feat: Allow custom part converters in A2A classes

This change introduces type descriptions for the functions which convert between A2A and GenAI `Part`s. It then allows passing instances of those functions to the various A2A-related functions/classes, effectively allowing users to inject their own logic for how part conversion should occur. The benefit of this pattern is that users can create decorators around the core `Part` conversion logic, which allows them to intercept the cases they care about while delegating the ones they do not to the core converter. This is a pattern we use a lot in the A2A Python SDK. One example where this type of logic is useful is for extensions: this allows extension logic to, for example, interpret an A2A DataPart into a FunctionResponse using extension-specific logic. PiperOrigin-RevId: 803186799

Author: google-genai-bot

src/google/adk/a2a/converters/event_converter.py

@@ -42,8 +42,10 @@
 from .part_converter import A2A_DATA_PART_METADATA_IS_LONG_RUNNING_KEY
 from .part_converter import A2A_DATA_PART_METADATA_TYPE_FUNCTION_CALL
 from .part_converter import A2A_DATA_PART_METADATA_TYPE_KEY
+from .part_converter import A2APartToGenAIPartConverter
 from .part_converter import convert_a2a_part_to_genai_part
 from .part_converter import convert_genai_part_to_a2a_part
+from .part_converter import GenAIPartToA2APartConverter
 from .utils import _get_adk_metadata_key
 
 # Constants
@@ -169,6 +171,7 @@ def convert_a2a_task_to_event(
  a2a_task: Task,
  author: Optional[str] = None,
  invocation_context: Optional[InvocationContext] = None,
+ part_converter: A2APartToGenAIPartConverter = convert_a2a_part_to_genai_part,
 ) -> Event:
  """Converts an A2A task to an ADK event.
 
@@ -177,6 +180,7 @@ def convert_a2a_task_to_event(
  author: The author of the event. Defaults to "a2a agent" if not provided.
  invocation_context: The invocation context containing session information.
  If provided, the branch will be set from the context.
+ part_converter: The function to convert A2A part to GenAI part.
 
  Returns:
  An ADK Event object representing the converted task.
@@ -203,7 +207,9 @@ def convert_a2a_task_to_event(
  # Convert message if available
  if message:
  try:
- return convert_a2a_message_to_event(message, author, invocation_context)
+ return convert_a2a_message_to_event(
+ message, author, invocation_context, part_converter=part_converter
+ )
  except Exception as e:
  logger.error("Failed to convert A2A task message to event: %s", e)
  raise RuntimeError(f"Failed to convert task message: {e}") from e
@@ -229,6 +235,7 @@ def convert_a2a_message_to_event(
  a2a_message: Message,
  author: Optional[str] = None,
  invocation_context: Optional[InvocationContext] = None,
+ part_converter: A2APartToGenAIPartConverter = convert_a2a_part_to_genai_part,
 ) -> Event:
  """Converts an A2A message to an ADK event.
 
@@ -237,6 +244,7 @@ def convert_a2a_message_to_event(
  author: The author of the event. Defaults to "a2a agent" if not provided.
  invocation_context: The invocation context containing session information.
  If provided, the branch will be set from the context.
+ part_converter: The function to convert A2A part to GenAI part.
 
  Returns:
  An ADK Event object with converted content and long-running tool metadata.
@@ -269,7 +277,7 @@ def convert_a2a_message_to_event(
 
  for a2a_part in a2a_message.parts:
  try:
- part = convert_a2a_part_to_genai_part(a2a_part)
+ part = part_converter(a2a_part)
  if part is None:
  logger.warning("Failed to convert A2A part, skipping: %s", a2a_part)
  continue
@@ -322,13 +330,18 @@ def convert_a2a_message_to_event(
 
 @a2a_experimental
 def convert_event_to_a2a_message(
- event: Event, invocation_context: InvocationContext, role: Role = Role.agent
+ event: Event,
+ invocation_context: InvocationContext,
+ role: Role = Role.agent,
+ part_converter: GenAIPartToA2APartConverter = convert_genai_part_to_a2a_part,
 ) -> Optional[Message]:
  """Converts an ADK event to an A2A message.
 
  Args:
  event: The ADK event to convert.
  invocation_context: The invocation context.
+ role: The role of the message.
+ part_converter: The function to convert GenAI part to A2A part.
 
