Cursor is the strongest fit here because it is a code-focused IDE that can inspect your function inline, explain why it returns unexpected values, suggest fixes, and let you iterate without leaving your editor. You paste the function and describe the symptom, and Cursor walks through the logic to pinpoint the bug. Deepseek Coder is a solid free alternative if you want a chat-based code model without a subscription. Claude Sonnet works well when you need a thorough, conversational explanation of subtle logic errors. GitHub Copilot is worth considering if you are already in VS Code and want inline suggestions as you step through the fix.