Building an MCP server in Python (and connecting it to Claude Code)

An MCP server is a small app that extends an AI model's capabilities by giving it access to custom tools, a particular set of data or workflows. It's based on the Model Context Protocol, which is an open standard for connecting AI apps with these external sources. The most straightforward way to create an MCP server is to use the official SDK, implement a single function and mark it as a tool and then expose it through stdio (standard input/output) which you can register in Claude Code; it basically boils down to a single Python file with a single tool and connecting it end-to-end took us around 10 minutes. Background Generally, the Model Context Protocol defines two sides: The server — it's the app you write that you use to publish tools/data The client — for example Claude Code; it finds and calls available tools based on your permission As for the main purpose of the Model Context Protocol — before it was introduced, every AI app needed its own custom integration with every tool; the Model Context Protocol replaces this with a single standard connector, so to say it's like USB-C for the AI world — you have a single standardised port instead of having to use a separate cable with every device. In terms of the protocol, a tool is just a function that the model can decide to call. So if you want to build an MCP server, you do it when you want your model to have access to some resources you have (like your internal API or database for example) which aren't available through any of the already-published servers. Let's have a look at a minimal example of what such server might look like — a single Python file with a single tool that returns the number of words, characters and lines in the input text. Scaffold the project To set up the project we used uv (a CLI for managing Python projects) and installed the official SDK: uv init word-count-mcp cd word-count-mcp uv add "mcp[cli]" uv init word-count-mcp — initialises a new project called "word-count-mcp" with an uv proje