┬──────────────────┬───────────────┤
│ Sidebar  │  Request Builder │   Response    │
│ • Tools  │  • Form inputs   │   • JSON      │
│ • History│  • JSON editor   │   • Errors    │
│          │  • Schema view   │   • Timing    │
├──────────┴──────────────────┴───────────────┤
│              API Routes                      │
│  /api/stdio/*    — stdio process manager    │
│  /api/tools/*    — HTTP/SSE transport       │
│  /api/history    — SQLite persistence       │
│  /api/summarize  — AI-powered summarization │
└─────────────────────────────────────────────┘

Tech Stack:

• Next.js 15 + React 19 (App Router)
• Tailwind CSS (GitHub dark theme)
• Prisma + SQLite (history + config persistence)
• Anthropic SDK (AI features)
• TypeScript throughout

Why This Exists

I built this while working with MCP servers and got tired of:

1. Running servers in one terminal, client in another, copying JSON-RPC payloads by hand
2. Digging through server source code to figure out what arguments a tool accepts
3. Postman/Insomnia not supporting stdio (which most MCP servers use)
4. Having no way to replay previous requests or debug failures

Open Source Postman for MCP is the dev tool I wish existed from day one.

Roadmap

• Export request collections (à la Postman)
• Environment variables for reusable configs
• Batch/scripted requests
• Syntax highlighting for tool descriptions (Markdown)
• Desktop app (Electron/Tauri) for better stdio handling

Contributing

PRs welcome! This project aims to be the standard testing tool for MCP.

Areas that need help:

• SSE transport (currently stubbed out)
• Better error messages for stdio failures
• Request diff/comparison view
• More sample servers

License

MIT — Use it however you want.

Credits

Built by @baristaGeek because the MCP ecosystem needed proper devtools.

If you think this is cool, give it a star ⭐