diff --git a/src/google/adk/events/event_actions.py b/src/google/adk/events/event_actions.py
index dd0ada936bc..3c833919a9b 100644
--- a/src/google/adk/events/event_actions.py
+++ b/src/google/adk/events/event_actions.py
@@ -42,6 +42,20 @@
 logger = logging.getLogger('google_adk.' + __name__)
 
 
+def _build_deferred_pydantic_serializers(obj: Any) -> None:
+  """Builds serializers for Pydantic models nested in state containers."""
+  if isinstance(obj, BaseModel):
+    obj.__class__.model_rebuild(raise_errors=False)
+    return
+  if isinstance(obj, dict):
+    for value in obj.values():
+      _build_deferred_pydantic_serializers(value)
+    return
+  if isinstance(obj, (list, tuple)):
+    for value in obj:
+      _build_deferred_pydantic_serializers(value)
+
+
 def _make_json_serializable(obj: Any) -> Any:
   """Converts an object into a JSON-serializable form.
 
@@ -52,6 +66,7 @@ def _make_json_serializable(obj: Any) -> Any:
   are replaced with their `repr` via `serialize_unknown=True` so the overall
   structure can still be persisted without crashing.
   """
+  _build_deferred_pydantic_serializers(obj)
   return to_jsonable_python(obj, serialize_unknown=True)
 
 
@@ -105,15 +120,19 @@ def _serialize_state_delta(
     try:
       return cast(dict[str, Any], handler(value))
     except Exception:  # pylint: disable=broad-except
-      logger.warning(
-          'Failed to serialize `state_delta`; some values are not'
-          ' JSON-serializable (e.g. callables) and will be replaced with a'
-          ' string representation in the persisted event.',
-          exc_info=True,
-      )
-      # Re-run the handler on the sanitized value so that caller `exclude` /
-      # `include` directives are still applied to the fallback output.
-      return cast(dict[str, Any], handler(_make_json_serializable(value)))
+      _build_deferred_pydantic_serializers(value)
+      try:
+        return cast(dict[str, Any], handler(value))
+      except Exception:  # pylint: disable=broad-except
+        logger.warning(
+            'Failed to serialize `state_delta`; some values are not'
+            ' JSON-serializable (e.g. callables) and will be replaced with a'
+            ' string representation in the persisted event.',
+            exc_info=True,
+        )
+        # Re-run the handler on the sanitized value so that caller `exclude` /
+        # `include` directives are still applied to the fallback output.
+        return cast(dict[str, Any], handler(_make_json_serializable(value)))
 
   artifact_delta: dict[str, int] = Field(default_factory=dict)
   """Indicates that the event is updating an artifact. key is the filename,
@@ -179,15 +198,19 @@ def _serialize_agent_state(
     try:
       return cast(Optional[dict[str, Any]], handler(value))
     except Exception:  # pylint: disable=broad-except
-      logger.warning(
-          'Failed to serialize `agent_state`; some values are not'
-          ' JSON-serializable (e.g. callables) and will be replaced with a'
-          ' string representation in the persisted event.',
-          exc_info=True,
-      )
-      # Re-run the handler on the sanitized value so that caller `exclude` /
-      # `include` directives are still applied to the fallback output.
-      return cast(dict[str, Any], handler(_make_json_serializable(value)))
+      _build_deferred_pydantic_serializers(value)
+      try:
+        return cast(Optional[dict[str, Any]], handler(value))
+      except Exception:  # pylint: disable=broad-except
+        logger.warning(
+            'Failed to serialize `agent_state`; some values are not'
+            ' JSON-serializable (e.g. callables) and will be replaced with a'
+            ' string representation in the persisted event.',
+            exc_info=True,
+        )
+        # Re-run the handler on the sanitized value so that caller `exclude` /
+        # `include` directives are still applied to the fallback output.
+        return cast(dict[str, Any], handler(_make_json_serializable(value)))
 
   rewind_before_invocation_id: Optional[str] = None
   """The invocation id to rewind to. This is only set for rewind event."""
diff --git a/tests/unittests/events/test_event_actions.py b/tests/unittests/events/test_event_actions.py
index ef864060db9..e3a36a5ba06 100644
--- a/tests/unittests/events/test_event_actions.py
+++ b/tests/unittests/events/test_event_actions.py
@@ -22,6 +22,7 @@
 from google.adk.events.event_actions import _make_json_serializable
 from google.adk.events.event_actions import EventActions
 from pydantic import BaseModel
+from pydantic import ConfigDict
 
 
 class _Sample(BaseModel):
@@ -71,6 +72,24 @@ def test_serializable_state_delta_round_trips(self):
     dumped = actions.model_dump(mode='json')
     assert dumped['state_delta'] == {'a': 1, 'b': [1, 2], 'c': {'d': 'e'}}
 
+  def test_deferred_pydantic_state_delta_round_trips(self):
+    """Nested deferred Pydantic models remain serializable in state."""
+
+    class _DeferredModel(BaseModel):
+      model_config = ConfigDict(defer_build=True)
+
+      value: str
+
+    class _Container(BaseModel):
+      model: _DeferredModel
+
+    deferred = _Container.model_validate({'model': {'value': 'ready'}}).model
+    actions = EventActions(state_delta={'deferred': deferred})
+
+    dumped = actions.model_dump(mode='json')
+
+    assert dumped['state_delta']['deferred'] == {'value': 'ready'}
+
   def test_non_serializable_state_delta_does_not_raise(self):
     actions = EventActions(state_delta={'cb': lambda: 1, 'ok': 2})
     dumped = actions.model_dump(mode='json')