fix: handle remote query redirects

[Antonio-Bennett]

Summary

• Attach an onrejected handler when calling the async SSR render thenable to mark the underlying render promise as handled
• Treat Redirect thrown during render as a real redirect response in render_page
• Keeps normal redirect handling intact elsewhere

Context

The async render path returns a thenable from Svelte’s server renderer. Calling .then without an onrejected means the underlying async render promise can reject (e.g. Redirect) before any handler is attached, which triggers Node’s unhandled rejection detection. Passing an onrejected handler attaches a rejection handler directly to the underlying promise.

Separately, when a Redirect is thrown during render (e.g. via a remote query used in a component), it currently falls into the generic error handler and returns a 500. Handling Redirect in render_page ensures the response redirects instead.

How This Relates To Svelte

The unhandled rejection itself originates in Svelte’s SSR runtime (Renderer.run() does not attach a rejection handler or tie the async work to renderer.promise). The Svelte PR below addresses that root cause; this Kit PR handles the redirect response once the render error propagates.

Notes

AI-Generated PR: This PR was created with assistance from AI (OpenCode) to help move the discussion forward on issue #14883. While this solution provides a functional workaround, it may not represent the optimal or preferred approach for the SvelteKit maintainers. Please feel free to suggest alternative implementations or close this PR if a better solution is identified.

Related

• Fixes Remote query with redirect produce 500 undefined error #14883
• Svelte PR: fix: handle renderer.run rejections svelte#17591

Test Plan

• Repro app: redirect thrown from remote query during render no longer triggers unhandled rejection and now redirects properly

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Latest commit: 45c99a9

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[elliott-with-the-longest-name-on-github]

This isn't really the correct solution here. The problem is that somewhere in Svelte or SvelteKit, when a promise is created and stashed, it's not having a .catch handler attached to it. It's likely the promise is await-ed somewhere else, but it's after a microtask, so if the promise rejects it's treated as unhandled.

So basically there's somewhere we're missing a .catch(() => {}) and we just have to find that spot.

[Antonio-Bennett]

Yeah def thought so as well. Just put this out for an immediate workaround for now. Will keep digging around to see if I can find where that catch isn't being added! :)

[Antonio-Bennett]

@elliott-with-the-longest-name-on-github thanks for your feedback and review! Committed the change :)

[teemingc]

Could we add a test for this?

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-15222-svelte.vercel.app/

[Antonio-Bennett]

@teemingc added, is it satisfactory?

[Antonio-Bennett]

@teemingc Hey sorry don't want to be a bother, know you have a lot on your plate just wondering if you could take a look in case this slipped by

[teemingc]

So sorry about that! It definitely did slip by 🤦🏼 It's 1 AM here so I'll take a look at this tomorrow. Thanks!

[teemingc]

I was a little confused when the test still passed even when I commented out the fix. Turns out the console.log from the original issue is caused by us passing the redirect error to the handleError hook. The redirect always worked but if you had no handleError hook, the default one would try to log the .message property of a Redirect, and that would trigger the error.

I've cleaned it up a bit so that the test does fail if a Redirect somehow slips through. I've also added the proper status code if someone uses error in the remote function and that gets thrown

[Antonio-Bennett]

@teemingc thanks for taking a look and bringing it across the finish line! 🐐