Environment

• Operating System: Darwin
• Node Version: v22.14.0
• Nuxt Version: 3.17.5
• CLI Version: 3.25.1
• Nitro Version: 2.11.12
• Package Manager: pnpm@10.11.0
• Builder: -
• User Config: compatibilityDate, devtools
• Runtime Modules: -
• Build Modules: -

Reproduction

Couldn't do it in StackBlitz because it's a cookie thing but here is a minimal reproduction to run locally: https://github.com/pgraug/nuxt-repro-forgets-set-cookie-on-error

Describe the bug

When setting multiple cookies during SSR and encountering an error, only the last cookie you set will be included in the response headers instead of one set-cookie header per cookie.

This is due to one use of setResponseHeader in Nuxt's Nitro error handler (permalink to line) that overwrites the previous set-cookie headers each time it sees a new one.

I propose two possible solutions:

• Using appendResponseHeader inside the loop instead since it handles multiple cookies with the same by converting them to an array. Only thing I'm not sure about with this is whether there are some unintended side-effects like content-type becoming an array as well because the response already had one.
• Doing an Array.reduce on Array.from(res.headers.entries()) to group headers of the same name into an array and then using setResponseHeaders to set them all at once. This should be safe from any unintended side-effects but is more complex.

The first solution seems the simplest but I would love your thoughts on whether it would have any unintended consequences. I can make the PR for it if you're okay with my solution.

Additional context

No response

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