Proxys inside $state don't work

[GuillaumeDerval]

Describe the bug

Proxys are somehow ignored once they are inside a $state.

This occurs both on Proxies being directly $stated (as shown below) but also on proxies that are deeply nested somewhere inside an object (a more useful usecase :-))

Reproduction

Playground link: https://svelte.dev/playground/78baf2231a9647d09b52743fe53b8437?version=5.19.6.

<script>
	let thing = {
		entry: "one"
	}

	let proxyThing = new Proxy(thing, {
		set(target, prop, value) {
				console.log("set", prop, value);
				return Reflect.set(...arguments);
		}
	})

	proxyThing.entry = "two"
	
	let stateProxyThing = $state(proxyThing)

	stateProxyThing.entry = "three";
</script>

<input type=text bind:value={stateProxyThing.entry}/>

Current behavior:

• Log (in console) set entry two
• "three" appears in the input field

Expected:

• Log (in console) set entry two
• Log set entry three
• "three" appears in the input field
• When the input field is modified set entry X appears in the console, with X the new content of the field

Logs

System Info

System:
    OS: macOS 15.2
    CPU: (10) arm64 Apple M1 Pro
    Memory: 102.95 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 23.6.1 - /opt/homebrew/bin/node
    npm: 10.9.2 - /opt/homebrew/bin/npm
  Browsers:
    Chrome: 132.0.6834.160
    Safari: 18.2
  npmPackages:
    svelte: ^5.7 => 5.19.6

Severity

annoyance

[7nik]

That's because $state lazily copies the value. Instead, you should proxify the $state.

Talking about the second case, it's impossible to detect whether the object is proxied. So I see the only way - also $statify the value because currently stateful state isn't copied.

[GuillaumeDerval]

Thanks. Are these two behaviors ($state lazyly copying and $state not copying $statefied objects) documented somewhere?