I'm interested in free-threaded no-GIL builds.

From what little research I've done, I suspect the issue is in CFFI itself and that issue might've already been fixed but not in an official release. So I suspect that you don't need to remove references to Py_LIMITED_API from tcod's build files, instead you need to fetch and built cffi directly from its latest source and then tcod will build normally.

Otherwise I'll need to install a GIL-less version of Python on the GitHub workflows in order to verify any fixes.

I tried

pip install git+https://github.com/python-cffi/cffi.git

but did not help if you have meant that ...

I tried

pip install git+https://github.com/python-cffi/cffi.git

but did not help if you have meant that ...

I did mean that. I'm unsure what to try next.

clang -UNDEBUG -UPy_LIMITED_API ...
/Users/xxx/.pyenv/versions/3.13-dev/include/python3.13t/Python.h:51:4: error: "The limited API is not currently supported in the free-threaded build"

somehow it remains, although I changed the build script.

This limitied api is only needed due to older python versions, right ? Probably you do not want to discard those...

I assume you've also removed the py_limited_api=True line from build_libtcod.py?

yes

py_limited_api=not sysconfig.get_config_var("Py_GIL_DISABLED"),

i will look again tomorrow...maybe AI has also some further hints

this is what the AI analzed:

⚠️ Python-tcod Build Issue with Python 3.13 No-GIL

Problem Summary

Even though the project no longer sets Py_LIMITED_API in the ffi.set_source call, building python-tcod against Python 3.13 (No-GIL variant) still triggers errors related to the limited API. Specifically, compilation fails with:

#error "The limited API is not currently supported in the free-threaded build"

Why This Happens

1. CFFI Always Includes <Python.h>

   • CFFI-generated .c files (like tcod._libtcod.c) always include the Python C API via #include <Python.h>, regardless of your build flags.

2. Python 3.13 No-GIL Is Strict About the Limited API

   • Python 3.13’s headers explicitly disallow use of the limited API in no-GIL builds.
   • Even without Py_LIMITED_API being defined, some internal logic or missing defines can still trigger the "limited API" checks.

3. The Issue Is Triggered Inside Python’s Headers

   • This is not due to an explicit mistake in your own code, but due to changes in Python’s internal headers and how CFFI-generated code interacts with them.

Workarounds and Suggestions

• Use the standard (GIL-enabled) CPython 3.13 to build python-tcod, if you're not specifically targeting No-GIL compatibility.
• Add a check in build_libtcod.py to detect unsupported Python builds and fail early with a clear message.
• Investigate CFFI-level solutions: If possible, suppress or replace #include <Python.h> in the generated C code (this is non-trivial).
• Upstream Watch: Monitor the status of CFFI and CPython 3.13 development to see if proper No-GIL compatibility will be added.

Conclusion

This issue isn’t caused by incorrect flags in your project, but by deeper incompatibilities between how CFFI-generated files interact with the Python C API headers in No-GIL builds. A future-proof fix will likely require changes in either Python itself or CFFI.

Sounds reasonable to me, but i cannot judge the cffi/c stuff...

A workaround for my project is now to use Rust for multi-core algorithms and call it from python...should be doable, I guess

This is not due to an explicit mistake in your own code, but due to changes in Python’s internal headers and how CFFI-generated code interacts with them.

There is a blatant lack of specificity here from the AI. The AI seems to state stuff that is relevant but is unable to properly explain how everything interacts with each other.

CFFI needs to include #include <Python.h> in its generated code _libtcod.c. The issue is that CFFI might set Py_LIMITED_API before it includes that header. Looking at _libtcod.c shows how this actually happens as well as some minor documentation on how CFFI uses Py_LIMITED_API.

What I notice is that CFFI internally uses _CFFI_NO_LIMITED_API to disable adding Py_LIMITED_API. Perhaps you can try adding a define for _CFFI_NO_LIMITED_API which should prevent it from defining Py_LIMITED_API in the generated code.

I have tried that also but no success....so I guess there is no easy solution for now...