> I have no idea what you mean by this. First of all, of course you don't have to put the entire copyright license into each source file; this is solely about copyright notices, which typically point to a more complete LICENSE/NOTICE file.

You don't even need a copyright notice. Under current US law (and that of many other countries that have harmonized their copyright regimes), copyright is automatic, and requires no notice at all. Obviously it is better/safer for you to put a copyright notice on everything you create, but it is not required. You will hold copyright on a work you create and distribute today, even if you don't put your name or the magic "Copyright 2024 $NAME" bit anywhere on it.

I think there's some confusion here about copyright vs. licensing: those two things are independent, except of course that you need to hold the copyright to be able to set licensing terms. The default, without any provided license, is the most restrictive of one: that no one else has any rights toward the covered work.

> While registering copyrights or including copyright notices explicitly is not necessary to have protection under copyright law, my layperson understanding is that it does indeed afford you additional protection under law in some cases.

Registering does, yes. In the US, at least, you cannot file suit to protect your copyright unless it is registered. I don't believe the presence or absence of a copyright notice matters one bit; ultimately regardless of the presence or absence of a copyright notice, if it ends up in court, everyone will be providing more documentation as to the provenance of the work in question.

The dates you list in the copyright notice are perhaps useful for readers, but if a dispute ends up in court, part of the proceedings may involve determining the creation date/year of any bits under dispute, independent of what you put in the copyright notice. But in practice, with software, considering the stupid-long time things remain under copyright these days, the exact date doesn't matter much, as pretty much anything anyone is going to bring a dispute about is going to still be under copyright.

> However, if you ever actually wind up in court over copyright issues, the lack of clarity on what licenses go where could possibly create reasonable doubt

"Beyond a reasonable doubt" is a standard applied to criminal cases; civil cases have a lower burden of proof. But yes, you're always better off with things documented than not. A judge/jury will certainly appreciate a plaintiff or defendant who has all their ducks in a row vs. one that is disorganized. But they'll also be cognizant of (and try to determine) what the copyright holder intended to do, even if the documentation doesn't spell it out as clearly as one would like.

The law is not a computer; you're not likely to get away with copyright infringement just because the copyright holder missed some detail.

> I am mainly concerned about the copyright notices because they explicitly denote the copyright license

No they don't. A "copyright notice"[0] is simply something that says "Copyright $YEAR $NAME". A "copyright license" is a list of terms that give people more rights to use and distribute the work than they would get under copyright law.

You don't need to put licensing information in each source file. If everything in a project is released under the same license, noting the license in a single place is fine. If different files have different licenses, of course you'll need to note things specifically; though, again, if you don't want to put licensing information in each file, you can still note in a single place which files are under which license.

But as I mentioned above, more documentation is better/safer than less. Personally I put licensing information in every source file of the things I release, even if every file is under the same license as the project as a whole.

[0] https://en.wikipedia.org/wiki/Copyright_notice#Form_of_notic...