How to Add MCP Support to an Existing ASP.NET Core REST API Without Breaking Auth

A software engineering guide explains how to integrate the Model Context Protocol (MCP) into an existing ASP.NET Core REST API, enabling AI clients like Claude and Gemini to discover and call API endpoints as tools. MCP acts as a thin translation layer between AI models and backend services, using JSON-RPC to forward requests without requiring any changes to the existing API or its JWT authentication setup. Developers can implement this by adding the official C# MCP SDK to a new minimal ASP.NET Core project and decorating methods with tool attributes that AI models read to understand when and how to invoke them. The guide emphasizes that clear, explicit method descriptions are critical, as vague tool definitions can cause AI models to hallucinate incorrect arguments. Authentication handling between the MCP server and the secured backend API is flagged as a key production concern requiring careful design.