TL;DR Understanding a large legacy codebase requires more than search and navigation. The tools available in 2026 fall into four categories: navigation tools (Sourcegraph, OpenGrok) that help you find code, AI-assisted explorers (GitHub Copilot Chat, Cursor, Cody) that explain code in context, visualization tools (CodeScene, Understand by SciTools) that show structure, and code intelligence platforms (CoreStory) that build a persistent, queryable model of what the system does and why. This guide covers each category honestly, including what works for COBOL and mainframe systems. Why Legacy Codebase Understanding Is Hard You’ve just been assigned to a system that was built 15 years ago. The original developers are gone. The documentation, if it exists, describes the system as it was in 2015. The codebase spans three languages, two databases, and a mainframe component nobody wants to touch. This is the reality for most enterprise engineering teams.…