Exploring a Codebase and Fixing Bugs with Claude Code

The most common real-world use case: you join a new project, or there's fairly old code you didn't write yourself — and you need to fix a bug in it. Claude Code speeds this workflow up considerably.

Step 1: Understand First, Fix Later

> Where does payment processing happen in this codebase?
> Walk me through the flow at a high level — from the
> request all the way to the database save.

A good pattern is: ask for exploration/explanation first, verify the understanding is correct yourself, then move toward the fix — this keeps the agent from changing the wrong place.

Step 2: Describe the Bug — With Reproduction Steps

> When a user checks out without selecting a shipping
> method, the app throws a 500 error. Error log:
> "Undefined property: shipping_method_id". Fix this.

The more specific your error message and steps are, the more accurate the fix will be — exactly the same way you'd file a bug report for a human developer.

Step 3: Review the Fix

Whatever files Claude Code changes, it shows you the diff before or after applying it — always read the diff, especially for logic-heavy code. If you don't like the approach, just say so directly: "fix this with validation, not a default value" — the agent will adjust its approach.

Step 4: Verify — Run the Tests

> Run the related tests after the fix and confirm
> everything is passing.

The agent can run commands like php artisan test or npm test itself and report whether the fix actually works — saving you the manual verification time.

Key Takeaways

• Have it explore/understand first, then fix — this improves accuracy.
• The more specific your bug report (error message, steps), the more accurate the fix.
• Always review the diff for each change, and verify with tests.