Svelte 5: state_unsafe_mutation Updating state inside a derived is forbidden. If the value should not be reactive, declare it without $state

[niemyjski]

Describe the bug

After upgrading from 193 to latest I started to get the following error message when I use derived.

state_unsafe_mutation Updating state inside a derived is forbidden. If the value should not be reactive, declare it without $state

This seems like a bug because looking at the component source there is no updating inside of a derived value or in my $derived: https://github.com/svecosystem/runed/blob/main/packages/runed/src/lib/utilities/MediaQuery/MediaQuery.svelte.ts#L60

Reproduction

https://svelte-5-preview.vercel.app/#H4sIAAAAAAAACo2QQU_DMAyF_4qxJrWTygaIU7cOcQcJtCPhkDXuFqlNoiTdmKr8dxTaQatduMX2y3v212Ela3KYf3SoeEOY47MxmKE_m1i4I9WeMEOnW1vGztqVVhq_YYp52RhtPXTwSkLy95bsGQJUVjeQ2FaRSFYj2VYK2nE7zBfLoV70GUlUMgVQauU8SPfC7Z62pSVSvXMBik6jqDRJG6luT1L4Qw73dw-P5muezFdMrZd_S6r1JXdiWXTXCYuG-_JALsBygxk2WshKksDc25ZC9ktoMPwHJai52hcMvWO4iccBSOXJVrwkeLPaOOj69s9ovNFTDjuta-Jq1StCPCY-aorIJ2II-WBXwMzERzofvvU43UGfLhwKmAmy8kgivZm4XKGbZhQwZRaYmtp2ozJcA_wM37tto4xuAgAA

Logs

at state_unsafe_mutation (http://localhost:5173/next/node_modules/.vite/deps/chunk-PNZBZ7D7.js?v=28b39c12:187:19)
    at set (http://localhost:5173/next/node_modules/.vite/deps/chunk-PNZBZ7D7.js?v=28b39c12:1901:5)
    at get matches (http://localhost:5173/next/node_modules/.vite/deps/runed.js?v=28b39c12:602:7)
    at get isLargeScreen (http://localhost:5173/next/src/routes/(app)/+layout.svelte?t=1721699576709:182:27)
    at http://localhost:5173/next/src/routes/(app)/(components)/layouts/Sidebar.svelte?t=1721699576709:42:47
    at update_reaction (http://localhost:5173/next/node_modules/.vite/deps/chunk-PNZBZ7D7.js?v=28b39c12:1276:23)

### System Info

```shell
System:
    OS: Windows 11 10.0.22631
    CPU: (24) x64 AMD Ryzen 9 5900X 12-Core Processor
    Memory: 9.11 GB / 31.91 GB
  Binaries:
    Node: 22.5.1 - C:\Program Files\nodejs\node.EXE
    npm: 10.6.0 - ~\AppData\Roaming\npm\npm.CMD
    pnpm: 9.5.0 - C:\ProgramData\chocolatey\bin\pnpm.EXE
  Browsers:
    Edge: Chromium (126.0.2592.113)
    Internet Explorer: 11.0.22621.3527

Severity

blocking all usage of svelte

[paoloricciuti]

As you can see the getter you are accessing is actually updating #matches both in the case the effect has not been registered yet or if effect.tracking is false.

Since it's the first time that setter is accessed you are getting this error because it's actually accessing state. So this could be a bug of the implementation in runed (that was my idea so blame me eventually lol). At the same time this feels like something it should be possible to solve. I'll think about this a bit.

[trueadm]

This looks like a bug in runed. They're mutating state in the getter, which feels a bit off here – or it should be tracked. Either way, It's not a Svelte 5 bug so closing :)