Thumb-First · Platform

The technical-verification layer of the thumb-first suite. Where thumb-first-design decides what the mobile design should be (platform-agnostic judgment), this skill verifies that the implementation holds up on the actual device — objective defects with file:line and fixes, rated by severity.

Run /thumb-first for a combined design + platform review; use this skill directly for technical checks only.

Step 0 · Detect the Target (do this first)

The relevant defects are completely different for a web/PWA vs. a native app. Determine the target before doing anything else.

Sniff the repo:

• Web / PWA → next.config.*, vite.config.*, an index.html, a web app manifest.json/manifest.webmanifest, a service worker, Tailwind/CSS. This is this skill's home turf — proceed to the principles and routing below.
• Native → react-native/expo in package.json, app.json/app.config.* (Expo), an ios/+android/ pair, *.xcodeproj/Package.swift/*.swift (SwiftUI/UIKit), or pubspec.yaml (Flutter).
• Ambiguous or both (e.g. an Expo app with a web target) → ask which target to audit.

If the target is NATIVE — do not fake a web audit. This skill's checks (Safari quirks, viewport units, service workers, web manifest) do not apply. Instead: