(I assume this happens across platforms)
Sounds like a WontFix to me, as asynchronous functions return a promise, which is not a valid value to return for your expected outcome.
Unless this is a feature request.

Hello,

Thanks for your quick response and triage !

Actually, since the part of the spec allowing to return a Promise resolving to a BlockingResponse does not seem to be implemented as well (see 4th paragraph here https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/webRequest/onBeforeSendHeaders), I don't see how to implement my use case in a Chromium extension

Indeed, this piece of code does not work as expected whereas it works as expected in Firefox :

function promiseReturningFunction() {
    return new Promise((resolved) => {
        setTimeout(() => {
            resolved('Boum Paf Paf');
        }, 300);
    });
}

function createListener() {
    return function (request) {
        return promiseReturningFunction().then((result) => {
            const headers = [{ name: 'X-SPECIAL-HEADER', value: result }];

            return { requestHeaders: request.requestHeaders.concat(headers) };
        });
    };
}

chrome.webRequest.onBeforeSendHeaders.addListener(createListener(), { urls: ['<all_urls>'] }, ['requestHeaders', 'blocking']);


If you think I forgot something, misuse or did something wrong, let me know. If you have any pointer, comments or advice, I'd love to hear them.

Thanks again,

(FYI I've updated the async/await implementation of my opening ticket description in favor of a Promise resolving to a BlockingResponse implementation in my latest comment)

Looking at MDN for Chrome extensions can be misleading, because Firefox implements the Chrome extension API with some deviations and missing features. I think you caught one of the deviations here.

https://developer.chrome.com/extensions/webRequest#event-onBeforeSendHeaders does not mention any support for promises. Sorry.

Then it is a feature request 

I doubt no one has filed such a request already, but here you go.

Currently AFAIK all extension APIs use callbacks. Merging with crbug.com/328932.