Using readBody with Nuxt internal routing fails to parse URLSearchParams

[phoenix-ru]

Environment

• Operating System: Linux
• Node Version: v22.5.1
• Nuxt Version: 3.13.2
• CLI Version: 3.13.2
• Nitro Version: 2.9.7
• Package Manager: npm@10.8.2
• Builder: -
• User Config: compatibilityDate, devtools
• Runtime Modules: -
• Build Modules: -

Reproduction

https://github.com/phoenix-ru/h3-urlsearchparams-reproduction

Describe the bug

Users of @sidebase/nuxt-auth are experiencing the following problem with our module, described here:

ERROR  [nuxt] [request error] [unhandled] [500] The first argument must be of type string or an instance of Buffer, ArrayBuffer, or Array or an Array-like Object. Received an instance of URLSearchParams

We managed to pinpoint the problem to this piece of code:
https://github.com/sidebase/nuxt-auth/blob/a0704470583c5fea47c26c14b8673f8bb6a39a5f/src/runtime/composables/authjs/useAuth.ts#L120-L133

It works well when being called using HTTP, but fails when calling Nuxt endpoints from within a server. As you know,$fetch calls are translated to function calls in server code.

Upon some investigation, I was able to find the exact culprit - it is the readRawBody method and its Buffer.from return statement:

h3/src/utils/body.ts

Line 93 in 112fa33

return Buffer.from(_resolved);

This is what happens:

• during a regular HTTP call it looks like $fetch serializes URLSearchParams and is therefore capable to deserialize them later;
• this doesn't happen during a function call - readRawBody receives URLSearchParams as-is;
• URLSearchParams successfully evades all the if checks and ends up in the last return statement.

I fixed it locally and verified the patch, I will submit a PR shortly.

Additional context

No response

Logs

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[pi0]

fixed on edge and next release thanks.