In this direction, I think #4811 should probably also be merged asap as 3.14 beta is very close and we should start testing 3.14 beta in CI.

Hearty +1.

Also OK with allowing early testing with 3.15.

Hmm, is there a bug here? I'm seeing the:

DEBUG   error: the configured Python interpreter version (3.14) is newer than PyO3's maximum supported version (3.13)
DEBUG   = help: please check if an updated version of PyO3 is available. Current version: 0.24.2
DEBUG   = help: set PYO3_USE_ABI3_FORWARD_COMPATIBILITY=1 to suppress this check and build anyway using the stable ABI

error for a build that has features = ["abi3"] (but does not specify a particular abi3 version). It seems like defaulting to the highest available abi3 is a safe and sensible behavior?

I don't think there's a bug; at the moment the logic is:

• we don't know what the 3.14 ABI and runtime behaviour actually is
• you can choose to attempt to use abi3 to build and run anyway

I think for this case of an in-dev version where you request to build with abi3 we could just build and assume abi3 works fine, maybe with warnings that PyO3 doesn't officially support this version yet.

From my perspective, the theory of abi3 is: it must be valid to build abi3-py313 and use it on a Python 3.14, therefore it should be ok to, when running on Python 3.14, also build for 3.13-abi3 :-)

So from that perspective, there should be no correctness risk, it's just a question of whether downgrading like this is too unintuitive to be the default behavior.

FWIW, abi3-py313 on an in-dev Python 3.14 would have been my assumption of expected behavior on a generic abi3 build that targets "current version".

I think:

diff --git a/pyo3-build-config/src/impl_.rs b/pyo3-build-config/src/impl_.rs
index 9485b8a03..56baaf4e6 100644
--- a/pyo3-build-config/src/impl_.rs
+++ b/pyo3-build-config/src/impl_.rs
@@ -730,6 +730,8 @@ print("gil_disabled", get_config_var("Py_GIL_DISABLED"))
             );
 
             self.version = version;
+        } else if self.version > SUPPORTED_VERSIONS_CPYTHON.max {
+            self.version = SUPPORTED_VERSIONS_CPYTHON.max;
         }
 
         Ok(())

is roughly the desired behavior, but SUPPORTED_VERSIONS_CPYTHON isn't actually defined in this file, so I haven't tested.

(And with apologies, I'm now walking away from my computer for most of the day :-))

If you have already opted for abi3, building for "newest known" abi3 version makes a ton of sense for future compatibility, I support that.

I guess the only question is whether it makes sense to warn, maybe just at build time?