Debugging

This code isn't working as expected. Expected behavior: [WHAT SHOULD HAPPEN]. Actual behavior: [WHAT HAPPENS / ERROR]. Walk through the likely cause, point to the exact line(s), and give me a corrected version with a one-line note on what was wrong. Code: [PASTE]

Review the code below like a thoughtful senior engineer. Cover: correctness, readability, performance, security, and edge cases. Prioritize your feedback (must-fix vs. nice-to-have) and be specific with line references. Don't nitpick style if there's a linter. Code: [PASTE]

Explain this error in plain language: what it means, the most common causes, and how to fix it. Give me the 2–3 most likely fixes in order of probability. Error: [PASTE ERROR + relevant code]