Cursor is the strongest fit for adding unit tests to an existing codebase because it operates directly inside your editor, reads your actual files, and can generate contextually accurate tests for functions and classes it can see. It understands project structure, imports, and dependencies, so the tests it writes are ready to run with minimal editing. GitHub Copilot is a solid alternative if you prefer a lighter IDE plugin that suggests tests inline as you type. Claude Code works well when you want to describe the codebase in a conversation and get full test files back in one shot. Deepseek Coder is worth considering if cost is a priority, since it is free and handles code-focused tasks competently.