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Free-threaded and GIL-enabled Windows builds should share an abi3 libpython dll #148690

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Free-threaded and GIL-enabled Windows builds should share an abi3 libpython dll#148690
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3.15bugs and security fixesOS-windowsrelease-blockertopic-free-threadingtype-bugAn unexpected behavior, bug, or error

Description

@ngoldbaum
ngoldbaum
opened on Apr 17, 2026

Bug description:

Currently, the free-threaded interpreter ships python3t.dll and the GIL-enabled interpreter ships python3.dll. If the guidance is for abi3t extensions to link against python3t.dll, then that means abi3t extensions won't be importable on the GIL-enabled build without shipping python3t.dll alongside python3.dll.

Another possible fix is to make the GIL-enabled build ship python3.dll and make abi3t extensions like against python3.dll.

Either way, fixing this will require touching the file layout of the Windows installation, which @zooba tells me has to happen before beta1.

You can reproduce this by cloning the stable-abi-testing repository and building the setuptools/c example on the free-threaded build and then trying to import the wheel you get on the GIL-enabled build:

Details 
PS C:\Users\goldbaum\stable-abi-testing> uv venv --python=3.15.0a8t
Using CPython 3.15.0a8+freethreaded
Creating virtual environment at: .venv
Activate with: .venv\Scripts\activate
PS C:\Users\goldbaum\stable-abi-testing> .\.venv\Scripts\activate
(stable-abi-testing) PS C:\Users\goldbaum\stable-abi-testing> uv pip install "git+https://github.com/ngoldbaum/setuptools@abi3.abi3t"
Resolved 1 package in 390ms
Installed 1 package in 378ms
 + setuptools==82.0.0 (from git+https://github.com/ngoldbaum/setuptools@f7b3c3c5ad49dd070cacf16599776c07be234449)
 (stable-abi-testing) PS C:\Users\goldbaum\stable-abi-testing> uv pip install "git+https://github.com/ngoldbaum/setuptools@abi3.abi3t"
Resolved 1 package in 390ms
Installed 1 package in 378ms
 + setuptools==82.0.0 (from git+https://github.com/ngoldbaum/setuptools@f7b3c3c5ad49dd070cacf16599776c07be234449)
 (stable-abi-testing) PS C:\Users\goldbaum\stable-abi-testing\setuptools\c> python -m build --no-isolation
* Getting build dependencies for sdist...
C:\Users\goldbaum\stable-abi-testing\.venv\Lib\site-packages\setuptools\config\pyprojecttoml.py:72: _ExperimentalConfiguration: `[tool.setuptools.ext-modules]` in `pyproject.toml` is still *experimental* and likely to change in future releases.
  config = read_configuration(filepath, True, ignore_option_errors, dist)
running egg_info
creating limited_api.egg-info
writing limited_api.egg-info\PKG-INFO
writing dependency_links to limited_api.egg-info\dependency_links.txt
writing top-level names to limited_api.egg-info\top_level.txt
writing manifest file 'limited_api.egg-info\SOURCES.txt'
reading manifest file 'limited_api.egg-info\SOURCES.txt'
writing manifest file 'limited_api.egg-info\SOURCES.txt'
* Building sdist...
C:\Users\goldbaum\stable-abi-testing\.venv\Lib\site-packages\setuptools\config\pyprojecttoml.py:72: _ExperimentalConfiguration: `[tool.setuptools.ext-modules]` in `pyproject.toml` is still *experimental* and likely to change in future releases.
  config = read_configuration(filepath, True, ignore_option_errors, dist)
running sdist
running egg_info
writing limited_api.egg-info\PKG-INFO
writing dependency_links to limited_api.egg-info\dependency_links.txt
writing top-level names to limited_api.egg-info\top_level.txt
reading manifest file 'limited_api.egg-info\SOURCES.txt'
writing manifest file 'limited_api.egg-info\SOURCES.txt'
warning: sdist: standard file not found: should have one of README, README.rst, README.txt, README.md

