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TypeError: cookies is not iterable #9091

Description

@Vextil

Describe the bug

After updating kit from 1.5.5 to 1.7.1 I'm getting the following error on every request:

TypeError: cookies is not iterable
    at Headers.get [headers map sorted] (/project/app/node_modules/undici/lib/fetch/headers.js:430:29)
    at /project/app/node_modules/undici/lib/fetch/headers.js:476:21
    at Headers Iterator.next (/project/app/node_modules/undici/lib/fetch/util.js:731:22)
    at Module.serialize_data (/node_modules/@sveltejs/kit/src/runtime/server/page/serialize_data.js:55:24)
    at eval (/node_modules/@sveltejs/kit/src/runtime/server/page/render.js:367:27)
    at Array.map (<anonymous>)
    at Module.render_response (/node_modules/@sveltejs/kit/src/runtime/server/page/render.js:366:5)
    at async Module.render_page (/node_modules/@sveltejs/kit/src/runtime/server/page/index.js:306:10)
    at async resolve (/node_modules/@sveltejs/kit/src/runtime/server/respond.js:371:17)
    at async Module.respond (/node_modules/@sveltejs/kit/src/runtime/server/respond.js:242:20)

Reproduction

Happens when making a server-side fetch to an endpoint that returns a Set-Cookie header.

Seems to be an issue with the new undici version:
nodejs/undici#1935

Fixed here, but no release yet:
nodejs/undici#1936

Logs

No response

System Info

System:
    OS: Linux 5.15 Ubuntu 22.04.1 LTS 22.04.1 LTS (Jammy Jellyfish)
    CPU: (8) x64 AMD Ryzen 5 5600G with Radeon Graphics
    Memory: 2.42 GB / 5.95 GB
    Container: Yes
    Shell: 5.1.16 - /bin/bash
  Binaries:
    Node: 16.17.0 - /usr/local/bin/node
    npm: 8.15.0 - /usr/local/bin/npm
  npmPackages:
    @sveltejs/adapter-auto: ^1.0.0 => 1.0.3 
    @sveltejs/adapter-cloudflare: ^2.0.1 => 2.0.2 
    @sveltejs/adapter-node: ^1.1.8 => 1.2.0 
    @sveltejs/kit: ^1.7.1 => 1.7.1 
    svelte: ^3.55.0 => 3.55.1 
    vite: ^4.1.0 => 4.1.2

Severity

blocking an upgrade

Additional Information

Probably nothing we can do other than waiting for a new undici version. Rollback not recommended because of security vulnerabilities.
Publishing the issue anyway so if anyone else runs into the same issue they can find this.

Activity

  1. added this to the soon milestone on Feb 17, 2023
  2. smblee commented on Feb 18, 2023

    @smblee
  3. Vextil commented on Feb 21, 2023

    @Vextil
    ContributorAuthor

    Fixed on #9106
    Latest 1.8.3 version of SvelteKit is working fine.

  4. added theissue type on Aug 4, 2026
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