[Bug]: Storybook fails when using async svelte

[jhechtf]

Describe the bug

Svelte just recently released the Async Svelte, allowing for usage of await in many places in Svelte components.

When trying to use Storybook in a project that is utilizing this feature, currently only available through an experimental compiler option, storybook breaks on every component, even the example ones storybook creates when initializing a project.

Reproduction link

https://github.com/jhechtf/storybook-svelte-5-repro

Reproduction steps

1. Create a new svelte project with pnpx sv create, choose minimal and tailwindcss.
2. Add in storybook with pnpx storybook@latest init
3. Edit svelte.config.js to have the following compilerOptions

const config = {
  preprocess: vitePreprocess(),
  compilerOptions: {
    // Enable run-time checks when not in production
     experimental: {
       async: true,
     }
  },
  kit: {
    adapter: adapter({
      fallback: "index.html",
    }),
  },
};
export default config

4. Try to run storybook with pnpm storybook. The page will open and you will find the following error

5. Stop the storybook process, comment out the `experimental: ...` and re-run, and you will get the following

System

System:
    OS: Windows 10 10.0.19045
    CPU: (16) x64 AMD Ryzen 7 3700X 8-Core Processor
  Binaries:
    Node: 24.4.1 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.5 - C:\Program Files (x86)\Yarn\bin\yarn.CMD
    npm: 11.4.2 - C:\Program Files\nodejs\npm.CMD
    pnpm: 9.6.0 - C:\Program Files\nodejs\pnpm.CMD <----- active
  Browsers:
    Chrome: 139.0.7258.128
    Edge: Chromium (139.0.3405.86)
  npmPackages:
    @storybook/addon-svelte-csf: 5.0.7 => 5.0.7
    @storybook/addon-vitest: 9.1.2 => 9.1.2
    @storybook/sveltekit: 9.1.2 => 9.1.2
    storybook: 9.1.2 => 9.1.2

Additional context

No response

[dosubot]

Hi @jhechtf! I'm Dosu and I’m helping the storybook team.

Storybook does not currently support Svelte's experimental async compiler option. The error you’re seeing (“setContext called after initialization”) happens because Storybook’s Svelte renderer assumes synchronous component setup, but async Svelte changes the timing of context initialization in a way that breaks this assumption. There are no official workarounds or patches for this yet, and supporting async Svelte would require significant changes to how Storybook mounts and manages Svelte components and context lifecycles. For now, you’ll need to disable the async experimental option to use Storybook with Svelte components. You can track related renderer and compatibility discussions here and here.

To reply, just mention @dosu.

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[yuki0418]

Is there any way to prevent Storybook from crashing?

[o2ba]

could it be related to sveltejs/svelte#16629?