Crash on circular reference assignment in $state with 5.34.0+

[rhiller]

Describe the bug

Assigning a circular reference into a $state object immediately crashes with a RangeError (maximum call stack size exceeded) in Svelte 5.34.0+, even without any recursion.

This worked as expected in Svelte 5.33.19.

Reproduction

<script>
  const filesState = $state({ files: {} });
  let nodes = { id: 1, items: [{ id: 2, items: [{ id: 3 }, { id: 4 }] }] };

  filesState.files = nodes;
  console.log('before assignment');
  filesState.files.items[0].parent = filesState.files; // 💥 crashes here
  console.log('after assignment'); // never reached
</script>

Logs

System Info

- Svelte version: 5.34.0 (and above)
- Previous working: 5.33.19

Severity

blocking an upgrade

[paoloricciuti]

Mmm does this also happen if you run it in production mode?

[rhiller]

Mmm does this also happen if you run it in production mode?

Just checked and it works fine in production mode with 5.34.0 (and with latest svelte@5.38.1).

Very hard to do development though.

[paoloricciuti]

Yeah It was just to pinpoint the issue, I think I know what this is