March 17, 2026 · 5 min read

Running a Debugging Session with Claude Code

Claude can debug effectively but the session needs structure. Here's the format that gets to root cause instead of guesses.

Unstructured debugging with Claude goes like this: you paste an error, Claude suggests a fix, you try it, it doesn't work, Claude suggests another fix, you try it, it half-works. Twenty minutes later you've made three changes and don't understand any of them.

Structured debugging sessions go much faster. Here's the format I use.

Phase 1: Reproduce before you fix

The first message in any debugging session is a reproduction, not a fix request. I give Claude:

Then I ask: "What are the three most likely causes of this?" Not "fix this." Causes first.

Phase 2: Isolate before changing code

Once Claude lists likely causes, I ask it to suggest how to test each one — with logging, with a small isolated reproduction, with a different input. The goal is to confirm the actual cause before writing any fix code.

This step is where most people skip ahead. They take Claude's first guess and implement it. Sometimes it works. Often it masks the real problem.

Phase 3: Fix the cause, not the symptom

After isolating the cause, the fix prompt is specific: "The cause is X. Write a fix that addresses X directly. Don't change anything else."

That scope constraint matters. Without it, Claude will sometimes fix the symptom by adding defensive code around it, leaving the root cause in place.

The question that unsticks hard bugs

When Claude and I are both stuck — suggestions aren't working, the cause isn't obvious — I ask: "What would need to be true for this error to occur that we haven't considered yet?"

This forces a different angle. It often surfaces assumptions we've both been carrying (that the data is always this shape, that this function always runs in this order, that this config is always set). Some of those assumptions are wrong, and that's where the bug is.

When to stop using Claude for debugging

If we've gone three rounds of hypothesis-test-fail with no progress, I switch to reading the source code directly. Claude is working from context I've given it — it doesn't have access to the full runtime state, the exact library version behavior, or whatever undocumented quirk is the actual culprit.

At that point the most useful thing Claude can do is help me search docs, read through library source, or explain what a specific function is supposed to do. That's still valuable. But the diagnostic work shifts to me.