How to Secure Your MCP Server Against Prompt Injection (Practical Guide)

MCP (Model Context Protocol) servers are powerful — they give Claude Code real capabilities: reading files, querying databases, calling APIs. But that power comes with a real attack surface that most developers aren't thinking about yet. Prompt injection through tool descriptions is the most underappreciated threat in the MCP ecosystem right now. This guide walks you through what it looks like, how to audit your own server, and the patterns that keep you safe. What Is Prompt Injection in an MCP Server? When Claude Code connects to your MCP server, it reads your tool definitions — names, descriptions, parameter schemas. Claude uses that information to decide when and how to call your tools. This is the attack surface. A malicious or misconfigured tool description can inject instructions directly into Claude's reasoning. The model doesn't have a separate "system layer" that's immune to tool metadata — tool descriptions land in the same context window that Claude reasons over. This means