  Returns:
  An A2A Message if the event has content, None otherwise.
@@ -347,7 +360,7 @@ def convert_event_to_a2a_message(
  try:
  a2a_parts = []
  for part in event.content.parts:
- a2a_part = convert_genai_part_to_a2a_part(part)
+ a2a_part = part_converter(part)
  if a2a_part:
  a2a_parts.append(a2a_part)
  _process_long_running_tool(a2a_part, event)
@@ -477,6 +490,7 @@ def convert_event_to_a2a_events(
  invocation_context: InvocationContext,
  task_id: Optional[str] = None,
  context_id: Optional[str] = None,
+ part_converter: GenAIPartToA2APartConverter = convert_genai_part_to_a2a_part,
 ) -> List[A2AEvent]:
  """Converts a GenAI event to a list of A2A events.
 
@@ -485,6 +499,7 @@ def convert_event_to_a2a_events(
  invocation_context: The invocation context.
  task_id: Optional task ID to use for generated events.
  context_id: Optional Context ID to use for generated events.
+ part_converter: The function to convert GenAI part to A2A part.
 
  Returns:
  A list of A2A events representing the converted ADK event.
@@ -509,7 +524,9 @@ def convert_event_to_a2a_events(
  a2a_events.append(error_event)
 
  # Handle regular message content
- message = convert_event_to_a2a_message(event, invocation_context)
+ message = convert_event_to_a2a_message(
+ event, invocation_context, part_converter=part_converter
+ )
  if message:
  running_event = _create_status_update_event(
  message, invocation_context, event, task_id, context_id

src/google/adk/a2a/converters/part_converter.py

@@ -19,6 +19,7 @@
 from __future__ import annotations
 
 import base64
+from collections.abc import Callable
 import json
 import logging
 from typing import Optional
@@ -51,6 +52,14 @@
 A2A_DATA_PART_METADATA_TYPE_EXECUTABLE_CODE = 'executable_code'
 
 
+A2APartToGenAIPartConverter = Callable[
+ [a2a_types.Part], Optional[genai_types.Part]
+]
+GenAIPartToA2APartConverter = Callable[
+ [genai_types.Part], Optional[a2a_types.Part]
+]
+
+
 @a2a_experimental
 def convert_a2a_part_to_genai_part(
  a2a_part: a2a_types.Part,

src/google/adk/a2a/converters/request_converter.py

@@ -31,6 +31,7 @@
 
 from ...runners import RunConfig
 from ..experimental import a2a_experimental
+from .part_converter import A2APartToGenAIPartConverter
 from .part_converter import convert_a2a_part_to_genai_part
 
 
@@ -50,6 +51,7 @@ def _get_user_id(request: RequestContext) -> str:
 @a2a_experimental
 def convert_a2a_request_to_adk_run_args(
  request: RequestContext,
+ part_converter: A2APartToGenAIPartConverter = convert_a2a_part_to_genai_part,
 ) -> dict[str, Any]:
 
  if not request.message:
@@ -60,10 +62,7 @@ def convert_a2a_request_to_adk_run_args(
  'session_id': request.context_id,
  'new_message': genai_types.Content(
  role='user',
- parts=[
- convert_a2a_part_to_genai_part(part)
- for part in request.message.parts
- ],
+ parts=[part_converter(part) for part in request.message.parts],
  ),
  'run_config': RunConfig(),
  }

src/google/adk/a2a/executor/a2a_agent_executor.py

@@ -53,6 +53,10 @@
 from typing_extensions import override
 
 from ..converters.event_converter import convert_event_to_a2a_events
+from ..converters.part_converter import A2APartToGenAIPartConverter
+from ..converters.part_converter import convert_a2a_part_to_genai_part
+from ..converters.part_converter import convert_genai_part_to_a2a_part
+from ..converters.part_converter import GenAIPartToA2APartConverter
 from ..converters.request_converter import convert_a2a_request_to_adk_run_args
 from ..converters.utils import _get_adk_metadata_key
 from ..experimental import a2a_experimental
@@ -65,12 +69,18 @@
 class A2aAgentExecutorConfig(BaseModel):
  """Configuration for the A2aAgentExecutor."""
 
