[A question on Homebrew internals] Why is CLT required on non-latest macOS releases

[673123]

Output of brew config

N/A

Output of brew doctor

N/A

Description of issue

Hello. I'm someone who's trying to learn how computer systems work. Through reading the code, I saw that when the macOS is older than a hard-coded latest version, CLT is required and the user it notified "Xcode alone is not sufficient" . So I'm very curious about the reason behind it, why can't Xcode be used when the macOS platform is not latest?

Update of extra info

This feature was added in pr-3453, which is stated to help "avoid the continuous workarounds for SDK weirdness".

[SMillerDev]

I saw that when the macOS is older than a hard-coded latest version, CLT is required and it say "Xcode alone is not sufficient" .

Where did you see that? Because it's hard to comment on the reason behind such a broad statement.

[gromgit]

I'm guessing in the output of brew doctor, which @673123 didn't provide.

[673123]

Where did you see that? Because it's hard to comment on the reason behind such a broad statement.

I read the code and it says so in Library/Homebrew/os/mac/xcode.rb, needs_clt_installed?.

[SMillerDev]

What does that have to do with the requirements of Homebrew as a whole though?

[ZhongRuoyu]

This is because Xcode does not ship SDKs for older macOS versions, and you need the SDK to be able to build stuffs from source.

Even if you're on the latest macOS, installing some formulae from bottle still requires you to have CLT installed, because those formulae have baked-in references to SDK paths from the CLT.

[673123]

Thank you for responding. I also have a follow up question. If I download the old Xcode corresponding to the old macOS, will I be able to compile things?
Plus that I'm curious about what would happen if I compile things for older macOS using the new SDK. I tried to compile a simple C++ program with the combination macOS 15+Xcode 26 and it works fine

[ZhongRuoyu]

If I download the old Xcode corresponding to the old macOS, will I be able to compile things?

Yes, you should be able to compile things.

Plus that I'm curious about what would happen if I compile things for older macOS using the new SDK.

Things should work, but the point above regarding bottles requiring CLT still applies.

[673123]

Plus that I'm curious about what would happen if I compile things for older macOS using the new SDK.

Things should work, but the point above regarding bottles requiring CLT still applies.

Thanks. Also what's the "continuous workarounds for SDK weirdness" stated in pr-3458?

[ZhongRuoyu]

Also what's the "continuous workarounds for SDK weirdness" stated in pr-3458?

That was too long ago so I'm really not sure, sorry.