Introduce of custom transports leads to `deserialize` not working on server side

[gurpreetatwal]

Describe the bug

Prior to @sveltejs/kit@2.11.0 I was able to use the deserialize function in server side routes to parse the response returned by calling svelte kit action methods using fetch. If relevant, my specific use case is a route that runs automates filling out some forms for the user based on the response from an LLM. I use deserialize to parse the response from the action.

Attempting to upgrade to any version >= 2.11.0 (including latest 2.20.3 as of April 3) results in all server side usages of deserialize throwing the following error:

Cannot read properties of undefined (reading 'decoders')
TypeError: Cannot read properties of undefined (reading 'decoders')
    at deserialize (/node_modules/@sveltejs/kit/src/runtime/app/forms.js:37:84)
    at load (/src/routes/load/+page.server.js:17:74)
    at async load_server_data (/node_modules/@sveltejs/kit/src/runtime/server/page/load_data.js:45:18)
    at async Promise.all (index 1)
    at async render_data (/node_modules/@sveltejs/kit/src/runtime/server/data/index.js:101:17)
    at async resolve (/node_modules/@sveltejs/kit/src/runtime/server/respond.js:443:17)
    at async respond (/node_modules/@sveltejs/kit/src/runtime/server/respond.js:329:20)
    at async file:///node_modules/@sveltejs/kit/src/exports/vite/dev/index.js:521:22

I ran npm run build and npm run preview results in the same error as well.

deserialize still works when used client side (.svelte) files but does not work in any server.js/ts files.

Reproduction

• 2.10.1 (last working version): https://stackblitz.com/edit/deserialize2-10-1
• 2.11.0 (first failing version): https://stackblitz.com/edit/deserialize2-11-0
• 2.20.3 (latest): https://stackblitz.com/edit/deserialize2-20-3

Logs

Cannot read properties of undefined (reading 'decoders')
TypeError: Cannot read properties of undefined (reading 'decoders')
    at deserialize (/node_modules/@sveltejs/kit/src/runtime/app/forms.js:37:84)
    at load (/src/routes/load/+page.server.js:17:74)
    at async load_server_data (/node_modules/@sveltejs/kit/src/runtime/server/page/load_data.js:45:18)
    at async Promise.all (index 1)
    at async render_data (/node_modules/@sveltejs/kit/src/runtime/server/data/index.js:101:17)
    at async resolve (/node_modules/@sveltejs/kit/src/runtime/server/respond.js:443:17)
    at async respond (/node_modules/@sveltejs/kit/src/runtime/server/respond.js:329:20)
    at async file:///node_modules/@sveltejs/kit/src/exports/vite/dev/index.js:521:22

Cannot read properties of undefined (reading 'decoders')
TypeError: Cannot read properties of undefined (reading 'decoders')
    at deserialize (file:///.svelte-kit/output/server/chunks/forms.js:6:50)
    at load (file:///.svelte-kit/output/server/entries/pages/load/_page.server.js:12:37)
    at async load_server_data (file:///.svelte-kit/output/server/index.js:621:18)
    at async Promise.all (index 1)
    at async render_data (file:///.svelte-kit/output/server/index.js:1965:19)
    at async resolve2 (file:///.svelte-kit/output/server/index.js:2977:22)
    at async respond (file:///.svelte-kit/output/server/index.js:2882:22)
    at async file:///node_modules/@sveltejs/kit/src/exports/vite/preview/index.js:197:5

System Info

System:
    OS: Linux 5.15 Ubuntu 22.04.5 LTS 22.04.5 LTS (Jammy Jellyfish)
    CPU: (8) x64 Intel(R) Core(TM) i7-4790K CPU @ 4.00GHz
    Memory: 11.96 GB / 31.28 GB
    Container: Yes
    Shell: 5.8.1 - /usr/bin/zsh
  Binaries:
    Node: 22.13.1 - ~/.local/share/nodejs/bin/node
    Yarn: 1.22.22 - ~/.local/share/nodejs/bin/yarn
    npm: 10.9.2 - ~/.local/share/nodejs/bin/npm
    pnpm: 9.1.0 - ~/.local/share/nodejs/bin/pnpm
  Browsers:
    Chrome: 135.0.7049.52
  npmPackages:
    @sveltejs/adapter-node: ^5.2.12 => 5.2.12
    @sveltejs/kit: ^2.10.0 => 2.10.0
    @sveltejs/vite-plugin-svelte: ^3.1.2 => 3.1.2
    svelte: ^4.2.19 => 4.2.19
    vite: ^5.4.10 => 5.4.11

Severity

blocking an upgrade

Additional Information

Probably related to #13149