- pass
+ a2a_part_converter: A2APartToGenAIPartConverter = (
+ convert_a2a_part_to_genai_part
+ )
+ gen_ai_part_converter: GenAIPartToA2APartConverter = (
+ convert_genai_part_to_a2a_part
+ )
 
 
 @a2a_experimental
 class A2aAgentExecutor(AgentExecutor):
  """An AgentExecutor that runs an ADK Agent against an A2A request and
+
  publishes updates to an event queue.
  """
 
@@ -82,7 +92,7 @@ def __init__(
  ):
  super().__init__()
  self._runner = runner
- self._config = config
+ self._config = config or A2aAgentExecutorConfig()
 
  async def _resolve_runner(self) -> Runner:
  """Resolve the runner, handling cases where it's a callable that returns a Runner."""
@@ -183,7 +193,9 @@ async def _handle_request(
  runner = await self._resolve_runner()
 
  # Convert the a2a request to ADK run args
- run_args = convert_a2a_request_to_adk_run_args(context)
+ run_args = convert_a2a_request_to_adk_run_args(
+ context, self._config.a2a_part_converter
+ )
 
  # ensure the session exists
  session = await self._prepare_session(context, run_args, runner)
@@ -217,7 +229,11 @@ async def _handle_request(
  async with Aclosing(runner.run_async(**run_args)) as agen:
  async for adk_event in agen:
  for a2a_event in convert_event_to_a2a_events(
- adk_event, invocation_context, context.task_id, context.context_id
+ adk_event,
+ invocation_context,
+ context.task_id,
+ context.context_id,
+ self._config.gen_ai_part_converter,
  ):
  task_result_aggregator.process_event(a2a_event)
  await event_queue.enqueue_event(a2a_event)

src/google/adk/agents/remote_a2a_agent.py

@@ -57,7 +57,10 @@
 from ..a2a.converters.event_converter import convert_a2a_message_to_event
 from ..a2a.converters.event_converter import convert_a2a_task_to_event
 from ..a2a.converters.event_converter import convert_event_to_a2a_message
+from ..a2a.converters.part_converter import A2APartToGenAIPartConverter
+from ..a2a.converters.part_converter import convert_a2a_part_to_genai_part
 from ..a2a.converters.part_converter import convert_genai_part_to_a2a_part
+from ..a2a.converters.part_converter import GenAIPartToA2APartConverter
 from ..a2a.experimental import a2a_experimental
 from ..a2a.logs.log_utils import build_a2a_request_log
 from ..a2a.logs.log_utils import build_a2a_response_log
@@ -120,6 +123,8 @@ def __init__(
  description: str = "",
  httpx_client: Optional[httpx.AsyncClient] = None,
  timeout: float = DEFAULT_TIMEOUT,
+ genai_part_converter: GenAIPartToA2APartConverter = convert_genai_part_to_a2a_part,
+ a2a_part_converter: A2APartToGenAIPartConverter = convert_a2a_part_to_genai_part,
  **kwargs: Any,
  ) -> None:
  """Initialize RemoteA2aAgent.
@@ -149,6 +154,8 @@ def __init__(
  self._httpx_client_needs_cleanup = httpx_client is None
  self._timeout = timeout
  self._is_resolved = False
+ self._genai_part_converter = genai_part_converter
+ self._a2a_part_converter = a2a_part_converter
 
  # Validate and store agent card reference
  if isinstance(agent_card, AgentCard):
@@ -298,7 +305,7 @@ def _create_a2a_request_for_user_function_response(
  return None
 
  a2a_message = convert_event_to_a2a_message(
- ctx.session.events[-1], ctx, Role.user
+ ctx.session.events[-1], ctx, Role.user, self._genai_part_converter
  )
  if function_call_event.custom_metadata:
  a2a_message.task_id = (
@@ -355,7 +362,7 @@ def _construct_message_parts_from_session(
 
  for part in event.content.parts:
 
- converted_part = convert_genai_part_to_a2a_part(part)
+ converted_part = self._genai_part_converter(part)
  if converted_part:
  message_parts.append(converted_part)
  else:
@@ -380,7 +387,10 @@ async def _handle_a2a_response(
  if a2a_response.root.result:
  if isinstance(a2a_response.root.result, A2ATask):
  event = convert_a2a_task_to_event(
- a2a_response.root.result, self.name, ctx
+ a2a_response.root.result,
+ self.name,
+ ctx,
+ self._a2a_part_converter,
  )
  event.custom_metadata = event.custom_metadata or {}
  event.custom_metadata[A2A_METADATA_PREFIX + "task_id"] = (
@@ -389,7 +399,10 @@ async def _handle_a2a_response(
 
  else:
  event = convert_a2a_message_to_event(
- a2a_response.root.result, self.name, ctx
+ a2a_response.root.result,
+ self.name,
+ ctx,
+ self._a2a_part_converter,
  )
  event.custom_metadata = event.custom_metadata or {}
  if a2a_response.root.result.task_id: