# Mobile Field Readiness > Take a mobile app or PWA feature from "works on my laptop" to "works on a phone in the field" — camera, GPS and geofencing, permissions, install and update, poor signal, one-handed use. Use when building or reviewing anything a person uses standing up, outdoors, on their own device. - Skill: `chinthakat/mobile-field-readiness` (Agent Skill) - Install (CLI): `npx skillmds@latest add chinthakat/mobile-field-readiness` - Raw SKILL.md: https://api.skillmd.com/api/skills/chinthakat/mobile-field-readiness/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: chinthakat (https://skillmd.com/u/chinthakat) - Updated: 2026-09-22 - Page: https://skillmd.com/skills/chinthakat/mobile-field-readiness --- # Mobile field readiness A desktop browser at a desk on wifi is the least representative test environment available. The failures below do not appear there at all. ## 1. Secure context first, because it silently gates everything Camera, microphone, geolocation, service workers and notifications require HTTPS. `localhost` is exempt, which is exactly why this gets discovered late: everything works on the dev machine and nothing works the moment you open the LAN address on a real phone. ``` http://localhost:5173 camera works (localhost exemption) http://192.168.1.42:5173 camera silently unavailable https://.trycloudflare.com camera works ``` Set up a real HTTPS path for device testing on day one — a local certificate tool or a tunnel — and put the command in the README. Otherwise every person who joins loses an afternoon to it. ## 2. Test on a real device early, and on a cheap one Desktop responsive mode gets you layout and nothing else. It does not reproduce: - Touch versus mouse, and the absence of hover. - The on-screen keyboard covering the field being typed into, and the viewport resize that follows. - Safe areas: notches, rounded corners, the home indicator, the URL bar that appears and disappears as you scroll. - Real camera latency and real photo file sizes. - Memory pressure — a mid-range Android will kill a tab that a laptop runs comfortably. - Sunlight, which makes low-contrast greys invisible. Test on the worst device your users actually have, not the newest one you own. ## 3. Every permission has a denied path, and it is permanent Camera, location and notifications can be denied, denied permanently, or restricted by policy. On iOS a second prompt is often not shown at all. For each one: - Explain **before** prompting why you need it, in context, at the moment it is needed. A prompt on first launch with no explanation gets denied. - Handle denial with something usable — upload from the gallery instead of the camera, type an address instead of using GPS. - When denied permanently, say so and point at the OS settings. "Location unavailable" leaves the user stuck; "Location is off for this site — turn it on in Settings > Safari > Location" does not. - Never block the whole flow on an optional permission. ## 4. GPS is approximate, and you must decide how approximate is acceptable A position comes with an **accuracy radius**, and it is routinely 20–50 m outdoors and much worse indoors, near tall buildings, or on a device with wifi positioning only. - Read `accuracy` and use it. A check-in inside a 100 m geofence with 500 m accuracy has not been verified by anything. - Size the geofence for the site, not for the map pin. Loading docks, car parks and multi-building sites need a bigger radius than feels right. - Take the first fix, then watch for a better one — the first is often the coarse network fix, with the GPS fix arriving seconds later. - **Verify on the server.** Coordinates from a client are a claim, not a fact; any of them can be spoofed. Decide whether that matters for your use, and if it does, treat GPS as one signal among several rather than proof. - Provide a manual override with a reason, and log it. Otherwise a genuine failure leaves a worker unable to do their job. ## 5. Service worker updates are the top support issue A cached shell that will not update produces "I see the old version" reports that are impossible to reproduce, because the reporter's device is the only one affected. - Serve the entry document and the service worker with no-cache; serve hashed assets with long cache. Backwards, and users are pinned on a broken build. - Have an update path: detect the new worker, then either prompt ("A new version is ready — reload") or skip waiting and reload, but choose one and test it. - Never cache API responses in the same policy as the shell. - Be able to answer "which build is this user on?" — put the build id in the UI and in error reports. - Test the upgrade, not just the install: load the old version, deploy, reload, confirm the new one arrives. ## 6. Design for one hand, outdoors, in a hurry - Primary actions in the bottom third, where a thumb reaches. - Touch targets at least 44×44 px with real spacing. Two destructive actions should never be adjacent. - High contrast; do not rely on colour alone for state. - Never lose typed input on rotation, backgrounding, or an incoming call. - Confirm destructive actions, and make them undoable where you can. - Keep the primary flow reachable in as few taps as possible. Count them. ## 7. Throttle rather than disconnect The field is rarely cleanly offline. It is 2G, a half-open connection, a captive portal, or a connection that accepts the request and never answers. - Test at "slow 3G" throttling, not just offline. - Set request timeouts. A hung fetch with no timeout is an app that appears frozen. - Show progress for anything over a second, and make it cancellable. - Retry with backoff; give up and tell the user rather than spinning forever. If the flow must survive a real loss of coverage, that is a design decision on its own — see `offline-first-sync`. ## 8. Battery, data and storage are the user's, not yours - Continuous GPS drains a battery in hours. Sample, do not stream, and stop when backgrounded. - Downscale photos on capture. A modern phone camera produces files that are both slow to upload on 3G and expensive on a metered plan. - Cap local caches and evict. - Release the camera and any wake lock the moment the screen is left. ## Checklist - [ ] HTTPS available for device testing, documented in the README - [ ] Tested on a real, low-end device - [ ] Every permission has an explanation, a denied path and a permanent-denial message - [ ] GPS accuracy read and acted on; geofence sized for the site; server-side check; manual override logged - [ ] Service worker update path tested by upgrading, not installing - [ ] Build id visible in the UI - [ ] Reachable one-handed; 44 px targets; no input lost on rotate or background - [ ] Tested throttled, not only offline; timeouts on every request - [ ] Photos downscaled; GPS sampled; caches capped