running check
creating limited_api-1.2.3
creating limited_api-1.2.3\limited_api.egg-info
copying files to limited_api-1.2.3...
copying limited.c -> limited_api-1.2.3
copying pyproject.toml -> limited_api-1.2.3
copying setup.cfg -> limited_api-1.2.3
copying limited_api.egg-info\PKG-INFO -> limited_api-1.2.3\limited_api.egg-info
copying limited_api.egg-info\SOURCES.txt -> limited_api-1.2.3\limited_api.egg-info
copying limited_api.egg-info\dependency_links.txt -> limited_api-1.2.3\limited_api.egg-info
copying limited_api.egg-info\top_level.txt -> limited_api-1.2.3\limited_api.egg-info
copying limited_api.egg-info\SOURCES.txt -> limited_api-1.2.3\limited_api.egg-info
Writing limited_api-1.2.3\setup.cfg
Creating tar archive
removing 'limited_api-1.2.3' (and everything under it)
* Building wheel from sdist
* Getting build dependencies for wheel...
C:\Users\goldbaum\stable-abi-testing\.venv\Lib\site-packages\setuptools\config\pyprojecttoml.py:72: _ExperimentalConfiguration: `[tool.setuptools.ext-modules]` in `pyproject.toml` is still *experimental* and likely to change in future releases.
  config = read_configuration(filepath, True, ignore_option_errors, dist)
running egg_info
writing limited_api.egg-info\PKG-INFO
writing dependency_links to limited_api.egg-info\dependency_links.txt
writing top-level names to limited_api.egg-info\top_level.txt
reading manifest file 'limited_api.egg-info\SOURCES.txt'
writing manifest file 'limited_api.egg-info\SOURCES.txt'
* Building wheel...
C:\Users\goldbaum\stable-abi-testing\.venv\Lib\site-packages\setuptools\config\pyprojecttoml.py:72: _ExperimentalConfiguration: `[tool.setuptools.ext-modules]` in `pyproject.toml` is still *experimental* and likely to change in future releases.
  config = read_configuration(filepath, True, ignore_option_errors, dist)
running bdist_wheel
running build
running build_ext
building 'limited' extension
creating build\temp.win-amd64-cpython-315t\Release
"C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.44.35207\bin\HostX86\x64\cl.exe" /c /nologo /O2 /W3 /GL /DNDEBUG /MD -DPy_GIL_DISABLED=1 -IC:\Users\goldbaum\stable-abi-testing\.venv\include -IC:\Users\goldbaum\AppData\Roaming\uv\python\cpython-3.15+freethreaded-windows-x86_64-none\include -IC:\Users\goldbaum\AppData\Roaming\uv\python\cpython-3.15+freethreaded-windows-x86_64-none\Include "-IC:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.44.35207\include" "-IC:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.44.35207\ATLMFC\include" "-IC:\Program Files\Microsoft Visual Studio\2022\Community\VC\Auxiliary\VS\include" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.26100.0\ucrt" "-IC:\Program Files (x86)\Windows Kits\10\\include\10.0.26100.0\\um" "-IC:\Program Files (x86)\Windows Kits\10\\include\10.0.26100.0\\shared" "-IC:\Program Files (x86)\Windows Kits\10\\include\10.0.26100.0\\winrt" "-IC:\Program Files (x86)\Windows Kits\10\\include\10.0.26100.0\\cppwinrt" "-IC:\Program Files (x86)\Windows Kits\NETFXSDK\4.7.1\include\um" /Tclimited.c /Fobuild\temp.win-amd64-cpython-315t\Release\limited.obj -DPy_LIMITED_API=0x030f0000 -DPy_TARGET_ABI3T=0x030f0000
limited.c
creating C:\Users\goldbaum\AppData\Local\Temp\build-via-sdist-g3tcaad2\limited_api-1.2.3\build\lib.win-amd64-cpython-315t
