Module 12 · Lesson 03

Debugging Workflows — Using Claude as a Thinking Partner

Reading time: 17 minutes Track: Claude Fluency for Teams · Developer path

The wrong way to debug with Claude

The least effective debugging pattern:

1. Paste error + some code
2. Ask "why is this broken?"
3. Apply Claude's suggested fix
4. Get a different error
5. Repeat until something works or you give up

This approach treats Claude as a fix machine. The outputs are fixes to symptoms you don't understand, which means you can't maintain them and the same category of bug will happen again.

The right framework: hypothesis-driven debugging

Effective debugging with Claude follows the same scientific method good debugging always uses — Claude just accelerates each step.

Step 1: Establish what you know

Before asking Claude anything, state what you know:

• What is the expected behavior?
• What is the actual behavior?
• When does it happen and when doesn't it?
• What have you already ruled out?

Expected: When user submits form, their profile is updated in the DB.
Actual: The PUT /profile endpoint returns 200 but no DB change occurs.
When it happens: All users, all browsers. Started after yesterday's deploy.
Already ruled out: Network issues (other endpoints work), auth (user is authenticated).

Step 2: Generate hypotheses

Ask Claude to generate candidate root causes, not a fix.

Given this information, what are the 3 most likely root causes?
Rank them by probability. For each, what evidence would confirm or rule it out?

This gives you a prioritized investigation plan, not a random change to try.

Step 3: Test hypotheses with Claude

For each hypothesis, work with Claude to design a targeted test:

Hypothesis 1: The request body is not being parsed correctly.
How do I confirm this? What specific log line or test would prove or disprove it?

Step 4: Implement the fix with understanding

Once you've identified the root cause, ask Claude to propose a fix and explain it:

Root cause confirmed: body-parser middleware is not applied to this route.
Propose a fix and explain why it works and whether there are any side effects.

Running tests in the debugging loop

Claude Code can run your test suite directly:

Run the tests for the profile update feature and tell me what's failing and why.

Claude will execute the tests, read the output, and interpret the results. This tight observe-analyze-fix loop is faster than switching between terminal and editor manually.

Useful debugging prompts

When you have an error message:

Error: [paste error]. Relevant code: [paste code].
What are the possible causes of this error? Start with the most likely.

When behavior is wrong but no error:

Expected behavior: [X]. Actual behavior: [Y]. No error thrown.
Read [relevant files] and identify where the logic diverges from the expected behavior.

When you've fixed a bug and want to prevent recurrence:

I just fixed [bug description]. What test would have caught this before it reached production?
Write that test.

When debugging a performance issue:

This [operation] is taking [X ms]. Expected: [Y ms].
Read the relevant code and identify the likely performance bottleneck.
What profiling would confirm it?

The "explain the fix" discipline

Before accepting any bug fix Claude proposes:

Before I apply this — explain to me in plain terms why this fix works.
What was wrong and why does this change correct it?

If Claude can't explain it clearly, or if the explanation doesn't fully make sense to you, don't apply the fix. The fix may be correct, but without understanding why, you can't maintain it.