How I Built a 55-Tool MCP Server That Runs an Entire Business

Every ERP has a REST API. Ours has one too. But what made everything click was building an MCP server on top of it — 55 tools that let any AI agent manage invoices, expenses, clients, taxes, and accounting through natural language. This is the story of how I built it, what I learned, and why MCP might be the most underrated protocol in business software right now. Why MCP, not just REST I had a perfectly good REST API. Cursor pagination, field selection, 6 filter types, OpenAPI spec. But here is the thing: APIs are designed for developers. MCP is designed for AI agents. With REST, you need to: Read the docs Figure out the auth Chain multiple endpoints Parse responses Handle errors With MCP, an AI agent gets a tool catalog with typed parameters, calls what it needs, and gets structured responses. No docs required. The schema IS the documentation. // REST: 4 calls, manual orchestration const clients = await fetch('/api/v1/clients?q=Acme', { headers }); const client = await clients.json()