`__data.json` page invalidation returns corrupted JSON in some rare cases

[MSDev201]

Describe the bug

In some cases, specially on slower network devices (also happens simulating slow network in chrome dev tools), the page invalidation call to __data.json contains invalid json making JSON.parse fail.

`{"id":239,�"url":240,`

Reproduction

I've only experienced this issue on routes having a streamed promise from a load function.
This issue has been poping up in my sentry issues for almoast a year and kept happening about 10 to 20 times a day for various users. I could never reproduce the issue my self so i put it of as a user issue with their network. Today this exact issue happened to me in my local dev environment which implies it most likely is not a user network issue and made me open this bug report.

Logs

__Local error:__

VM26528:1 Uncaught (in promise) SyntaxError: Expected ',' or '}' after property value in JSON at position 57040 (line 1 column 57041)
    at JSON.parse (<anonymous>)
    at client.js?v=55e50156:2943:23


VM26529:1 Uncaught (in promise) SyntaxError: Expected double-quoted property name in JSON at position 11079 (line 1 column 11080)
    at JSON.parse (<anonymous>)
    at client.js?v=55e50156:2943:23


happening in the `function load_data(url, invalid)` function

__One of the errors reported in Sentry:__

SyntaxError
Unexpected token '�', "�{"type":""... is not valid JSON

System Info

System:
    OS: Windows 11 10.0.26200
    CPU: (24) x64 13th Gen Intel(R) Core(TM) i7-13700KF
    Memory: 28.95 GB / 63.86 GB
  Binaries:
    Node: 22.17.1 - C:\nvm4w\nodejs\node.EXE
    npm: 10.9.2 - C:\nvm4w\nodejs\npm.CMD
  Browsers:
    Chrome: 145.0.7632.160
    Edge: Chromium (145.0.3800.82)
    Internet Explorer: 11.0.26100.7309
  npmPackages:
    @sveltejs/adapter-node: ^5.5.3 => 5.5.3
    @sveltejs/kit: ^2.53.0 => 2.53.0
    svelte: ^5.53.2 => 5.53.2
    vite: ^6.3.0 => 6.4.1

Severity

annoyance

Additional Information

I can't verify it reliably but it seems like its happening more often when more requests to the invalidation endpoint are being issued.