How to Add a Custom Tool to Your MCP Server (Step by Step)

You've got a running MCP server. Now what? The gap between "it works" and "it does something useful" is where most tutorials abandon you. This one won't. We're going to add a real tool to a real MCP server — one that reads a JSON file, validates the shape, and returns structured data. The kind of thing you'd actually build. Prerequisites A working MCP server. If you don't have one yet: npx @webbywisp/create-mcp-server my-server cd my-server && npm install && npm run build Done. Now let's add a tool. What We're Building A read_config tool that: Accepts a file path argument Reads a JSON file from disk Validates it's actually valid JSON Returns the parsed content + metadata Simple but genuinely useful — the kind of tool you'd wire up to let Claude read your agent's config files. Step 1: Create the Tool File In a tool-server project, create src/tools/readConfig.ts : import { z } from ' zod ' ; import { readFile } from ' fs/promises ' ; import { resolve , basename } from ' path ' ; export c