Describe the bug

Since SvelteKit 2.39.0 calling error in a server load function while one of the parent layouts is streaming a promise causes the request to fail instead of responding with the error page

Reproduction

Reproduction: https://github.com/jackgdll/sveltekit-promise-streaming-error

1. Start the dev server and visit http://localhost:5173/page/subpage
2. The connection to the server is reset instead of showing the error page
3. Doing one of the following will restore the expected behaviour:
   • await the promise in src/routes/page/+layout.server.js instead of streaming it
   • pnpm i @sveltejs/kit@<2.39.0 and restart the server
   • move src/routes/page/+layout.server.js to src/routes/+layout.server.js

Logs

no logs, it just fails silently.

System Info

System:
    OS: macOS 26.0.1
    CPU: (12) arm64 Apple M2 Max
    Memory: 77.66 MB / 32.00 GB
    Shell: 4.0.8 - /opt/homebrew/bin/fish
  Binaries:
    Node: 22.14.0
    npm: 10.9.2
    pnpm: 10.17.1
  Browsers:
    Chrome: 141.0.7390.66
    Firefox: 135.0
    Safari: 26.0.1
  npmPackages:
    @sveltejs/adapter-auto: ^6.1.1 => 6.1.1 
    @sveltejs/kit: ~2.39.0 => 2.39.1 
    @sveltejs/vite-plugin-svelte: ^6.2.1 => 6.2.1 
    svelte: ^5.39.11 => 5.39.11 
    vite: ^7.1.9 => 7.1.9

Severity

blocking an upgrade

Additional Information

No response