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__data.json page invalidation returns corrupted JSON in some rare cases #15511
Description
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 JSONSystem 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 => 6.4.1Severity
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.
Activity
elliott-with-the-longest-name-on-github commentedon Mar 12, 2026
I'll throw this one at an agent to see what it comes up with, but without a reproduction this is going to be very hard to track down 😓
fix: reject malformed streamed data encoding (#16423)