"C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.44.35207\bin\HostX86\x64\link.exe" /nologo /INCREMENTAL:NO /LTCG /DLL /MANIFEST:EMBED,ID=2 /MANIFESTUAC:NO /LIBPATH:C:\Users\goldbaum\stable-abi-testing\.venv\libs /LIBPATH:C:\Users\goldbaum\AppData\Roaming\uv\python\cpython-3.15+freethreaded-windows-x86_64-none\libs /LIBPATH:C:\Users\goldbaum\AppData\Roaming\uv\python\cpython-3.15+freethreaded-windows-x86_64-none /LIBPATH:C:\Users\goldbaum\stable-abi-testing\.venv\PCbuild\amd64 "/LIBPATH:C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.44.35207\ATLMFC\lib\x64" "/LIBPATH:C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.44.35207\lib\x64" "/LIBPATH:C:\Program Files (x86)\Windows Kits\NETFXSDK\4.7.1\lib\um\x64" "/LIBPATH:C:\Program Files (x86)\Windows Kits\10\lib\10.0.26100.0\ucrt\x64" "/LIBPATH:C:\Program Files (x86)\Windows Kits\10\\lib\10.0.26100.0\\um\x64" /EXPORT:PyInit_limited build\temp.win-amd64-cpython-315t\Release\limited.obj /OUT:build\lib.win-amd64-cpython-315t\limited.pyd /IMPLIB:build\temp.win-amd64-cpython-315t\Release\limited.lib
   Creating library build\temp.win-amd64-cpython-315t\Release\limited.lib and object build\temp.win-amd64-cpython-315t\Release\limited.exp
Generating code
Finished generating code
installing to build\bdist.win-amd64\wheel
running install
running install_lib
creating build\bdist.win-amd64\wheel
copying build\lib.win-amd64-cpython-315t\limited.pyd -> build\bdist.win-amd64\wheel\.
running install_egg_info
running egg_info
writing limited_api.egg-info\PKG-INFO
writing dependency_links to limited_api.egg-info\dependency_links.txt
writing top-level names to limited_api.egg-info\top_level.txt
reading manifest file 'limited_api.egg-info\SOURCES.txt'
writing manifest file 'limited_api.egg-info\SOURCES.txt'
Copying limited_api.egg-info to build\bdist.win-amd64\wheel\.\limited_api-1.2.3-py3.15.egg-info
running install_scripts
creating build\bdist.win-amd64\wheel\limited_api-1.2.3.dist-info\WHEEL
creating 'C:\\Users\\goldbaum\\stable-abi-testing\\setuptools\\c\\dist\\.tmp-t0bncj0f\\limited_api-1.2.3-cp315-abi3.abi3t-win_amd64.whl' and adding 'build\\bdist.win-amd64\\wheel' to it
adding 'limited.pyd'
adding 'limited_api-1.2.3.dist-info/METADATA'
adding 'limited_api-1.2.3.dist-info/WHEEL'
adding 'limited_api-1.2.3.dist-info/top_level.txt'
adding 'limited_api-1.2.3.dist-info/RECORD'
removing build\bdist.win-amd64\wheel
Successfully built limited_api-1.2.3.tar.gz and limited_api-1.2.3-cp315-abi3.abi3t-win_amd64.whl
(stable-abi-testing) PS C:\Users\goldbaum\stable-abi-testing\setuptools\c> deactivate
PS C:\Users\goldbaum\stable-abi-testing\setuptools\c> uv venv --python=3.15.0a8
Using CPython 3.15.0a8
Creating virtual environment at: .venv
Activate with: .venv\Scripts\activate
PS C:\Users\goldbaum\stable-abi-testing\setuptools\c> .\.venv\Scripts\activate
(c) PS C:\Users\goldbaum\stable-abi-testing\setuptools\c> uv pip install .\dist\limited_api-1.2.3-cp315-abi3.abi3t-win_amd64.whl
Resolved 1 package in 16ms
Prepared 1 package in 20ms
Installed 1 package in 11ms
 + limited-api==1.2.3 (from file:///C:/Users/goldbaum/stable-abi-testing/setuptools/c/dist/limited_api-1.2.3-cp315-abi3.abi3t-win_amd64.whl)
 (c) PS C:\Users\goldbaum\stable-abi-testing\setuptools\c> python
Python 3.15.0a8 (main, Apr 14 2026, 14:44:01) [MSC v.1950 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import limited
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    import limited
ImportError: DLL load failed while importing limited: The specified module could not be found.

