Annotating the openai.Omit type so that ModelSettings can be serialized by pydantic (#938)

Because `openai.Omit` is not a type which can be serialized by
`pydantic`, it produces difficulty in consistently
serializing/deserializing types across the agents SDK, many of which are
`pydantic` types. This adds an annotation to enable `pydantic` to
serialize `Omit`, in particular in the case of `ModelSettings` which
contains `Omit` in its `extra_headers`

Author: tconley1428

src/agents/model_settings.py

@@ -1,14 +1,50 @@
 from __future__ import annotations
 
 import dataclasses
+from collections.abc import Mapping
 from dataclasses import dataclass, fields, replace
-from typing import Any, Literal
+from typing import Annotated, Any, Literal, Union
 
-from openai._types import Body, Headers, Query
+from openai import Omit as _Omit
+from openai._types import Body, Query
 from openai.types.responses import ResponseIncludable
 from openai.types.shared import Reasoning
-from pydantic import BaseModel
-
+from pydantic import BaseModel, GetCoreSchemaHandler
+from pydantic_core import core_schema
+from typing_extensions import TypeAlias
+
+
+class _OmitTypeAnnotation:
+    @classmethod
+    def __get_pydantic_core_schema__(
+            cls,
+            _source_type: Any,
+            _handler: GetCoreSchemaHandler,
+    ) -> core_schema.CoreSchema:
+        def validate_from_none(value: None) -> _Omit:
+            return _Omit()
+
+        from_none_schema = core_schema.chain_schema(
+            [
+                core_schema.none_schema(),
+                core_schema.no_info_plain_validator_function(validate_from_none),
+            ]
+        )
+        return core_schema.json_or_python_schema(
+            json_schema=from_none_schema,
+            python_schema=core_schema.union_schema(
+                [
+                    # check if it's an instance first before doing any further work
+                    core_schema.is_instance_schema(_Omit),
+                    from_none_schema,
+                ]
+            ),
+            serialization=core_schema.plain_serializer_function_ser_schema(
+                lambda instance: None
+            ),
+        )
+Omit = Annotated[_Omit, _OmitTypeAnnotation]
+Headers: TypeAlias = Mapping[str, Union[str, Omit]]
 
 @dataclass
 class ModelSettings:

tests/model_settings/test_serialization.py

@@ -2,6 +2,8 @@
 from dataclasses import fields
 
 from openai.types.shared import Reasoning
+from pydantic import TypeAdapter
+from pydantic_core import to_json
 
 from agents.model_settings import ModelSettings
 
@@ -132,3 +134,32 @@ def test_extra_args_resolve_both_none() -> None:
     assert resolved.extra_args is None
     assert resolved.temperature == 0.5
     assert resolved.top_p == 0.9
+
+def test_pydantic_serialization() -> None:
+
+    """Tests whether ModelSettings can be serialized with Pydantic."""
+
+    # First, lets create a ModelSettings instance
+    model_settings = ModelSettings(
+        temperature=0.5,
+        top_p=0.9,
+        frequency_penalty=0.0,
+        presence_penalty=0.0,
+        tool_choice="auto",
+        parallel_tool_calls=True,
+        truncation="auto",
+        max_tokens=100,
+        reasoning=Reasoning(),
+        metadata={"foo": "bar"},
+        store=False,
+        include_usage=False,
+        extra_query={"foo": "bar"},
+        extra_body={"foo": "bar"},
+        extra_headers={"foo": "bar"},
+        extra_args={"custom_param": "value", "another_param": 42},
+    )
+
+    json = to_json(model_settings)
+    deserialized = TypeAdapter(ModelSettings).validate_json(json)
+
+    assert model_settings == deserialized