Debugging with Claude Code

The Investigation Mindset

The biggest debugging mistake is guessing. You see an error, you think you know what caused it, and you start changing code. Twenty minutes later you have changed six files, the original error is gone, and two new ones have appeared.

Claude Code is at its best when you treat debugging as investigation, not guessing. Give it evidence, let it trace the path, and fix what is actually broken.

The Six-Step Debugging Workflow

Every debugging session should follow this structure, whether you are doing it yourself or with Claude Code.

1. Reproduce

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

2. Isolate

Narrow down where the problem lives. Is it the frontend or the backend? Is it in your code or a dependency? Is it happening in development or only in production?

Claude Code is excellent at this. Ask it to trace the data flow from the entry point (API call, user action, cron job) through the codebase. It can read multiple files simultaneously and map the 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, including the stack trace. Do not summarize or paraphrase the error: paste the exact output.

If there are no logs, that is your first problem to solve. Add logging at the boundaries (API entry/exit, database calls, external service calls) before continuing.

4. Form a Hypothesis

Based on the evidence, what do you think is causing the issue? Claude Code can help here by analyzing the code path, checking recent changes (git log, git diff), and looking for common patterns that cause the type of error you are seeing.

5. Test the Fix

Make the smallest possible change to test your hypothesis. Do not refactor three files while fixing a bug. Change one thing, run the reproduction steps, and see if the behavior changes.

6. Verify

Confirm the fix works and has not broken anything else. Run your test suite. Check related functionality. If the bug was in an API endpoint, test other endpoints that share the same code path.

Common Debugging Patterns

Here are the questions people most often bring to Claude Code, and how to approach them.

"Why is this API returning 500?"

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

Common culprits: missing environment variables, null values where an object is expected, database connection failures, and unhandled promise rejections.

"Why is my component not rendering?"

Check the browser console first. React and other frameworks surface rendering errors there. Share the console output with Claude Code. Common causes: a parent component is not passing the right props, conditional rendering logic is excluding it, or a data fetch is returning undefined.

Ask Claude Code to read the component file and its parent to trace the props and state.

"Why is the test failing?"

Paste the full test output, not just the error line. Claude Code needs to see the expected vs actual values, the test name, and the file path. Often the test itself is correct and the implementation has a subtle bug. Sometimes the test is outdated and tests the wrong behavior.

Ask Claude Code to read both the test file and the implementation file. It can usually spot the mismatch quickly.

Tips for Effective Debugging Sessions

Share the actual error message. "It is not working" gives Claude Code nothing to work with. The exact error text, status code, or stack trace gives it everything.

Share the full stack trace. The root cause is often three or four frames deep. The top-level error message is frequently a symptom, not the cause.

Let Claude Code read the files. Do not paste code snippets and ask "what is wrong?" Let it read the full file with surrounding context. Bugs often come from interactions between functions, not from a single line.

Use sub-agents for parallel investigation. If you are not sure whether the problem is in the frontend or backend, spawn two sub-agents: one to check the API logs and one to check the browser console output. This is faster than checking one at a time.

Check recent changes. Many bugs are introduced by recent commits. Ask Claude Code to run git log and git diff to see what changed recently in the affected files. The culprit is often 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 do not understand is not a fix; it is a time bomb.