What is universal linking?

What they replaced

Before universal links, iOS deep linking used custom URL schemes — `myapp://product/123`. Those had two serious problems: nothing happened at all if the app was not installed, and any app could claim any scheme, which was a genuine hijacking risk.

Universal links use ordinary HTTPS URLs. The same link opens the app if installed and the web page if not, and ownership is verified cryptographically rather than claimed.

How they work

1. You host an association file at a fixed path on your domain, listing which apps may open which URL patterns.
2. iOS fetches it when the app is installed, and caches the result.
3. A tap on a matching URL opens the app directly at the corresponding content.
4. If the app is not installed, the URL loads in the browser as a normal page.

How they break

• Association file problems. Wrong path, wrong content type, redirects, or not publicly reachable — all fail silently.
• Cached at install. Changes to the file are not picked up immediately, which makes testing confusing.
• User opt-out. Long-pressing a link and choosing to open in the browser sets a preference that persists, and the user does not know they set it.
• In-app browsers. Links opened inside a social app's own browser frequently do not trigger the app at all.
• The last two mean a meaningful share of universal links will not open the app regardless of correct implementation, which is worth accounting for rather than debugging indefinitely.