@ascorbic perhaps you might have some idea if there's some support we're missing?

Does the project build okay then? Only 404's when the site is live and trying to hit that endpoint?

I'm wondering if a redirect would work here as a workaround. I'll dig more into it, but this might be a temp fix.

• Create a route /admin
• Inside routes/admin create a +page.server.ts file with the following:

import { redirect } from '@sveltejs/kit';
import type { PageServerLoad } from './$types';

export const load: PageServerLoad = async () => {
	throw redirect(
		301,
		'https://example.com/.netlify/identity'
	);
};

• In netlify.toml:

[[redirects]]
  from = "/admin"
  to = "https://example.com/.netlify/identity"

The project did build okay the last time I tested it, but I haven't messed with that in a while now. I can repro my old project (with updated dependencies) some time this week to verify it it still causes issues.

The project did build okay the last time I tested it, but I haven't messed with that in a while now. I can repro my old project (with updated dependencies) some time this week to verify it it still causes issues.

Okay, I'm going to look into this. One thing I noticed is that @sveltejs/adapter-netlify was not installed and specified in the svelte.config.js file in the Stackblitz reproduction. The adapter-auto does not look at the configuration, so it would not pick up edge: true in the adapter config.

I'm pretty sure my actual project had adapter-netlify, but I don't have the original project anymore

I can confirm the issue is still occurring. It works on this commit, but not this one. Navigating to /admin is fine, but the login doesn't work.

This is kind of an issue how edge functions are handled by Netlify. Issue is that by using ^/.*$ regex in the manifest.json you will catch all the request and pipe them through Sveltekit. With lambda functions this is fine, and /.netlify/* doesn't get caught.

(I opened at Netlify's forum but their soution is a bit overcomplicated imho: https://answers.netlify.com/t/how-to-use-netlify-identity-with-edge-functions-sveltekit/75634)

I wrote a very dirty solution for it. I'm replacing the regex pattern created by the adapter to ^/[^.].*$. This way all the routes starting with . will not get caught by the edge function. This awful solution comes with a caveat that the aformentioned regex excludes / too... So unless you prerender the home page anyway this would break your app. There might be a better regex for this but since edge functions doesn't support look aheads I couldn't think of a better one. (https://docs.netlify.com/edge-functions/create-integration/#generate-declarations)

Here is how I did this with a onBuild build plugin:

import { readFile, writeFile } from 'fs/promises'

/**
 * Keep /.netlify/* endpoints getting caught by edge function pattern
 */
export const onBuild = async function ({ build }) {
	try {
		const file = await readFile('.netlify/edge-functions/manifest.json', {encoding: 'utf8'}).then(s => JSON.parse(s))
		// https://regex101.com/r/ldVrTI/1
		// This will actually ignore all routes starting with ".". => /.*/*
		file.functions[0].pattern = '^/[^.].*$'
		await writeFile('.netlify/edge-functions/manifest.json', JSON.stringify(file))
	} catch (err) {
		build.failBuild(err)
	}
}

We're adding support for negative matchers. It's currently experimental, but can add it to the adapter when it's a stable. It would be good to think of which other routes would need excluding.

Any updates on this, I can't see any workaround documented

A workaround is listed here: #10584 (point 4 of alternatives considered)

EDIT: Looks like this is already mentioned above