MCP ProtoLab

A Postman-like web UI for Model Context Protocol servers

Connect, explore, and debug any MCP server directly from your browser. No local setup required.

Get Started · Features · Contributing

Features

🔌 Connect to Any MCP Server

Enter a server URL and connect in seconds
Supports Streamable HTTP (modern, session-aware) and SSE (legacy) transports
Authentication: Bearer Token, API Key, Basic Auth, or none

🛠 Explore All Server Capabilities

Tools — Execute server tools with forms auto-generated from JSON Schema
Prompts — Invoke prompts with arguments and preview generated messages
Resources — Browse and preview resources with MIME type detection
Live discovery — All capabilities fetched and displayed instantly on connect

📋 Real-Time Request Logs

Watch every JSON-RPC request and response as it happens
Timing data per request, visual status indicators (pending / success / error)
Full history of up to 200 requests with complete payloads

🗂 MCP Server Registry

Browse official MCP servers built into the app
Filter by transport (HTTP, SSE, STDIO) or search by name
One-click connect to any listed server

⚡ Built for Developer Productivity

Dark mode UI — comfortable for long sessions
3-panel layout: server list · capability explorer · live logs
Monaco Editor (same as VS Code) for JSON payload editing
Session ID persistence — no manual header management
Built-in CORS proxy — no browser extension needed

Advantages

Compared to CLI Tools

Visual interface — No need to remember JSON-RPC commands
Auto-generated forms — Tool parameters presented as form fields, not raw JSON
Multiple connections — Test several servers simultaneously
Shareable URLs — Send connections to teammates

Compared to Other MCP Clients

Full protocol support — Tools, prompts, and resources in one place
Transport flexibility — Switch between HTTP and SSE without code changes
Zero setup — Open in browser and start testing
Built-in registry — Discover official servers without searching documentation

Developer Experience

Type exploration — See tool schemas and prompt arguments visually
Error visibility — Clear error messages with full response bodies
Session debugging — Inspect session IDs and transport behavior

Get Started

Option 1 — Use the Hosted Version

No setup required. Open MCP ProtoLab in your browser:

https://jac-mcp-playground.jaseci.org/

Option 2 — Run Locally

Prerequisites

pip install jaclang jac-client

Clone and start

git clone https://github.com/jaseci-labs/jac-mcp-playground.git
cd jac-mcp-playground
jac start main.jac

Open http://localhost:8000 in your browser.

Usage Examples

Connect to an MCP Server

Navigate to Playground → Add Server
Enter server URL (e.g., https://mcp.example.com/mcp/)
Select transport (Streamable HTTP recommended)
Add authentication if required
Click Connect

Execute a Tool

Select a connected server from the sidebar
Open the Tools tab and click any tool
Fill in the auto-generated form and submit
See the full JSON-RPC response in the logs panel

Browse the Server Registry

Navigate to MCP Servers List
Browse or search available servers
Click Test Server — the connection form opens pre-filled

Architecture

┌─────────────────────────────────────────────────────────────┐
│                         Browser                             │
│  ┌───────────┐  ┌──────────────┐  ┌─────────────────────┐   │
│  │ jac-client│  │   MCP Store  │  │  MCP Client Hook    │   │
│  │           |  │ (Context)    │  │  (JSON-RPC)         │   │
│  └─────┬─────┘  └──────┬───────┘  └──────────┬──────────┘   │
│        └───────────────┴─────────────────────┘              │
│                           │                                 │
│                       HTTP POST                             │
└───────────────────────────┼─────────────────────────────────┘
                            │
┌───────────────────────────▼─────────────────────────────────┐
│                    Jac Runtime                              │
│  ┌─────────────────────────────────────────────────────┐    │
│  │         mcp_proxy Walker (CORS Handler)             │    │
│  │  • Forward requests to MCP server                   │    │
│  │  • Handle SSE endpoint discovery                    │    │
│  │  • Stream responses line-by-line                    │    │
│  └──────────────────┬──────────────────────────────────┘    │
└─────────────────────┼───────────────────────────────────────┘
                      │
              ┌───────▼────────┐
              │  MCP Server    │
              │  (Remote)      │
              └────────────────┘

Tech Stack

Framework: Jac (Jaseci) — Full-stack language
Frontend: jac-client
Styling: Tailwind CSS v4 with custom theme
Editor: Monaco Editor (VS Code editor component)
Icons: Lucide React
Deployment: Kubernetes via jac-scale

Why Jac?

This project is one of the first production tools built entirely in Jac — a full-stack language from Jaseci Labs that eliminates the frontend/backend split. The entire application — UI components, server logic, routing, and the MCP proxy — lives in a single language with a unified developer experience.

Contributing

Contributions are welcome and appreciated! Here's how to get involved:

Reporting Bugs

Search existing issues first. If it's new, open one with:

A clear title and description
Steps to reproduce
Expected vs. actual behavior
Browser/OS details if relevant

Suggesting Features

Open a GitHub Discussion or an issue tagged enhancement. Describe the use case, not just the solution.

Submitting a Pull Request

Fork the repository and create a branch from main
```
git checkout -b feat/your-feature-name
```

Set up your local environment

pip install jaclang jac-client
git clone https://github.com/your-username/jac-mcp-playground.git
cd jac-mcp-playground
jac start main.jac

Make your changes — keep commits focused and write clear messages
Test your changes locally against at least one MCP server
Open a PR against main with:
- A description of what changed and why
- Screenshots or a short recording if it's a UI change
- Reference to the related issue (e.g. Closes #42)

What to Work On

Check the open issues for things labeled good first issue or help wanted.

License

MIT — see LICENSE for details.

Live Demo · MCP Specification · Jac Docs · Jaseci Labs

_{Built with ❤️ by Jaseci Labs}

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