[5.43.9] Reading async data in `{@attach}` causes `Cannot read properties of undefined (reading 'f')`

[sillvva]

Describe the bug

Reading remote function data in an attachment causes the error:

TypeError: Cannot read properties of undefined (reading 'f')

This error started in 5.43.9. It works in 5.43.8, although the boundary content is duplicated (an issue fixed in 5.43.9).

Current workaround is passing the data to the attachment in a callback function.

  <button
    {@attach download(() => ({
      filename: "data.json",
      blob: new Blob([JSON.stringify(data)])
    }))}
  >
    Export
  </button>

Reproduction

https://stackblitz.com/edit/sveltejs-kit-template-default-3yuabjmd?file=src%2Flib%2Fcomponent.svelte,src%2Froutes%2F%2Bpage.svelte

Logs

System Info

System:
    OS: macOS 26.1
    CPU: (16) arm64 Apple M4 Max
    Memory: 309.19 MB / 48.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.11.0 - /Users/sillvva/.nvm/versions/node/v24.11.0/bin/node
    npm: 11.6.1 - /Users/sillvva/.nvm/versions/node/v24.11.0/bin/npm
    pnpm: 10.21.0 - /Users/sillvva/.nvm/versions/node/v24.11.0/bin/pnpm
    bun: 1.2.20 - /Users/sillvva/.bun/bin/bun
  Browsers:
    Firefox: 144.0
    Safari: 26.1
  npmPackages:
    svelte: 5.43.9 => 5.43.9

Severity

annoyance

[7nik]

REPL

[dummdidumm]

Also happens at the top level - we need to add logic to attachments to await blockers.