Update Windows RTools support and add Windows ARM64 by ndabas · Pull Request #864 · stan-dev/cmdstanpy

ndabas · 2026-09-18T09:40:37Z

Submission Checklist

Run unit tests
Declare copyright holder and open-source license: see below

Summary

I have updated the RTools installation support on Windows, so versions up to 4.5 are now supported. Critically, that now adds native support for Windows on ARM64. I have updated the rest of the build-related code to work on both AMD64 and ARM64 versions of Windows.

The one thing worth pointing out is that I ran into 4 tests failing only on Windows ARM; this is because those particular tests were running an exe and immediately trying to delete it. On Windows 11 ARM, Windows Defender cannot be disabled, per the readme; and Windows Defender tries to scan every executable when it is run, so deleting it immediately fails.

We can choose to not add Windows ARM to the main workflow, as that workflow has a subset of supported platforms anyway. In that case I can remove the changes to main.yml and the delete-retry workaround, let me know.

I also added lots of tests and a new workflow to test the toolchain installation on Windows.

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

Nikhil Dabas

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

WardBrian

Thanks @ndabas! This looks very nice overall.

My main concerns are about the overlap this has with the large refactor currently happening in #851. I think that together they will actually clean up a lot of the code, but it will likely require some changes here once that one is merged.

WardBrian · 2026-09-18T14:05:18Z

+on:
+  workflow_dispatch:
+  schedule:
+    # weekly: catches CRAN retiring the pinned installer revisions


My recollection is this is part of why we've stuck with rtools40 for so long, because there is not a stable link for the newer ones. Is that what this comment means?

Yes that is what this comment was about. However I noticed that in the CmdStan docs, they point to stable download URLs from the R-Hub project (like https://github.com/r-hub/rtools45/releases/download/latest/rtools45.exe) so I incorporated those in the code. Let me know if that sounds fine -- and I will remove this comment and the weekly run.

That does sound better to me

WardBrian · 2026-09-18T14:06:02Z

 EXTENSION = '.exe' if platform.system() == 'Windows' else ''


+def delete_file(path: str, timeout: float = 5.0) -> None:


I think this function could be in the test utilities, rather than the actual package

Agreed, I'll move it over.

WardBrian · 2026-09-18T14:07:50Z

+            # clang treats backslashes in the -include path as escapes
+            self._user_header = Path(self._user_header).absolute().as_posix()


It might make sense to improve the path handling for this in cmdstan itself:
https://github.com/stan-dev/cmdstan/blob/d3d5df6a22565edbe13edbd4eb40762cc8c5a4d6/make/program#L47

For other variables we use subst to swap backslashes:
https://github.com/stan-dev/cmdstan/blob/d3d5df6a22565edbe13edbd4eb40762cc8c5a4d6/make/program#L83

Thanks for the pointers, I'll see how to clean this up a bit.

WardBrian · 2026-09-18T14:08:50Z

+# Windows ARM64 needs the Stan Math support added in stan-dev/math#3051
+MIN_WINDOWS_ARM64_VERSION = (2, 35)


This is happening in parallel to #851, which will be defining a minimum cmdstan version greater than this for support anyway, so I'm not sure we need to check this specifically here

Okay that sounds great -- I'll remove this check then.

WardBrian · 2026-09-18T14:09:25Z

    print(
        """Arguments:
-        -v (--version) :CmdStan version
+        -v (--version) : RTools version: 3.5, 4.0, 4.4 or 4.5


Since we will be releasing version 2.0 soon, we can definitely remove the old 3.5 support, if not also 4.0

It's probably harmless to leave the old ones around, but yes, we can eliminate some test burden by at least getting rid of 3.5. I'll do that.

WardBrian · 2026-09-18T14:10:05Z

+# CRAN embeds build revisions in the RTools 4.2+ installer filenames, so we
+# use the r-hub mirror, which publishes them under a stable 'latest' tag.
+# These are the builds the CmdStan guide points users at.


This seems to go against the comment in the ci workflow about the links changing

Yes indeed, when I was checking the CmdStan docs I discovered that they have these stable links. I'll remove the CI comment as mentioned.

WardBrian · 2026-09-18T14:11:02Z

+ARM64 build of RTools 4.4 or 4.5, which CmdStanPy installs and detects
+automatically. CmdStan 2.35 or later is required.
+
+A Python interpreter built for Intel/AMD and running under emulation reports an


I was wondering about emulation, since we don't provide windows/aarch64 builds for the stanc3 compiler -- but I guess it works out?

Yes since it's just a compiler of sorts it works fine on ARM64 under emulation. I can look at that as well after this PR to get native support.

WardBrian · 2026-09-18T14:14:32Z

    EXTENSION,
+    RTOOLS_LAYOUTS,
+    RToolsLayout,
    cmdstan_path,


It doesn't look like all these new imports are actually used outside of the other utilities, so probably don't need to appear here

I'll verify that and remove these if needed.

ndabas added 6 commits September 17, 2026 15:20

Add support for newer RTools releases

d7211ed

Add tests for Windows toolchain installation

bafe58d

Add workflow for testing Windows toolchain install

6717c2d

Add Windows ARM64 support

65db8cb

Retry executable delete on Windows

943d6cd

Update install docs for Windows

d0860d5

WardBrian reviewed Sep 18, 2026

View reviewed changes

amas0 mentioned this pull request Sep 18, 2026

Explore alternatives to RTools #832

Open

		EXTENSION = '.exe' if platform.system() == 'Windows' else ''


		def delete_file(path: str, timeout: float = 5.0) -> None:

		# clang treats backslashes in the -include path as escapes
		self._user_header = Path(self._user_header).absolute().as_posix()

		# Windows ARM64 needs the Stan Math support added in stan-dev/math#3051
		MIN_WINDOWS_ARM64_VERSION = (2, 35)

Uh oh!

Conversation

ndabas commented Sep 18, 2026

Submission Checklist

Summary

Copyright and Licensing

Uh oh!

WardBrian left a comment

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants