feat(client): allow specifying history_length via call-site MessageSendConfiguration in BaseClient.send_message (#500)

## Description
This PR implements the client-side enhancement described in #499.
It allows callers to specify `history_length` (and other message
configuration options)
directly via the optional configuration parameter in
`BaseClient.send_message`,
complementing the server-side support added in #497.

### Summary of Changes
- Added optional argument `configuration: MessageSendConfiguration |
None` to `BaseClient.send_message`.
- When provided, merges call-site configuration with `ClientConfig`
defaults.
- Allows partial overrides (e.g., setting `historyLength` only).
- Updated docstrings to reflect the new parameter behavior.
- Added unit tests covering both non-streaming and streaming scenarios.

### Motivation
Previously, `BaseClient.send_message` built `MessageSendConfiguration`
internally from `ClientConfig`,
but `ClientConfig` does not include `history_length`.
This prevented clients from specifying it on a per-call basis, unlike
`get_task`,
which already supports `history_length` via `TaskQueryParams`.
After this change, client behavior becomes consistent across both
methods.

### Related Issues
Fixes #499
Complements #497 (server-side historyLength support)

Release-As: 0.3.17

---------

Co-authored-by: tadaki <[email protected]>
Co-authored-by: Holt Skinner <[email protected]>
Co-authored-by: TadakiAsechi <[email protected]>
Co-authored-by: TadakiAsechi <[email protected]>

Author: 5 people

src/a2a/client/base_client.py

@@ -47,6 +47,7 @@ async def send_message(
         self,
         request: Message,
         *,
+        configuration: MessageSendConfiguration | None = None,
         context: ClientCallContext | None = None,
         request_metadata: dict[str, Any] | None = None,
         extensions: list[str] | None = None,
@@ -59,14 +60,15 @@ async def send_message(
 
         Args:
             request: The message to send to the agent.
+            configuration: Optional per-call overrides for message sending behavior.
             context: The client call context.
             request_metadata: Extensions Metadata attached to the request.
             extensions: List of extensions to be activated.
 
         Yields:
             An async iterator of `ClientEvent` or a final `Message` response.
         """
-        config = MessageSendConfiguration(
+        base_config = MessageSendConfiguration(
             accepted_output_modes=self._config.accepted_output_modes,
             blocking=not self._config.polling,
             push_notification_config=(
@@ -75,6 +77,15 @@ async def send_message(
                 else None
             ),
         )
+        if configuration is not None:
+            update_data = configuration.model_dump(
+                exclude_unset=True,
+                by_alias=False,
+            )
+            config = base_config.model_copy(update=update_data)
+        else:
+            config = base_config
+
         params = MessageSendParams(
             message=request, configuration=config, metadata=request_metadata
         )

tests/client/test_base_client.py

@@ -9,6 +9,7 @@
     AgentCapabilities,
     AgentCard,
     Message,
+    MessageSendConfiguration,
     Part,
     Role,
     Task,
@@ -125,3 +126,78 @@ async def test_send_message_non_streaming_agent_capability_false(
     assert not mock_transport.send_message_streaming.called
     assert len(events) == 1
     assert events[0][0].id == 'task-789'
+
+
+@pytest.mark.asyncio
+async def test_send_message_callsite_config_overrides_non_streaming(
+    base_client: BaseClient, mock_transport: MagicMock, sample_message: Message
+):
+    base_client._config.streaming = False
+    mock_transport.send_message.return_value = Task(
+        id='task-cfg-ns-1',
+        context_id='ctx-cfg-ns-1',
+        status=TaskStatus(state=TaskState.completed),
+    )
+
+    cfg = MessageSendConfiguration(
+        history_length=2,
+        blocking=False,
+        accepted_output_modes=['application/json'],
+    )
+    events = [
+        event
+        async for event in base_client.send_message(
+            sample_message, configuration=cfg
+        )
+    ]
+
+    mock_transport.send_message.assert_called_once()
+    assert not mock_transport.send_message_streaming.called
+    assert len(events) == 1
+    task, _ = events[0]
+    assert task.id == 'task-cfg-ns-1'
+
+    params = mock_transport.send_message.call_args[0][0]
+    assert params.configuration.history_length == 2
+    assert params.configuration.blocking is False
+    assert params.configuration.accepted_output_modes == ['application/json']
+
+
+@pytest.mark.asyncio
+async def test_send_message_callsite_config_overrides_streaming(
+    base_client: BaseClient, mock_transport: MagicMock, sample_message: Message
+):
+    base_client._config.streaming = True
+    base_client._card.capabilities.streaming = True
+
+    async def create_stream(*args, **kwargs):
+        yield Task(
+            id='task-cfg-s-1',
+            context_id='ctx-cfg-s-1',
+            status=TaskStatus(state=TaskState.completed),
+        )
+
+    mock_transport.send_message_streaming.return_value = create_stream()
+
+    cfg = MessageSendConfiguration(
+        history_length=0,
+        blocking=True,
+        accepted_output_modes=['text/plain'],
+    )
+    events = [
+        event
+        async for event in base_client.send_message(
+            sample_message, configuration=cfg
+        )
+    ]
+
+    mock_transport.send_message_streaming.assert_called_once()
+    assert not mock_transport.send_message.called
+    assert len(events) == 1
+    task, _ = events[0]
+    assert task.id == 'task-cfg-s-1'
+
+    params = mock_transport.send_message_streaming.call_args[0][0]
+    assert params.configuration.history_length == 0
+    assert params.configuration.blocking is True
+    assert params.configuration.accepted_output_modes == ['text/plain']