GitLab Duo can be transformed into a personalized coding expert by creating custom rules. These rules automatically enforce development standards, preventing common AI errors like incorrect Java versions or style violations. The guide covers version control, style enforcement, and DevSecOps automation with practical examples. You can lock AI to specific Java versions, manage Python environments, and generate multi-platform C++ code. The system can enforce style rules such as avoiding C goto anti-patterns and ensuring VueJS design patterns. DevSecOps automation includes bootstrapping projects with proper CI/CD security scanning and documentation. Custom rules are created in a .gitlab/duo/chat-rules.md file, supporting Markdown for structure and clarity. Simple rules can be implemented quickly, like disallowing goto statements in C code. Developers can ask Duo Chat about existing style guides or have it summarize them from CI/CD configurations. The guide provides detailed use cases for version support, development environments, design patterns, and DevSecOps workflows. Examples include enforcing Java 8 for banking systems and ensuring multi-platform C++ code generation.