I built ZIP-code autofill into AI US Citizenship Test 2026 , a Flutter app that helps green card holders prepare for the naturalization interview with an AI tutor, realistic N-400 mock interviews, and a full practice test engine. Several civics answers are state-specific — your representative, your senators, your governor — so asking for a ZIP code and auto-filling the city and state removed an entire screen of dropdowns from the onboarding flow.

The feature took an afternoon to build and a week to get right. Every shortcut I took in the first version — validating with int.parse\ , firing the API on each keystroke, trusting the first response that arrived — produced a real bug that a real user found. What follows is the hardened pattern, so you can skip the week of bug reports.

It looks like a small feature. A text field, an API call, two filled fields.…