Cursor is the strongest fit for adding integration tests to legacy code, as it operates directly inside your editor, understands your full codebase context, and can generate, run, and iterate on test files while you work. It handles the messy reality of legacy code well by reading existing patterns and inferring dependencies before writing tests. Claude Code is worth considering if you prefer a terminal-first agentic workflow that can reason deeply about complex codebases. GitHub Copilot is a lighter-weight alternative that works well for suggesting test scaffolding inline as you type. Deepseek Coder is a free option that specializes in code tasks and can generate integration test boilerplate at no cost.