It's not possible to proxy URLs with square brackets because URI does not like them:

irb(main):001> URI.parse('http://example.com/[id]/page.svelte')
(irb):1:in `<main>': bad URI(is not URI?): "http://example.com/[id]/page.svelte" (URI::InvalidURIError)

This is especially handy while proxying through Svelte apps, which use square brackets to identify parameters in URLs while routing: https://svelte.dev/docs/kit/routing