How to verify domain on Microsoft Azure and CloudFlare Pages

Default domain verification  on Microsoft Azure requires placing a /.well-known/microsoft-identity-associaion.json file on your web server with a specific content:

The trouble is, if you deploy with CloudFlare, for example, it fails verification before even checking the content, because someone at Microsoft decided they need to verify Content-Length header first, which CloudFlare doesn't set for this request:

It is also impossible to set Content-Length header yourself for a custom URL through CloudFlare interface, and it is not possible to set it "as is" via a CloudFlare Worker/Function, but there is a trick you can do: convert response to be fixed length.

Here is a working function code that successfully passes Azure domain verification:

const overrideMicrosoft = async ({ request, next, env }) => {
    const targetPath = '/assets/microsoft-idendity-association.json';
    const url = new URL(request.url);

    if (url.pathname === targetPath) {
        const response = await next();
        const json = await response.text();
        const { writable, readable } = new FixedLengthStream(json.length);

        const enc = new TextEncoder();
        const writer = writable.getWriter();
        writer.write(enc.encode(json));

        return new Response(readable);
    } else {
        return next();
    }
}

export const onRequest = [overrideMicrosoft];

Just add it to functions/_middleware.js and you're done.