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# Which Problems Are Solved The `/.well-known/apple-app-site-association` and `/.well-known/assetlinks.json` responses are instance specific: the body depends on the requested host. The responses did not declare this, so a shared cache whose cache key does not include the host could store one instance's file and serve it on another instance's domain. This could result in a wrong or empty file being returned intermittently, breaking native passkey verification. Dropping the `public` directive (#12634) was not sufficient: depending on its cache mode, a shared cache may store any response carrying a `max-age`. The docs page for native app links also still documented the old `Cache-Control: public` header and only the single `get_login_creds` relation (missing `handle_all_urls`, added in #12634). # How the Problems Are Solved - Send **`Vary: Host`** on both well-known responses, declaring that the body depends on the host: a spec-compliant shared cache must store per-host variants or skip caching. Clients addressing a single host (the Apple and Google verifiers, browsers) are unaffected and keep honoring the configured `max-age`. - Correct the outdated caching reasoning in the code comments. - Guard the header with unit and integration tests. - Update the native app links docs page: both Digital Asset Links relations and the current response headers. # Additional Changes None. `WellKnown.AppLinksCacheControlMaxAge` behavior is unchanged (`0` still results in `no-store`). # Additional Context - Follow-up to #12634; relates to #12497. - Verified with interleaved requests against two instances: responses are no longer reused across domains, and each domain consistently serves its own configuration. - To be cherry-picked to `v4.x` from `main` once merged. 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Co-authored-by: Claude Fable 5 <[email protected]>