diff --git a/litellm/types/llms/base.py b/litellm/types/llms/base.py
index e63e1b040d0..43b4adc099c 100644
--- a/litellm/types/llms/base.py
+++ b/litellm/types/llms/base.py
@@ -1,3 +1,4 @@
+import threading
 from collections.abc import Mapping
 from typing import TYPE_CHECKING, Any, Final
 
@@ -6,6 +7,8 @@ from pydantic import BaseModel, ConfigDict
 
 from litellm.constants import DEFER_PYDANTIC_BUILD
 
+_SCHEMA_BUILD_LOCK: Final = threading.RLock()
+
 
 class LiteLLMBaseModel(BaseModel):
     model_config = ConfigDict(defer_build=DEFER_PYDANTIC_BUILD)
@@ -25,12 +28,13 @@ class LiteLLMBaseModel(BaseModel):
     ) -> bool | None:
         # Resolve names from the model's own module, never a caller frame: a deferred first-use build
         # reads f_locals 5 frames up, and on Python < 3.13 that rewrites the dict the caller's locals() returned
-        return super().model_rebuild(
-            force=force,
-            raise_errors=raise_errors,
-            _parent_namespace_depth=0,
-            _types_namespace=_types_namespace,
-        )
+        with _SCHEMA_BUILD_LOCK:
+            return super().model_rebuild(
+                force=force,
+                raise_errors=raise_errors,
+                _parent_namespace_depth=0,
+                _types_namespace=_types_namespace,
+            )
 
     def model_post_init(self, context: object, /) -> None:
         # Instances built by a parent's validator or by model_construct skip this class's own
diff --git a/tests/unit/types/llms/test_types_llms_base.py b/tests/unit/types/llms/test_types_llms_base.py
index 132fb95cad1..964689cdeeb 100644
--- a/tests/unit/types/llms/test_types_llms_base.py
+++ b/tests/unit/types/llms/test_types_llms_base.py
@@ -1,10 +1,13 @@
 import os
 import subprocess
 import sys
+import threading
+from concurrent.futures import ThreadPoolExecutor
+from itertools import chain
 from typing import Final
 
 import pytest
-from pydantic import ConfigDict
+from pydantic import ConfigDict, create_model
 
 from litellm.types.llms.base import LiteLLMBaseModel
 
@@ -90,3 +93,52 @@ def test_deferred_first_use_build_leaves_caller_locals_snapshot_untouched() -> N
     assert not DeferredProbe.__pydantic_complete__
     assert build(DeferredProbe) == ["model"]
     assert DeferredProbe.__pydantic_complete__
+
+
+_RACE_ROUNDS: Final = 100
+_RACE_THREADS: Final = 16
+_RACE_VALIDATIONS_PER_THREAD: Final = 5
+_RACE_FIELD_COUNT: Final = 20
+
+
+class _Deferred(LiteLLMBaseModel):
+    model_config = ConfigDict(defer_build=True)
+
+
+def _fresh_deferred_subclass(round_id: int) -> tuple[type[LiteLLMBaseModel], type[LiteLLMBaseModel]]:
+    fields: Final = {f"field_{index}": (str | int | None, None) for index in range(_RACE_FIELD_COUNT)}
+    parent: Final = create_model(f"Parent{round_id}", __base__=_Deferred, **fields)
+    child: Final = create_model(f"Child{round_id}", __base__=parent, extra_flag=(bool | None, False))
+    return parent, child
+
+
+def _first_use_outcome(child: type[LiteLLMBaseModel]) -> str:
+    try:
+        return type(child.model_validate({"field_0": "x"})).__name__
+    except AttributeError as error:
+        return f"{type(error).__name__}: {error}"
+
+
+def _validate_after_barrier(child: type[LiteLLMBaseModel], gate: threading.Barrier) -> tuple[str, ...]:
+    gate.wait()
+    return tuple(_first_use_outcome(child) for _ in range(_RACE_VALIDATIONS_PER_THREAD))
+
+
+def _concurrent_first_use_outcomes(child: type[LiteLLMBaseModel]) -> frozenset[str]:
+    gate: Final = threading.Barrier(_RACE_THREADS)
+    with ThreadPoolExecutor(max_workers=_RACE_THREADS) as executor:
+        per_thread: Final = tuple(executor.map(lambda _: _validate_after_barrier(child, gate), range(_RACE_THREADS)))
+    return frozenset(chain.from_iterable(per_thread))
+
+
+def test_concurrent_first_use_of_a_deferred_subclass_always_builds_that_subclass() -> None:
+    previous_switch_interval: Final = sys.getswitchinterval()
+    sys.setswitchinterval(1e-6)
+    try:
+        for round_id in range(_RACE_ROUNDS):
+            parent, child = _fresh_deferred_subclass(round_id)
+            parent.model_validate({})
+            assert not child.__pydantic_complete__
+            assert _concurrent_first_use_outcomes(child) == {child.__name__}, f"round {round_id}"
+    finally:
+        sys.setswitchinterval(previous_switch_interval)