A deep link opens a specific screen inside an installed app rather than a website. Three mechanisms exist: a custom URL scheme, Apple's Universal Links and Android's App Links — and only the last two degrade gracefully when the app is not installed.
The three mechanisms.
| Custom scheme | whatsapp://send?phone= — fails with an error if the app is absent |
|---|---|
| Universal Links (iOS) | A normal https:// URL, verified by apple-app-site-association |
| App Links (Android) | A normal https:// URL, verified by assetlinks.json |
| Verification file location | Served from /.well-known/ on the site's own domain |
| Verification fetch | Both files are read over HTTPS and must return HTTP 200 |
| Fallback behaviour | Verified https links open the website when no app is installed |
| In-app browsers | Often intercept links, and may ignore the verification entirely |
Prefer the https form on any public page. A custom scheme on a page a stranger opens is a dead end for everyone who does not already have the app.
Where this shows up.
Every social link on a bio page faces this choice. Linking instagram.com/yourname opens the app for people who have it and the website for people who do not; linking the custom scheme strands the second group.
The common mistake.
Testing only on your own phone, where every app is installed and every link appears to work. The failure only shows up on a device without the app, which is most of your visitors.
Sources.
Every figure above is from one of these. They are the standard, the registry or the platform’s own documentation — read them rather than take our word for it.
- Supporting universal links in your app Apple Developer
- Verify Android App Links Android Developers
- Mobile deep linking Wikipedia