Part five of the Handmade Claude Code campaign: the agent plugs into the world. The Model Context Protocol is how tools that live in other processes — a database, a browser, a ticket tracker — offer themselves to any agent that speaks it. This part makes yours speak it: start the servers, shake hands, put their tools in front of the model, call them, survive their failures, use their prompts and resources, and shut them down cleanly.

This session continues the agent you built in parts one to four, in the same repository. The MCP client is yours too — JSON-RPC over stdio is small enough to write, and writing it is the point. An MCP SDK is out of contract.

Servers

.mcp.json  (in the working directory)
{"mcpServers": {"<name>": {"command": "sh", "args": ["srv.sh"], "env": {"K": "V"}}}}

At launch, before the first model call, every server is started as a child process — command with args, the working directory as its directory, env added to its environment — and spoken to over its stdin and stdout in JSON-RPC 2.0, one JSON object per line.

The handshake: an initialize request (with protocolVersion, capabilities and clientInfo), then the notifications/initialized notification, then tools/list. A server that fails to start, or fails the handshake, is reported on stderr by name and skipped; the run goes on without it.

Tools

Every tool a server lists is advertised to the model as mcp__<server>__<tool>, with the server's description and its inputSchema as the input_schema. A call becomes tools/call with name (the server's tool name) and arguments (the model's input). The result's content text blocks, joined, are the tool result content; a result with isError: true, or a JSON-RPC error reply, is an error result with the message as content. The loop goes on.

Prompts

A server that lists prompts (prompts/list) contributes slash commands: /mcp__<server>__<prompt> <args> calls prompts/get with the arguments mapped positionally onto the prompt's declared arguments, and the messages it returns become the user message (their text, joined).

Resources

A token @<server>:<uri> in the prompt reads a resource: resources/read with that uri, and the text of the returned contents travels with the prompt like a mentioned file.

Shutdown

When the run ends, every server's stdin is closed; a server still alive a moment later is killed. No orphans.

The judges

The quality panel — architecture, performance, code quality, test quality, technical governance and DX review — sits on the rungs where its subject is decided, one verdict per judge per rung, on top of the rung's own points. There is no closing review: what you build is judged as you build it, and a rung you never reach is a verdict you never get.

The ladder

• Set up and carry parts one to four forward (10)
• The handshake (20)
• Tools are advertised (30)
• A tool call goes through (30)
• Errors come back as results (20)
• A server that will not start (20)
• Prompts are commands (20)
• Resources are mentions (20)
• Environment and shutdown (20)