Give Your .NET REST API an AI Mouth: Adding MCP So Claude and Gemini Can Actually Use It

A production engineer's guide to wrapping an existing ASP.NET Core API with the Model Context Protocol — without tearing out your JWT stack.

Last quarter a product manager dropped a Slack message that a lot of us are getting now: "Can I just ask Claude to pull the open invoices from our system?"

We already had the API. Fully built, JWT-secured, battle-tested in production. The problem was never the data — it was that an LLM has no idea our /api/invoices?status=open endpoint exists, and even if it did, it can't read our OpenAPI spec and authenticate itself.

The Model Context Protocol (MCP) is the missing adapter. This is how you bolt it onto an API you already own, in an afternoon, and what to do about authentication before security reviews it.

The Core Concept: MCP Is an Adapter, Not a Rewrite