diff --git a/python/packages/core/agent_framework/_agents.py b/python/packages/core/agent_framework/_agents.py
index a3f4570b6e..47be14653d 100644
--- a/python/packages/core/agent_framework/_agents.py
+++ b/python/packages/core/agent_framework/_agents.py
@@ -935,6 +935,13 @@ def _propagate_conversation_id(update: AgentResponseUpdate) -> AgentResponseUpda
                 session.service_session_id = conv_id
             return update
 
+        def _stream_finalizer(updates: Sequence[AgentResponseUpdate]) -> AgentResponse:
+            # Read response_format from merged chat_options (includes default_options)
+            # so that response_format set via the constructor is respected.
+            ctx = ctx_holder["ctx"]
+            response_format = ctx["chat_options"].get("response_format") if ctx else None
+            return self._finalize_response_updates(updates, response_format=response_format)
+
         return (
             ResponseStream
             .from_awaitable(_get_stream())
@@ -943,9 +950,7 @@ def _propagate_conversation_id(update: AgentResponseUpdate) -> AgentResponseUpda
                     map_chat_to_agent_update,
                     agent_name=self.name,
                 ),
-                finalizer=partial(
-                    self._finalize_response_updates, response_format=options.get("response_format") if options else None
-                ),
+                finalizer=_stream_finalizer,
             )
             .with_transform_hook(_propagate_conversation_id)
             .with_result_hook(_post_hook)
diff --git a/python/packages/core/tests/core/test_agents.py b/python/packages/core/tests/core/test_agents.py
index 627987a1f2..63628b206d 100644
--- a/python/packages/core/tests/core/test_agents.py
+++ b/python/packages/core/tests/core/test_agents.py
@@ -97,6 +97,32 @@ async def test_chat_client_agent_run_streaming(client: SupportsChatGetResponse)
     assert result.text == "test streaming response another update"
 
 
+async def test_chat_client_agent_run_streaming_response_format_via_default_options(
+    client: SupportsChatGetResponse,
+) -> None:
+    """response_format set in default_options must be available on the streamed AgentResponse.value (issue #3970)."""
+    from pydantic import BaseModel
+
+    class Greeting(BaseModel):
+        greeting: str
+
+    json_text = '{"greeting":"hello"}'
+    client.streaming_responses = [  # type: ignore[attr-defined]
+        [ChatResponseUpdate(contents=[Content.from_text(json_text)], role="assistant")],
+    ]
+
+    agent = Agent(client=client, default_options={"response_format": Greeting})
+    stream = agent.run("Hello", stream=True)
+    async for _ in stream:
+        pass
+    result = await stream.get_final_response()
+
+    assert result.text == json_text
+    assert result.value is not None
+    assert isinstance(result.value, Greeting)
+    assert result.value.greeting == "hello"
+
+
 async def test_chat_client_agent_create_session(client: SupportsChatGetResponse) -> None:
     agent = Agent(client=client)
     session = agent.create_session()
diff --git a/python/packages/core/tests/core/test_function_invocation_logic.py b/python/packages/core/tests/core/test_function_invocation_logic.py
index f278afaeac..319d35f152 100644
--- a/python/packages/core/tests/core/test_function_invocation_logic.py
+++ b/python/packages/core/tests/core/test_function_invocation_logic.py
@@ -937,8 +937,7 @@ def ai_func(arg1: str) -> str:
 
     orphaned_calls = all_call_ids - all_result_ids
     assert not orphaned_calls, (
-        f"Response contains orphaned FunctionCallContent without matching "
-        f"FunctionResultContent: {orphaned_calls}."
+        f"Response contains orphaned FunctionCallContent without matching FunctionResultContent: {orphaned_calls}."
     )
 
 
@@ -1123,8 +1122,7 @@ def browser_snapshot(url: str) -> str:
 
     orphaned_calls = all_call_ids - all_result_ids
     assert not orphaned_calls, (
-        f"Response contains orphaned function calls {orphaned_calls}. "
-        f"This would cause API errors on the next call."
+        f"Response contains orphaned function calls {orphaned_calls}. This would cause API errors on the next call."
     )