feat: add traceability extension support

src/a2a/client/base_client.py

@@ -11,6 +11,7 @@
 from a2a.client.errors import A2AClientInvalidStateError
 from a2a.client.middleware import ClientCallInterceptor
 from a2a.client.transports.base import ClientTransport
+from a2a.extensions.base import Extension
 from a2a.types import (
     AgentCard,
     GetTaskPushNotificationConfigParams,
@@ -41,6 +42,12 @@ def __init__(
         self._card = card
         self._config = config
         self._transport = transport
+        self._extensions: list[Extension] = []
+
+    def install_extension(self, extension: Extension) -> None:
+        """Installs an extension on the client."""
+        extension.install(self)
+        self._extensions.append(extension)
 
     async def send_message(
         self,
@@ -61,6 +68,9 @@ async def send_message(
         Yields:
             An async iterator of `ClientEvent` or a final `Message` response.
         """
+        for extension in self._extensions:
+            extension.on_client_message(request)
+
         config = MessageSendConfiguration(
             accepted_output_modes=self._config.accepted_output_modes,
             blocking=not self._config.polling,

src/a2a/extensions/__init__.py

@@ -0,0 +1,6 @@
+"""A2A extensions."""
+
+from .base import Extension
+from . import common, trace
+
+__all__ = ['Extension', 'common', 'trace']

src/a2a/extensions/base.py

@@ -0,0 +1,26 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, Any
+
+if TYPE_CHECKING:
+    from a2a.client.client import A2AClient
+    from a2a.server.server import A2AServer
+
+
+class Extension:
+    """Base class for all extensions."""
+
+    def __init__(self, **kwargs: Any) -> None:
+        ...
+
+    def on_client_message(self, message: Any) -> None:
+        """Called when a message is sent from the client."""
+        ...
+
+    def on_server_message(self, message: Any) -> None:
+        """Called when a message is received by the server."""
+        ...
+
+    def install(self, client_or_server: A2AClient | A2AServer) -> None:
+        """Called when the extension is installed on a client or server."""
+        ...

src/a2a/extensions/trace.py

@@ -0,0 +1,81 @@
+from __future__ import annotations
+
+from datetime import datetime
+from enum import Enum
+from typing import Any
+
+from a2a._base import A2ABaseModel
+from a2a.extensions.base import Extension
+
+
+class CallTypeEnum(str, Enum):
+    """The type of the operation a step represents."""
+
+    AGENT = 'AGENT'
+    TOOL = 'TOOL'
+
+
+class ToolInvocation(A2ABaseModel):
+    """A tool invocation."""
+
+    tool_name: str
+    parameters: dict[str, Any]
+
+
+class AgentInvocation(A2ABaseModel):
+    """An agent invocation."""
+
+    agent_url: str
+    agent_name: str
+    requests: dict[str, Any]
+    response_trace: ResponseTrace | None = None
+
+
+class StepAction(A2ABaseModel):
+    """The action of a step."""
+
+    tool_invocation: ToolInvocation | None = None
+    agent_invocation: AgentInvocation | None = None
+
+
+class Step(A2ABaseModel):
+    """A single operation within a trace."""
+
+    step_id: str
+    trace_id: str
+    parent_step_id: str | None = None
+    call_type: CallTypeEnum
+    step_action: StepAction
+    cost: int | None = None
+    total_tokens: int | None = None
+    additional_attributes: dict[str, str] | None = None
+    latency: int | None = None
+    start_time: datetime
+    end_time: datetime
+
+
+class ResponseTrace(A2ABaseModel):
+    """A trace message that contains a collection of spans."""
+
+    trace_id: str
+    steps: list[Step]
+
+
+class TraceExtension(Extension):
+    """An extension for traceability."""
+
+    def on_client_message(self, message: Any) -> None:
+        """Appends trace information to the message."""
+        # This is a placeholder implementation.
+        if message.metadata is None:
+            message.metadata = {}
+        message.metadata['trace'] = 'client-trace'
+
+    def on_server_message(self, message: Any) -> None:
+        """Processes trace information from the message."""
+        # This is a placeholder implementation.
+        if hasattr(message, 'metadata') and 'trace' in message.metadata:
+            print(f"Received trace: {message.metadata['trace']}")
+
+
+AgentInvocation.model_rebuild()

src/a2a/server/request_handlers/default_request_handler.py

@@ -2,7 +2,7 @@
 import logging
 
 from collections.abc import AsyncGenerator
-from typing import cast
+from typing import Any, cast
 
