diff --git a/chatkit/agents.py b/chatkit/agents.py
index 7f435e3..38e674b 100644
--- a/chatkit/agents.py
+++ b/chatkit/agents.py
@@ -438,8 +438,11 @@ async def _convert_content(
 async def _convert_annotation(
     raw_annotation: object, converter: ResponseStreamConverter
 ) -> Annotation | None:
-    # There is a bug in the OpenAPI client that sometimes parses the annotation delta event into the wrong class
-    # resulting into annotation being a dict or untyped object instead instead of a ResponsesAnnotation
+    # OpenAI clients can use event-specific models for annotation delta events.
+    # Normalize those models before validating them as response annotations.
+    if isinstance(raw_annotation, BaseModel):
+        raw_annotation = raw_annotation.model_dump()
+
     annotation = TypeAdapter[ResponsesAnnotation](ResponsesAnnotation).validate_python(
         raw_annotation
     )
diff --git a/tests/test_agents.py b/tests/test_agents.py
index 1611162..afdf3e6 100644
--- a/tests/test_agents.py
+++ b/tests/test_agents.py
@@ -3,7 +3,7 @@
 from collections.abc import AsyncIterator
 from datetime import datetime
 from importlib import import_module
-from typing import cast
+from typing import Literal, cast
 from unittest.mock import AsyncMock, Mock
 
 import pytest
@@ -57,6 +57,7 @@
 )
 from openai.types.responses.response_text_delta_event import ResponseTextDeltaEvent
 from openai.types.responses.response_text_done_event import ResponseTextDoneEvent
+from pydantic import BaseModel
 
 from chatkit.agents import (
     AgentContext,
@@ -1128,6 +1129,14 @@ def add_annotation_event(annotation, sequence_number):
 
 
 async def test_stream_agent_response_annotation_added_normalizes_annotations():
+    # OpenAI 3.x defines this event model separately from the response-content model.
+    class EventAnnotationURLCitation(BaseModel):
+        end_index: int
+        start_index: int
+        title: str
+        type: Literal["url_citation"]
+        url: str
+
     context = AgentContext(
         previous_response_id=None, thread=thread, store=mock_store, request_context=None
     )
@@ -1174,13 +1183,13 @@ def add_annotation_event(annotation, sequence_number):
         sequence_number=1,
     )
     add_annotation_event(
-        {
-            "type": "url_citation",
-            "url": "https://example.com",
-            "title": "Example",
-            "start_index": 1,
-            "end_index": 5,
-        },
+        EventAnnotationURLCitation(
+            type="url_citation",
+            url="https://example.com",
+            title="Example",
+            start_index=1,
+            end_index=5,
+        ),
         sequence_number=2,
     )