The point of reproducibility is that, given the exact same configuration, you should be able to reproduce the exact same results. That does not imply that if you change something - including the NixOS version - you get the same result as before (otherwise you would never be able to update anything; that’s obviously not reproducing the same result).
For example, maybe you’ve previously built chromium without noticing and simply been using a locally cached version since. With the change to 25.11 lots of base libraries updated; If this is the case, now you need to build a new chromium, because it’s obviously not the same anymore and therefore you need to produce something different (i.e., this isn’t re-production, you’re not trying to reproduce either your old chromium or the upstream chromium) - and maybe your build fails because your old changes no longer result in a correct build for the new chromium version.
Or maybe they *are* correct
I suspect you’re just running OOM and the build would in fact complete fine after a very long time if you ran it with fewer cores; Not that you should, given the symptoms I suspect it’s likely you are in fact running a release channel, despite the nix-channel output. For now I have to go off your word that that’s not the problem without your code.
You might’ve also just accidentally changed something unrelated, even if you think you made no other changes; I’d recommend using git to guard against that kind of thing.
Reproducibility does mean that if you shared your configuration we could perfectly reproduce your problem and solve it for you; that’s what it buys you (among other things that are usually less exciting to end users). It also means that we can pretty confidently say it’s a configuration mistake and not an upstream problem, because we can reproduce working chromium builds and download it from the cache.
Whatever it is, without the code nobody but you can tell what it is. We can keep guessing random stuff, but we don’t even know if you’re reading your own code correctly, let alone whether you actually understand the suggestions. This thread will be very frustrating if it continues as-is; those two guesses are the last random suggestions you’ll get from me, at any rate. If you create a GitHub or codeberg repo it’s pretty easy to share, but yeah, that’d be a bit more effort.
I guess the alternative is to start writing a new configuration from scratch, adding back bits until you find the mistake? Given you’re using channels, I’d recommend using a VM to ensure there’s no state messing with things.