How to Write Custom Semgrep Rules: Complete Tutorial

Why write custom Semgrep rules Semgrep ships with over 2,800 community rules and 20,000+ Pro rules that cover common security vulnerabilities, best practice violations, and correctness issues across more than 30 programming languages. For many teams, these pre-built rule sets are enough to catch the most critical problems. But every codebase has patterns, APIs, and conventions that are unique to its organization - and that is where custom rules become essential. Custom Semgrep rules let you codify institutional knowledge into automated checks. When a senior engineer discovers a subtle misuse of an internal API, they can write a rule that catches that mistake everywhere it appears and prevents it from being introduced again. When your security team identifies a vulnerability pattern specific to your framework, they can encode it as a rule that runs on every pull request. The result is a living, growing library of checks tailored to your exact codebase. This tutorial covers everything yo