diff --git a/temporalio/contrib/openai_agents/_temporal_openai_agents.py b/temporalio/contrib/openai_agents/_temporal_openai_agents.py
index 60b4b36ef..43594657f 100644
--- a/temporalio/contrib/openai_agents/_temporal_openai_agents.py
+++ b/temporalio/contrib/openai_agents/_temporal_openai_agents.py
@@ -1,16 +1,24 @@
 """Initialize Temporal OpenAI Agents overrides."""
 
 import dataclasses
+import json
 import typing
 from collections.abc import AsyncIterator, Callable, Iterator, Sequence
 from contextlib import asynccontextmanager, contextmanager
 from datetime import timedelta
 
+import pydantic
 from agents import ModelProvider, Trace, set_trace_provider
 from agents.run import get_default_agent_runner, set_default_agent_runner
 from agents.tracing import get_trace_provider
 from agents.tracing.provider import DefaultTraceProvider
 
+# construct_type is OpenAI's lenient (non-validating) model builder, the same
+# one the SDK uses to parse live API responses. It is in a private module but
+# has no public alias.
+from openai._models import construct_type
+
+import temporalio.api.common.v1
 from temporalio.contrib.openai_agents._invoke_model_activity import ModelActivity
 from temporalio.contrib.openai_agents._model_parameters import ModelActivityParameters
 from temporalio.contrib.openai_agents._openai_runner import (
@@ -25,12 +33,14 @@
 from temporalio.contrib.openai_agents.workflow import AgentsWorkflowError
 from temporalio.contrib.opentelemetry._tracer_provider import ReplaySafeTracerProvider
 from temporalio.contrib.pydantic import (
-    PydanticPayloadConverter,
+    PydanticJSONPlainPayloadConverter,
     ToJsonOptions,
 )
 from temporalio.converter import (
+    CompositePayloadConverter,
     DataConverter,
     DefaultPayloadConverter,
+    JSONPlainPayloadConverter,
 )
 from temporalio.plugin import SimplePlugin
 from temporalio.worker import WorkflowRunner
@@ -64,12 +74,72 @@ def _set_open_ai_agent_temporal_overrides(
         set_trace_provider(previous_trace_provider or DefaultTraceProvider())
 
 
-class OpenAIPayloadConverter(PydanticPayloadConverter):
+def _lenient_construct(type_: typing.Any, value: typing.Any) -> typing.Any:
+    """Build ``value`` into ``type_`` without enforcing required fields.
+
+    OpenAI's ``construct_type`` handles its own response models (and the
+    unions/lists thereof), but not the ``agents`` dataclasses that wrap them
+    (e.g. ``ModelResponse``), so the dataclass layer is reconstructed here and
+    each field delegated to ``construct_type``. ``include_extras`` preserves the
+    ``Annotated`` discriminators the unions rely on.
+    """
+    if (
+        isinstance(type_, type)
+        and dataclasses.is_dataclass(type_)
+        and isinstance(value, dict)
+    ):
+        hints = typing.get_type_hints(type_, include_extras=True)
+        return type_(
+            **{
+                field.name: _lenient_construct(
+                    hints.get(field.name, object), value[field.name]
+                )
+                for field in dataclasses.fields(type_)
+                if field.name in value
+            }
+        )
+    return construct_type(type_=type_, value=value)
+
+
+class _OpenAIJSONPlainPayloadConverter(PydanticJSONPlainPayloadConverter):
+    """Strict pydantic deserialization with a lenient fallback.
+
+    OpenAI's response models can drift from live API payloads (e.g. a
+    deprecated-but-required field the API has stopped sending). The SDK tolerates
+    this when parsing responses, but strict ``validate_json`` on the workflow
+    side does not, so fall back to lenient construction when validation fails.
+    """
+
+    def from_payload(
+        self,
+        payload: temporalio.api.common.v1.Payload,
+        type_hint: type | None = None,
+    ) -> typing.Any:
+        """See base class."""
+        try:
+            return super().from_payload(payload, type_hint)
+        except pydantic.ValidationError:
+            if type_hint is None:
+                raise
+            return _lenient_construct(type_hint, json.loads(payload.data))
+
+
+class OpenAIPayloadConverter(CompositePayloadConverter):
     """PayloadConverter for OpenAI agents."""
 
     def __init__(self) -> None:
         """Initialize a payload converter."""
-        super().__init__(ToJsonOptions(exclude_unset=True))
+        json_payload_converter = _OpenAIJSONPlainPayloadConverter(
+            ToJsonOptions(exclude_unset=True)
+        )
+        super().__init__(
+            *(
+                c
+                if not isinstance(c, JSONPlainPayloadConverter)
+                else json_payload_converter
+                for c in DefaultPayloadConverter.default_encoding_payload_converters
+            )
+        )
 
 
 def _data_converter(converter: DataConverter | None) -> DataConverter:
diff --git a/tests/contrib/openai_agents/test_openai.py b/tests/contrib/openai_agents/test_openai.py
index 294acc1d0..de0af3923 100644
--- a/tests/contrib/openai_agents/test_openai.py
+++ b/tests/contrib/openai_agents/test_openai.py
@@ -547,7 +547,9 @@ def research_mock_model():
                         id="",
                         status="completed",
                         type="web_search_call",
-                        action=ActionSearch(query="", type="search"),
+                        action=ActionSearch.model_construct(
+                            type="search", queries=[""]
+                        ),
                     ),
                     ResponseBuilders.response_output_message("Granada"),
                 ],