diff --git a/src/a2a/compat/v0_3/proto_utils.py b/src/a2a/compat/v0_3/proto_utils.py
index d7ac19085..b0c3f158f 100644
--- a/src/a2a/compat/v0_3/proto_utils.py
+++ b/src/a2a/compat/v0_3/proto_utils.py
@@ -307,9 +307,11 @@ def task_status_update_event(
     @classmethod
     def message_send_configuration(
         cls, config: types.MessageSendConfiguration | None
-    ) -> a2a_pb2.SendMessageConfiguration:
+    ) -> a2a_pb2.SendMessageConfiguration | None:
+        # An empty configuration reads as blocking=false, so send none and
+        # leave the server on its blocking default.
         if not config:
-            return a2a_pb2.SendMessageConfiguration()
+            return None
         return a2a_pb2.SendMessageConfiguration(
             accepted_output_modes=config.accepted_output_modes,
             push_notification=cls.push_notification_config(
diff --git a/tests/compat/v0_3/test_grpc_transport.py b/tests/compat/v0_3/test_grpc_transport.py
index 402a57000..51c443fc4 100644
--- a/tests/compat/v0_3/test_grpc_transport.py
+++ b/tests/compat/v0_3/test_grpc_transport.py
@@ -41,6 +41,34 @@ async def test_compat_grpc_transport_send_message_response_msg_parsing():
     assert response.message.message_id == 'msg-123'
 
 
+@pytest.mark.asyncio
+async def test_compat_grpc_transport_send_message_without_configuration():
+    """A request without a configuration must not send one.
+
+    The v0.3 `blocking` field has no presence, so an empty configuration is
+    read by the server as `blocking: false` and the call returns before the
+    task finishes. Without a configuration the server blocks by default, as
+    it does for the v0.3 JSON-RPC transport.
+    """
+    transport = CompatGrpcTransport(
+        channel=AsyncMock(spec=Channel), agent_card=None
+    )
+    mock_stub = MagicMock()
+    mock_stub.SendMessage = AsyncMock(
+        return_value=a2a_v0_3_pb2.SendMessageResponse()
+    )
+    transport.stub = mock_stub
+
+    await transport.send_message(
+        SendMessageRequest(
+            message=Message(message_id='msg-1', role=Role.ROLE_USER)
+        )
+    )
+
+    sent = mock_stub.SendMessage.call_args.args[0]
+    assert not sent.HasField('configuration')
+
+
 def test_compat_grpc_transport_mirrors_extension_metadata():
     """Compat gRPC client must also emit the legacy x-a2a-extensions metadata
     so that v0.3 servers (which only know that name) understand the request."""
diff --git a/tests/compat/v0_3/test_rest_transport.py b/tests/compat/v0_3/test_rest_transport.py
index 7ad838d9f..7f0ca2369 100644
--- a/tests/compat/v0_3/test_rest_transport.py
+++ b/tests/compat/v0_3/test_rest_transport.py
@@ -19,6 +19,7 @@
     ListTasksRequest,
     Message,
     Role,
+    SendMessageConfiguration,
     SendMessageRequest,
     SendMessageResponse,
     StreamResponse,
@@ -574,6 +575,56 @@ async def mock_send_request(*args, **kwargs):
     assert response == SendMessageResponse()
 
 
+@pytest.mark.asyncio
+async def test_compat_rest_transport_send_message_without_configuration(
+    transport,
+):
+    """A request without a configuration must not send one.
+
+    The v0.3 `blocking` field has no presence, so an empty configuration is
+    read by the server as `blocking: false` and the call returns before the
+    task finishes. Without a configuration the server blocks by default, as
+    it does for the v0.3 JSON-RPC transport.
+    """
+    sent: dict = {}
+
+    async def mock_execute_request(method, path, context=None, json=None):
+        sent.update(json)
+        return {}
+
+    transport._execute_request = mock_execute_request
+
+    req = SendMessageRequest(
+        message=Message(message_id='msg-1', role=Role.ROLE_USER)
+    )
+    await transport.send_message(req)
+
+    assert 'configuration' not in sent
+
+
+@pytest.mark.asyncio
+async def test_compat_rest_transport_send_message_with_configuration(
+    transport,
+):
+    sent: dict = {}
+
+    async def mock_execute_request(method, path, context=None, json=None):
+        sent.update(json)
+        return {}
+
+    transport._execute_request = mock_execute_request
+
+    req = SendMessageRequest(
+        message=Message(message_id='msg-1', role=Role.ROLE_USER),
+        configuration=SendMessageConfiguration(
+            accepted_output_modes=['text/plain']
+        ),
+    )
+    await transport.send_message(req)
+
+    assert sent['configuration']['blocking'] is True
+
+
 @pytest.mark.asyncio
 async def test_compat_rest_transport_get_task_no_history(transport):
     async def mock_execute_request(method, path, context=None, params=None):