Reactivity not reflected in legacy components after Svelte 5.34.4

[probablykasper]

Describe the bug

Deep reactivity from .svelte.js in non-runes mode components broke in 5.34.4.

I don't see anywhere in the docs that this is not supposed to work.

There are related issues to this, but they are about components that were implicitly/unexpectedly in non-runes mode:

• Deep tracking in legacy mode doesn't work since 5.34.4 #16199
• Reactivity issue since 5.34.4 #16203

Reproduction

https://svelte.dev/playground/78f5c3f4495b40d68def384072bce77e?version=5.50.1

Logs

System Info

System:
    OS: macOS 13.7.8
    CPU: (8) x64 Intel(R) Core(TM) i7-7700K CPU @ 4.20GHz
    Memory: 445.73 MB / 40.00 GB
    Shell: 4.3.3 - /Users/k/.nix-profile/bin/fish
  Binaries:
    Node: 24.11.1 - /Users/k/.local/share/mise/installs/node/24.11.1/bin/node
    npm: 11.6.2 - /Users/k/.local/share/mise/installs/node/24.11.1/bin/npm
    Deno: 2.5.6 - /Users/k/.local/share/mise/installs/deno/2.5.6/bin/deno
  Browsers:
    Brave Browser: 144.1.86.146
    Firefox: 147.0.3
    Safari: 18.6
  npmPackages:
    svelte: ^5.50.1 => 5.50.1

Severity

annoyance

[paoloricciuti]

This is expected-ish. It should not have worked before, we fixed it at some point but it was causing many issues so we "semi-reverted" it but only for components that are not explicitly legacy.

If you remove runes={false} it works because it's a component that is not in runes mode but also not explicitly legacy.

[probablykasper]

Are there any docs that indicate that it's not supposed to work?

iirc there was talk about logging warnings about it, but it having too many false positives

runes={false} is just there for illustration, my actual component has a bunch of classic syntax