Bonus 07 of 07
How to read a diff - and why you must before accepting any change
Goal: A diff shows you exactly what Claude Code changed, line by line. Reading it takes 30 seconds. Skipping it is how hours of work get accidentally deleted. Today you learn to read one confidently.
What to do
Ask Claude Code to make a small change
Start Claude Code. Ask it to make one small, specific change - for example, change a single heading text.
Ask Claude Code to show you the diff
Type:
Show me a diff of everything you just changed. List each file that was modified, how many lines were added and removed, and flag anything you changed that I did not explicitly ask for.Read the diff - step by step
Read the diff using these 3 questions: (1) How many lines changed, and does that feel right for what I asked? (2) Any red (-) lines I did not expect to see removed? (3) Any files changed I did not ask about?
Ask Claude Code to make a change that includes something you did not request
Ask Claude Code to make a second change, but tell it to also change one extra thing you did not ask for, just for practice:
Change the About page heading to say "About us" - and also, as part of this change, update the footer copyright text to say 2027.Spot the unexpected change and ask for it to be reverted
In the diff, find the footer change you did not originally request. Then type:
Revert only the change to the footer copyright text. Keep the About heading change. I did not ask for the footer to be changed.Check the diff one final time
Ask for the diff again to confirm only the heading change remains:
Show me a final diff of the current state. Confirm only the About heading change is present and the footer change has been reverted.Expected result
You read a diff, identified an unexpected change, and reverted only that change while keeping the one you wanted. You have the most important quality control habit in non-developer building.
Key takeaway
- You have completed the full NonDev Apps 21-Day Claude Code Challenge. You can now install, build, plan, branch, recover, automate carefully, use Skills, use subagents, manage context, connect MCP, and review diffs. That is a builder's skill set.