 from a2a.server.agent_execution import (
     AgentExecutor,
@@ -26,6 +26,7 @@
     TaskManager,
     TaskStore,
 )
+from a2a.extensions.base import Extension
 from a2a.types import (
     DeleteTaskPushNotificationConfigParams,
     GetTaskPushNotificationConfigParams,
@@ -101,6 +102,12 @@ def __init__(  # noqa: PLR0913
         # TODO: Likely want an interface for managing this, like AgentExecutionManager.
         self._running_agents = {}
         self._running_agents_lock = asyncio.Lock()
+        self._extensions: list[Extension] = []
+
+    def install_extension(self, extension: Extension, server: Any) -> None:
+        """Installs an extension on the server."""
+        extension.install(server)
+        self._extensions.append(extension)
 
     async def on_get_task(
         self,
@@ -182,6 +189,9 @@ async def _setup_message_execution(
         Returns:
             A tuple of (task_manager, task_id, queue, result_aggregator, producer_task)
         """
+        for extension in self._extensions:
+            extension.on_server_message(params.message)
+
         # Create task manager and validate existing task
         task_manager = TaskManager(
             task_id=params.message.task_id,

tests/extensions/test_trace.py

@@ -0,0 +1,63 @@
+from datetime import datetime, timezone
+
+from a2a.extensions.trace import (
+    AgentInvocation,
+    CallTypeEnum,
+    ResponseTrace,
+    Step,
+    StepAction,
+    ToolInvocation,
+)
+
+
+def test_trace_serialization():
+    start_time = datetime(2025, 3, 15, 12, 0, 0, tzinfo=timezone.utc)
+    end_time = datetime(2025, 3, 15, 12, 0, 0, 250000, tzinfo=timezone.utc)
+
+    trace = ResponseTrace(
+        trace_id='trace-example-12345',
+        steps=[
+            Step(
+                step_id='step-1-agent',
+                trace_id='trace-example-1234p',
+                call_type=CallTypeEnum.AGENT,
+                step_action=StepAction(
+                    agent_invocation=AgentInvocation(
+                        agent_name='weather_agent',
+                        agent_url='http://google3/some/agent/url',
+                        requests={
+                            'user_prompt': "What's the weather in Paris and what should I wear?"
+                        },
+                    )
+                ),
+                cost=150,
+                total_tokens=75,
+                additional_attributes={'user_country': 'US'},
+                latency=250,
+                start_time=start_time,
+                end_time=end_time,
+            ),
+            Step(
+                step_id='step-2-tool',
+                trace_id='trace-example-12345',
+                parent_step_id='step-1-agent',
+                call_type=CallTypeEnum.TOOL,
+                step_action=StepAction(
+                    tool_invocation=ToolInvocation(
+                        tool_name='google_map_api_tool',
+                        parameters={'location': 'Paris, FR'},
+                    )
+                ),
+                cost=50,
+                total_tokens=20,
+                latency=100,
+                start_time=start_time,
+                end_time=end_time,
+            ),
+        ],
+    )
+
+    trace_dict = trace.model_dump(mode='json')
+    deserialized_trace = ResponseTrace.model_validate(trace_dict)
+
+    assert trace == deserialized_trace

tests/extensions/test_trace_extension.py

@@ -0,0 +1,43 @@
+from unittest.mock import Mock
+
+import pytest
+
+from a2a.client.base_client import BaseClient
+from a2a.extensions.trace import TraceExtension
+from a2a.server.request_handlers.default_request_handler import DefaultRequestHandler
+from a2a.types import Message, TextPart
+
+
+@pytest.mark.asyncio
+async def test_trace_extension():
+    client = BaseClient(card=Mock(), config=Mock(), transport=Mock(), consumers=[], middleware=[])
+    server_handler = DefaultRequestHandler(
+        agent_executor=Mock(),
+        task_store=Mock(),
+    )
+
+    trace_extension = TraceExtension()
+    client.install_extension(trace_extension)
+    server_handler.install_extension(trace_extension, server=Mock())
+
+    message = Message(
+        message_id='test_message',
+        role='user',
+        parts=[TextPart(text='Hello, world!')],
+    )
+
+    # Simulate client sending a message
+    for extension in client._extensions:
+        extension.on_client_message(message)
+
+    assert 'trace' in message.metadata
+    assert message.metadata['trace'] == 'client-trace'
+
+    # Simulate server receiving a message
+    for extension in server_handler._extensions:
+        extension.on_server_message(message)
+
+    # Check that the server-side handler was called
+    # (in this case, it just prints a message)
+    # We can't easily check the output of print, so we'll just
+    # assume it worked if no exceptions were raised.