I think that is desired, so not a bug but a feature

If that is true, would that issue be a feature request to allow non-bound props to be mutated?
If yes, it should be deleted and created under the appropriate issue template

Not sure though

Update

Apparently, it indeed does seem like a bug 🐛

@Malix-off, what is the reasoning for the change? As far as I'm aware Rich was wanting backwards compatibility with Svelte 4 and this would break 90% of my projects on update and require some refactoring. It will also break most people's StoryBlok setup with Svelte I should imagine (https://www.storyblok.com/tp/add-a-headless-cms-to-svelte-in-5-minutes#listening-to-changes-in-the-visual-editor).

It looks like a bug to me
especially because modifying the prop data is currently the way to make optimistic updates
https://youtu.be/HdkJTOTY-Js?t=663

I've updated to blocking all usage of svelte, this has become critical for me. When dealing with routes for example [slug]/+page.svelte, data.story updates when changing routes which is correct, however, I need to update data.story for StoryBlok's live editor work, so to do this I wrap data.story in a $state as I can't directly mutate data.story. I found that changing between pages in the same route causes $state to stay the same as it doesn't detect the update of page. Makes completely sense, it needs to be $derived however, I can not reassign a $derived value for live update.

Currently this completely breaks StoryBlok with Svelte.

Can you give an example how this breaks StoryBlok with Svelte? In which way do you interact with StoryBlok and mutate its state so that it currently breaks?

Since #10464 this will only be a warning and not an error anymore.

@dummdidumm I think @PP-Tom meant the code from the Storyblok documentation (here Section: "Listening to changes in the Visual Editor"):

<script>
  import { onMount } from "svelte";
  import { useStoryblokBridge, StoryblokComponent } from "@storyblok/svelte";

  export let data;

  onMount(() => {
      useStoryblokBridge(
          data.story.id,
          // modifying data here
          (newStory) => (data.story = newStory)
      );
  });
</script>

<div>
  {#if data.story}
      <StoryblokComponent blok={data.story.content} />
  {/if}
</div>

But I think one could just put the story data into state?

Yeah looks like it. The other alternative would be to use reassignment (data = { ...data, story: newStory }) rather than mutation