[sushant-me]

Summary

A discriminated union caches its key → variant mapping on the union for the life of the process. _build_discriminated_union_meta reads each variant's __pydantic_core_schema__, and the SDK defers Pydantic builds — so a variant that another thread is still building has no readable schema yet. That variant is silently skipped, and the resulting incomplete mapping is cached anyway, which leaves the union decoding to the wrong variant until the process restarts.

The first-variant fallback in construct_type exists to absorb a discriminator/literal mismatch, but it cannot help here: the discriminator lookup succeeds and silently returns the wrong type.

The fix

An incomplete mapping is returned to the caller but not cached, so the next call rebuilds it once the deferred build has finished. Complete mappings cache exactly as before.

The part that makes this more than a one-line change, and worth a reviewer's attention: the if not field: continue in that loop conflates two different situations, so the None return from _extract_field_schema_pv2 could not simply be reused.

• a variant whose schema has not finished building cannot contribute → the mapping is incomplete
• a variant that genuinely does not declare the discriminator legitimately contributes nothing → the mapping is still cacheable

Treating both as incomplete would mean never caching any real mapping, so _has_built_model_schema distinguishes them by inspecting whether the variant's schema is a model/model-fields schema yet.

Tests

test_discriminated_unions_incomplete_mapping_is_not_cached is deterministic rather than threaded — it replaces one variant's __pydantic_core_schema__ with an unbuilt one ({"type": "definition-ref"}), which is exactly the state the racing thread sees, so nothing depends on thread timing. It asserts:

• the incomplete mapping is not cached, while a usable result is still returned for the variants that can be read
• once every schema is readable, the complete mapping is cached and contains both variants

Verified:

# with the fix
tests/test_models.py ....... 62 passed

# revert only the caching guard
E   AssertionError: an incomplete discriminator mapping must not be cached
FAILED tests/test_models.py::test_discriminated_unions_incomplete_mapping_is_not_cached

uv run --isolated --all-extras pytest tests/test_models.py -q → 62 passed.

Notes

• Pydantic v1 takes the same complete path via the missing-field_info branch.
• I did not change DEFER_PYDANTIC_BUILD, and this does not make the first concurrent decodes faster. It only stops one unlucky thread from poisoning the union permanently.

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This mirrors a bug reported against the equivalent generated SDK at openai/openai-python#4036, where the same code shape exists; that repository restricts PR creation to collaborators, so the analysis went into the issue there instead. I am flagging it here because the defect is present in this codebase independently of that one.