Thanks for this PR. I think it’s quite urgent since the current OpenAI API isn’t working on Python 3.14 because of this issue.

Thanks for the PR but this isn't the correct fix, the places that read the __discriminator__ property need to be updated as well

The discriminator property need to be updated as well

Would you like for me to do so?
My understanding was that some of the code was generated automatically and PR couldn't be approved directly like that, reason why I didn't put much effort fixing everything related to it.

🧩 The Root Cause

Starting with Python 3.14, typing.Union (and also types.UnionType for | syntax) is now fully immutable — you can no longer attach arbitrary attributes to them (they have no __dict__).
Hence this line fails with:

cast(CachedDiscriminatorType, union).__discriminator__ = details

With the error:

AttributeError: 'typing.Union' object has no attribute '__discriminator__' and no __dict__ for setting new attributes

This behavior worked in ≤ 3.13 because Union objects allowed dynamic attributes, but that’s gone.

✅ The Correct Fix

We can’t store metadata on the Union type itself anymore.
Instead, we can keep a sidecar cache (a WeakKeyDictionary) keyed by the union type.

Here’s how you can safely patch both OpenAI and Anthropic implementations:

Step 1 – Create a global cache

Add this near the top of the module (just after CachedDiscriminatorType is defined):

import weakref

# Global cache since typing.Union objects are now immutable (Python 3.14+)
_DISCRIMINATOR_CACHE: "weakref.WeakKeyDictionary[type, DiscriminatorDetails]" = weakref.WeakKeyDictionary()

Step 2 – Update _build_discriminated_union_meta

Replace

if isinstance(union, CachedDiscriminatorType):
    return union.__discriminator__

With

cached = _DISCRIMINATOR_CACHE.get(union)
if cached is not None:
    return cached

Step 3 - Update the ___discriminator__ set

Replace

cast(CachedDiscriminatorType, union).__discriminator__ = details
return details

With

_DISCRIMINATOR_CACHE[union] = details
return details

🧠 Why This Works

• WeakKeyDictionary lets cached entries disappear when their type objects are garbage-collected.
• Works across all supported Python versions.
• Preserves existing “caching” semantics without mutating Union objects.

@RobertCraigie I updated my PR implementing the suggested plan. Let me know if this works for you.