The dumpbin output says the extension is linked against python3t.dll:

PS C:\Program Files\Microsoft Visual Studio\2022\Community> dumpbin /dependents C:\Users\goldbaum\stable-abi-testing\.venv\Lib\site-packages\limited.pyd
Microsoft (R) COFF/PE Dumper Version 14.44.35225.0
Copyright (C) Microsoft Corporation.  All rights reserved.


Dump of file C:\Users\goldbaum\stable-abi-testing\.venv\Lib\site-packages\limited.pyd

File Type: DLL

  Image has the following dependencies:

    python3t.dll
    KERNEL32.dll
    VCRUNTIME140.dll
    api-ms-win-crt-runtime-l1-1-0.dll

  Summary

        1000 .data
        1000 .pdata
        1000 .rdata
        1000 .reloc
        1000 .rsrc
        1000 .text

See discussion in this comment and the replies: #146636 (comment).

CPython versions tested on:

3.15

Operating systems tested on:

Windows

Linked PRs

Activity

  1. ngoldbaum
    added
    type-bugAn unexpected behavior, bug, or error
    OS-windows
    topic-free-threading
    3.15bugs and security fixes
    on Apr 17, 2026

  2. mgorny commented on Apr 17, 2026

    @mgorny
    mgorny
    on Apr 17, 2026
    Contributor

    A related problem is that PEP 739 build-details.json specifies a libpython.dynamic_stableapi entry that — at least to my best understanding — would list python3.dll on GIL-enabled biuld, and python3t.dll on freethreading build. However, for abi3t builds from GIL-enable Python, we'd need another entry specifying python3t.dll.

    CC @FFY00

  3. encukou commented on Apr 23, 2026

    @encukou
    encukou
    on Apr 23, 2026
    Member

    Continuing from the thread in #146636 where we mapped the strategy:

    • GIL-ful build should add abi3t-compat\python3t.dll, linked to its python3XX.dll.
      • If the installation is potentially shared with free-threaded build, this should be installed in abi3t-compat\python3t.dll
      • Otherwise, it should be installed alongside python3.dll
    • abi3.abi3t extensions should be linked to python3t.dll

    I managed to do the building part in #148912. I'd welcome any reviews or pointers; it's the first time I looked at the XMLs seriously.

    I could not build the installer though; even on main, Tools\msi\build.bat gives me this error on my machine:1

    light.exe : error LGHT0217: Error executing ICE action 'ICE01'. The most common cause of this kind of ICE failure is 
an incorrectly registered scripting engine. See http://wixtoolset.org/documentation/error217/ for details and how to  
solve this problem. The following string format was not expected by the external UI message logger: "The Windows Inst 
aller Service could not be accessed. This can occur if the Windows Installer is not correctly installed. Contact your 
 support personnel for assistance.". [C:\Users\encukou\dev\cpython\Tools\msi\launcher\launcher.wixproj]

    I'd appreciate any tips
    cc @chris-eibl -- do you have experience here?

    Footnotes

    1. “Error executing ICE action” sounds nice, but I'd rather not run the action at all if it was up to me... ↩

  4. zooba commented on Apr 23, 2026

    @zooba
    zooba
    on Apr 23, 2026
    Member

    I could not build the installer though; even on main, Tools\msi\build.bat gives me this error on my machine

    I see a few suggestions that it means your environment is too big (>32KB for all variables), but you could try just adding SuppressValidation=true to your environment1 to skip those checks. They won't be so important for what you're doing here.

    Footnotes

    1. Yes, making it even bigger! ↩

  5. chris-eibl commented on Apr 23, 2026

    @chris-eibl
    chris-eibl
    on Apr 23, 2026
    Member

    cc @chris-eibl -- do you have experience here?

    Sorry, no idea, I have never seen this error before. Hopefull, Steve's hint already helped out ...

  6. encukou commented on May 1, 2026

    @encukou
    encukou
    on May 1, 2026
    Member

    Marking as potential release blocker for visibility.

  7. encukou
    added
    release-blocker
    on May 1, 2026
  8. github-project-automation
    added this to Release and Deferred blockers 🚫on May 1, 2026
  9. github-project-automation
    moved this to Todo in Release and Deferred blockers 🚫on May 1, 2026
  10. zooba
    added a commit that references this issue on May 4, 2026

    gh-148690: Build Windows freethreaded binaries into separate director…

    10f950c

  11. hugovk commented on May 4, 2026

    @hugovk
    hugovk
    on May 4, 2026
    Member

    Is this release blocker now resolved for 3.15?

  12. encukou commented on May 5, 2026

    @encukou
    encukou
    on May 5, 2026
    Member

    Yeah, this is as good as it'll get before the actual release.

  13. encukou
    closed this as completedon May 5, 2026
  14. github-project-automation
    moved this from Todo to Done in Release and Deferred blockers 🚫on May 5, 2026
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