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New MCP playground - Playground V2

Matthew Wang, Andrew Khadder4 min read

We're excited to announce our new version of MCPJam's MCP playground - Playground V2.

The MCP Playground has become an essential tool for MCP server developers to test their servers against any LLM all from a single chat interface. No more switching between clients to validate. We found the MCP playground incredibly useful to QA and manually end to end test MCP servers.

MCP-UI and the OpenAI Apps SDK have also gained widespread adoption. We've also made it a priority to provide a local-first development environment for UI testing.

What's new in Playground V2

  • Render MCP-UI and OpenAI Apps SDK. We have full spec support for servers built with MCP-UI and OpenAI Apps SDK.
  • View all JSON-RPC messages sent back and forth between the MCPJam client and MCP server for fine debugging.
  • Added free frontier models (GPT-5, Sonnet, Haiku, Gemini 2.5, Llama 3.2, Grok 4, GLM 4.6). Test with frontier models, no API key needed.
  • Upgraded Chat Interface: cleaner UI with visible tool input params, raw output inspection, better error handling.
  • More responsive and smoother chat streaming experience.

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Support for MCP-UI

MCP-UI was the first project in the ecosystem that brought rich UI and interactions to MCP clients. Popular MCP clients including ChatGPT, Goose, and Smithery can render MCP-UI.

Playground V2 supports the full client spec of MCP-UI, including rendering and UI actions.

Support for OpenAI Apps SDK

Developing an OpenAI app currently requires a ton of setup. Testing apps currently requires an OpenAI Plus plan to access developer mode, and they don't have any options for local testing. Playground V2 allows you to test your OpenAI app locally without having to ngrok to ChatGPT or buy ChatGPT Plus.

Playground V2 OpenAI Apps SDK demo
Testing OpenAI Apps SDK in Playground V2

We provide frontier models for free

Providing the best developer experience for our users is our highest priority. We're providing frontier proprietary/open source models from OpenAI, Anthropic, Gemini, and more to be used for free in Playground V2. You no longer have to bring your own API key to access the best MCP server testing experience. It's on us.

We're on a mission to help developers build high quality servers, and we believe that providing free models lowers the barrier to achieving that.

Free frontier models in Playground V2
Access to multiple frontier models at no cost

Get started

You can start up the MCPJam inspector by running the following command:

npx @mcpjam/inspector@latest

This will spin up the local React instance of MCPJam inspector. Start off by connecting to an MCP server in the "Servers" tab.

Once you've connected to an MCP server, navigate to the Playground tab to use Playground V2. You can bring your own API key in the Settings tab and come back to the Playground to use your models. You can also create an account / sign in to use MCPJam free models and not have to bring your own keys.