Debugging with Claude Code

Most debugging mistakes happen in the first 30 seconds.

You see an error. You think you know what caused it. You start changing code.

Twenty minutes later you've touched six files, the original error is gone, and two new ones appeared. Sound familiar?

Claude Code is at its best when you treat debugging as an investigation. Give it evidence, let it trace the path, fix what's actually broken. Not what you suspect is broken.

The Six Steps (In Order, Every Time)

1. Reproduce

Before anything else, confirm you can trigger the bug consistently. If you can't reproduce it, you can't verify your fix. Tell Claude Code exactly how: the URL, the input, the sequence of clicks.

2. Isolate

Narrow it down. Frontend or backend? Your code or a dependency? Dev only or also production?

Claude Code is excellent at this step. Ask it to trace the data flow from the entry point through the codebase. It can read multiple files simultaneously and map the entire path.

3. Read the Logs

This sounds obvious but most people skip it. Logs tell you what actually happened, not what you think happened.

Share the full error output with Claude Code. The entire stack trace. Don't summarize it, don't paraphrase it. Paste the exact output. The root cause is often 3-4 frames deep, and the top-level error is frequently a symptom.

If there are no logs, that's your first problem. Add logging at the boundaries (API entry/exit, database calls, external services) before continuing.

4. Form a Hypothesis

Based on the evidence, not your gut: what's causing this? Claude Code can analyze the code path, check recent changes with git log and git diff, and look for common patterns that cause the type of error you're seeing.

5. Test the Fix

Make the smallest possible change. One thing. Run the reproduction steps. See if behavior changes.

Do not refactor three files while fixing a bug. That's how you turn one bug into three.

6. Verify

Confirm the fix works AND hasn't broken anything else. Run your test suite. Check related functionality. If the bug was in an API endpoint, test other endpoints sharing the same code path.

The Hits: Common Debugging Scenarios

"Why is this API returning 500?"

Start with server logs. A 500 means an unhandled exception somewhere. Share the exact error message and stack trace. Ask Claude Code to read the file and line number from the trace, then walk backward to the root cause.

Common culprits: missing env vars, null where an object is expected, database connection failures, unhandled promise rejections.

"Why is my component not rendering?"

Browser console first. React and friends surface rendering errors there. Share the console output.

Common causes: parent not passing the right props, conditional rendering excluding it, data fetch returning undefined. Ask Claude Code to read both the component and its parent to trace props and state.

"Why is the test failing?"

Paste the full test output. Not just the error line. Claude Code needs expected vs actual values, the test name, and the file path.

Often the test is correct and the implementation has a subtle bug. Sometimes the test is outdated. Ask Claude Code to read both files. It usually spots the mismatch fast.

Tips That Make the Difference

Share the actual error. "It's not working" gives Claude Code nothing. The exact error text, status code, or stack trace gives it everything.

Let Claude Code read the files. Don't paste snippets and ask "what's wrong?" Let it read the full file with context. Bugs come from interactions between functions, not single lines.

Use sub-agents for parallel investigation. Not sure if it's frontend or backend? Spawn two agents: one checks API logs, one checks browser console output. Faster than sequential guessing.

Check recent changes. A lot of bugs come from recent commits. Ask Claude Code to run git log and git diff on the affected files. The culprit is usually in the last few commits.

Do not change code until you understand the problem. This is the hardest discipline. The urge to "try something" is strong. Resist it. Understand first, then fix. A fix you don't understand isn't a fix. It's a time bomb.