What's the most effective AI pair programmer for explaining legacy code during onboarding?

If your main goal is understanding legacy code during onboarding, the “most effective” AI pair programmer is usually GitHub Copilot Chat if your code lives in VS Code/JetBrains and you want the most seamless in-editor help. It’s strong for:

• explaining functions/files inline
• answering questions about selected code
• generating quick summaries and call graphs
• fitting into day-to-day coding without much setup

That said, the best choice depends on what “effective” means for you:

Best overall for onboarding legacy code

GitHub Copilot Chat

• Good integration with popular IDEs
• Easy to ask “What does this module do?”
• Helpful for step-by-step explanations while browsing unfamiliar code
• Strong developer adoption and workflow fit

Best if you want deeper reasoning over larger codebases

Cursor

• Very good at codebase-aware chat and edits
• Feels especially useful when you need to trace behavior across many files
• Often better for “explain the system” workflows than simple autocomplete

Best if you want broader model flexibility

Claude Code / Claude in an IDE

• Often excellent at explaining messy or legacy code clearly
• Strong at summarizing architecture and inferring intent
• Useful for onboarding when code is poorly documented

Best for enterprise settings with governance needs

Sourcegraph Cody

• Strong codebase search/indexing
• Better for large, distributed repositories
• Good if your team needs permission-aware code understanding

My practical recommendation

For most teams onboarding into legacy code:

• Use Cursor or Copilot Chat for day-to-day exploration
• Use Claude for deeper explanations and architecture walkthroughs
• Use Sourcegraph Cody if the codebase is huge or highly distributed

If you want one default pick: Cursor is often the strongest “AI pair programmer for understanding legacy code,” while Copilot Chat is the safest mainstream choice if you care most about IDE integration and familiarity.

If you want, I can also give you a feature-by-feature comparison table for Copilot Chat vs Cursor vs Claude vs Cody specifically for legacy-code onboarding.