Passing $state wrapped variables to native code bridge causes DataCloneError: The object can not be cloned

[jamestinman]

Describe the bug

This took me a while to figure out this morning (LLMs also have no idea)

When passing Svelte 5 $state variables to native bridged code (via Capacitor in this instance) you can get a "DataCloneError: The object can not be cloned." error on iOS

Reproduction

import { AdMob, BannerAdSize } from '@capacitor-community/admob';
bannerOptions = $state({
        adId: unitId,
        adSize: BannerAdSize.ADAPTIVE_BANNER,
        position: BannerAdPosition.BOTTOM_CENTER
      });
 await AdMob.showBanner(bannerOptions);

// Work around, use spread operator (or $state.snapshot):
await AdMob.showBanner({ ...bannerOptions });

Logs

[Error] Unhandled Promise Rejection: DataCloneError: The object can not be cloned.
	postMessage
	postToNative (user-script:2:847)
	(anonymous function) (user-script:2:904)
	(anonymous function) (user-script:2:993)
	Promise
	(anonymous function) (user-script:2:992)
	(anonymous function)

System Info

System:
    OS: macOS 15.7.1
    CPU: (14) arm64 Apple M3 Max
    Memory: 602.67 MB / 36.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.19.0
    Yarn: 1.22.22
    npm: 11.6.1
  Browsers:
    Chrome: 142.0.7444.162
    Firefox: 143.0.1
    Safari: 26.0.1
  npmPackages:
    svelte: ^5.0.0-next.1 => 5.16.2

Severity

annoyance

[jamestinman]

I know there is a workaround, but it is not obvious and requires hunting down specific code.
Ideally if possible, the compiler recognises calls to non-Svelte packages and automatically passes the intent i.e. the unwrapped $state var.

[Conduitry]

There's no reasonable way for the compiler to know whether a given function call is going to expect reactive values or not, and any heuristics around that sound like a nightmare. https://svelte.dev/docs/svelte/$state#$state.snapshot is not a workaround; it is the intended way to get at the underlying non-reactive data in cases where you need it.

See also #14599, and the other issues linked to from that (stalled) PR.