manifest.json

{
   "background": {
      "persistent": true,
      "scripts": [ "background.js" ]
   },
   "description": "Header Test",
   "homepage_url": "https://sermak.ca",
   "icons": {
      "128": "icon.png"
   },
   "manifest_version": 2,
   "name": "Header Test",
   "permissions": [ "webRequest", "webRequestBlocking", "tabs", "http://*/*", "https://*/*", "file:///*" ],
   "short_name": "HT",
   "version": "31.6"
}

background.js

// override User Agent
var OBSHOption = ["requestHeaders", "blocking"];
if (chrome.webRequest.OnBeforeSendHeadersOptions.hasOwnProperty("EXTRA_HEADERS")) OBSHOption.push("extraHeaders");
chrome.webRequest.onBeforeSendHeaders.addListener(function(details)
{
	details.requestHeaders = details.requestHeaders.filter(function(elem) { return !elem.name.toLowerCase().includes("sec-ch-ua"); });
	details.requestHeaders.find(function(elem) { if (elem.name.toLowerCase() == "user-agent") elem.value = "New Agent"; });

	return {requestHeaders:details.requestHeaders};
}, {urls:["<all_urls>"]}, OBSHOption);

It's not fixed in v3 either,- I'm guessing it has to do with the "allow access to search results" and a bug in the Opera code that does sets this permission too late for headers to be modified, or something like that ...

In the meantime I've switched to Brave, but submitted a bug report.

extension is installed that modifies the User-Agent header. For v2 extensions that use chrome.webRequest.onBeforeSendHeaders, as well as v3 extensions chrome.declarativeNetRequest via modifyHeaders, the extension is able to modify all requestHeaders as expected except headers on google.com domain, but the issue is only with the initial request, all subsequent modifications are fine.

Do you have minimal test extensions (v2 and v3) that show this?

A quick way to test for this error is to search "what is my user agent" on google.com and notice how the UA is never modified as expected.

I've noticed that too for a long time with https://addons.opera.com/en/extensions/details/user-agent-switcher-5/ for example (when set to Chrome for Windows and then clicking the pencil icon (edit) to set the string to Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/123.0.0.0 Safari/537.36 (what Chrome Stable says at the URL chrome://version)).

"Allow access to search page results" doesn't help and neither does going to the URL opera://browserjs and disabling browser.js (to disable any user-agent workarounds Opera might have for Google that might override an extension's).

I figured it might have to do with client hints that the extension isn't modifying or something to do with some Javascript navigator.stuff that the extension neglects to change., but not sure.

What does seem to work though is starting Opera like this:

"opera.exe" "--user-agent=Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/123.0.0.0 Safari/537.36"

and not using an extension at all. Downside with that is it messes with Opera's sites.