I’ve created a repository for this purpose to demonstrate what the problem is: https://github.com/cxplay/vercel-rewrite-issue

GitHub

GitHub - cxplay/vercel-rewrite-issue: vercel-rewrite-issue

vercel-rewrite-issue. Contribute to cxplay/vercel-rewrite-issue development by creating an account on GitHub.

The vercel.json expectation should rewrite /whichpath.vercel.app/web/path to https://whichpath.vercel.app/web/path so that the body of the HTTP response is /web/path.

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{
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  "$schema": "https://openapi.vercel.sh/vercel.json",
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  "rewrites": [
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    {
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      "source": "/(whichpath.vercel.app/web/path)",
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      "destination": "https://$1"
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    }
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  ]
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}

But the actual response is:

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❯ curl https://vercel-rewrite-issue-red.vercel.app/whichpath.vercel.app/web/path
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/web/path/

The problem is in the trailing slash, which causes some Web services that don’t adapt to the trailing slash to fail to find the target.