I was experimenting with writing writing unit tests for MCP servers. I find unit testing incredibly useful for writing assertions for every MCP primitive: tools, prompts, and resources.
This helps with ensuring that future changes aren't breaking the server's behavior. To do this, I used a simple set up with Jest and MCPClientManager, a custom MCP client I wrote for testing.
Hi folks, we've been working on a CLI tool to programatically test and eval MCP servers. Looking to get some initial feedback on the project.
Let's say you're testing PayPal MCP. You can write a test case prompt "Create a refund order for order 412". The test will run the prompt and check if the right PayPal tool was called.
The CLI helps with:
1. Test different prompts and observe how LLMs interact with your MCP server. The CLI shows a trace of the conversation.
2. Examine your server's tool name / description quality. See where LLMs are hallucinating using your server.
3. Analyze your MCP server's performance, like token consumption, and performance with different models.
4. Benchmarking your MCP server's performance to catch future regressions.
The nice thing about CLI is that you can run these tests iteratively! Please give it a try, and would really appreciate your feedback.
I've been seeing a bunch of posts on this subreddit where people are afraid to start contributing to open source or don't know how to start. To get y'all started, I made a couple of beginner friendly GitHub issues that are "good first issues". They're really easy to do, and I provided step by step instructions. Very simple things like "add an icon".
I've been building MCPJam, an open source LLM chat playground for MCP servers. It's a MCP server testing tool, like Postman for MCP servers. You'll learn a lot about building LLM clients, working with React, Hono, Vercel AI SDK, lots of AI product engineering concepts.
If you're interested in contributing, or checking out the project, here's the GitHub:
To start, you can take a look at the Issues tab and see if there's anything there that interests you. Easy tasks are labelled "good first issue". Leave a comment in the issue if you're interested in taking it on!
For context, I've been working on the MCPJam inspector. It's an open source dev tool to test and debug MCP servers. We did an entire rebuild from Vite + Express to Next.js two weeks ago. We did this out of personal preference - we've built stuff in Next.js before and like its routing system and built in backend.
Switching to Next was a mistake for our use case. We didn't consider that our users are starting MCPJam with npx. Our npm package size exploded to 280MB. Next.js was too heavyweight for a locally ran web app. Switching back to Vite + Hono brought our package size to 9MB, much more manageable.
This post isn't to bash Next.js. It's just to remind you that tech stack does matter. We didn't think about the consequence of switching to Next and didn't consider our users' use of npx. If MCPJam was a hosted webapp, it would probably matter less. Remember to think about your stack's tradeoffs before you commit to building!
Hey y’all, my name is Matt. I maintain the MCPJam inspector, open source Postman for MCP servers. It’s a fork of the original inspector with upgrades like LLM playground, multi-connection, and better design.
If you check out the repo, please drop a star on GitHub. We’re also building an active MCP dev community on GitHub.
New features
Ollama support in the LLM playground. Now you can test your MCP server against local models like Deepseek, Mistral, Llama, and many more. No more having to pay for tokens for testing.
Chat with all servers. LLM playground defaults to accepting all tools. You can select / deselect the tools you want fed to the LLM, just like how Claude’s tool selection works.
Smoother / clearer server connection flow.
Please consider checking out and starring our open source repo:
I’m building a MCPJam dev Discord community. We talk about MCPJam, but also share general MCP knowledge and news. Active every day. Please check it out!
Hi, this is Matt, maintainer of the MCPJam inspector. The project is an open source MCP inspector with built in LLM testing. Now you can test individual tools, or test your MCP server against a real LLM.
I was experimenting with writing writing unit tests for MCP servers. I find unit testing incredibly useful for writing assertions for every MCP primitive: tools, prompts, and resources.
This helps with ensuring that future changes aren't breaking the server's behavior. To do this, I used a simple set up with Jest and MCPClientManager, a custom MCP client I wrote for testing.
Hope you find the article an interesting read!
Matt