feat: implement gRPC rich error details for A2A errors, including new error types and client-side parsing.

Author: knapg

src/a2a/client/transports/grpc.py

@@ -1,11 +1,12 @@
 import logging
 
-from collections.abc import AsyncGenerator, Callable
+from collections.abc import AsyncGenerator, Callable, Iterable
 from functools import wraps
-from typing import Any, NoReturn
+from typing import Any, NoReturn, cast
 
+from a2a.client.errors import A2AClientError, A2AClientTimeoutError
 from a2a.client.middleware import ClientCallContext
-from a2a.utils.errors import JSON_RPC_ERROR_CODE_MAP
+from a2a.utils.errors import JSON_RPC_ERROR_CODE_MAP, A2AError
 
 
 try:
@@ -18,8 +19,12 @@
     ) from e
 
 
+from google.rpc import (  # type: ignore[reportMissingModuleSource]
+    error_details_pb2,
+    status_pb2,
+)
+
 from a2a.client.client import ClientConfig
-from a2a.client.errors import A2AClientError, A2AClientTimeoutError
 from a2a.client.middleware import ClientCallInterceptor
 from a2a.client.optionals import Channel
 from a2a.client.transports.base import ClientTransport
@@ -44,6 +49,7 @@
     TaskPushNotificationConfig,
 )
 from a2a.utils.constants import PROTOCOL_VERSION_CURRENT, VERSION_HEADER
+from a2a.utils.errors import A2A_REASON_TO_ERROR
 from a2a.utils.telemetry import SpanKind, trace_class
 
 
@@ -54,14 +60,44 @@
 }
 
 
+def _parse_rich_grpc_error(
+    value: bytes, original_error: grpc.aio.AioRpcError
+) -> None:
+    try:
+        status = status_pb2.Status.FromString(value)
+        for detail in status.details:
+            if detail.Is(error_details_pb2.ErrorInfo.DESCRIPTOR):
+                error_info = error_details_pb2.ErrorInfo()
+                detail.Unpack(error_info)
+
+                if error_info.domain == 'a2a-protocol.org':
+                    exception_cls = A2A_REASON_TO_ERROR.get(error_info.reason)
+                    if exception_cls:
+                        raise exception_cls(status.message) from original_error  # noqa: TRY301
+    except Exception as parse_e:
+        # Don't swallow A2A errors generated above
+        if isinstance(parse_e, (A2AError, A2AClientError)):
+            raise parse_e
+        logger.warning(
+            'Failed to parse grpc-status-details-bin', exc_info=parse_e
+        )
+
+
 def _map_grpc_error(e: grpc.aio.AioRpcError) -> NoReturn:
     if e.code() == grpc.StatusCode.DEADLINE_EXCEEDED:
         raise A2AClientTimeoutError('Client Request timed out') from e
 
+    metadata = e.trailing_metadata()
+    if metadata:
+        iterable_metadata = cast('Iterable[tuple[str, str | bytes]]', metadata)
+        for key, value in iterable_metadata:
+            if key == 'grpc-status-details-bin' and isinstance(value, bytes):
+                _parse_rich_grpc_error(value, e)
+
     details = e.details()
     if isinstance(details, str) and ': ' in details:
         error_type_name, error_message = details.split(': ', 1)
-        # TODO(#723): Resolving imports by name is temporary until proper error handling structure is added in #723.
+        # Leaving as fallback for errors that don't use the rich error details.
         exception_cls = _A2A_ERROR_NAME_TO_CLS.get(error_type_name)
         if exception_cls:
             raise exception_cls(error_message) from e

src/a2a/server/request_handlers/grpc_handler.py

@@ -3,7 +3,8 @@
 import logging
 
 from abc import ABC, abstractmethod
-from collections.abc import AsyncIterable, Awaitable
+from collections.abc import AsyncIterable, Awaitable, Callable
+from typing import cast
 
 
 try:
@@ -16,9 +17,8 @@
         "'pip install a2a-sdk[grpc]'"
     ) from e
 
-from collections.abc import Callable
-
-from google.protobuf import empty_pb2, message
+from google.protobuf import any_pb2, empty_pb2, message
+from google.rpc import error_details_pb2, status_pb2
 
 import a2a.types.a2a_pb2_grpc as a2a_grpc
 
@@ -33,7 +33,7 @@
 from a2a.types import a2a_pb2
 from a2a.types.a2a_pb2 import AgentCard
 from a2a.utils import proto_utils
-from a2a.utils.errors import A2AError, TaskNotFoundError
+from a2a.utils.errors import A2A_ERROR_REASONS, A2AError, TaskNotFoundError
 from a2a.utils.helpers import maybe_await, validate, validate_async_generator
 
 
@@ -419,10 +419,41 @@ async def abort_context(
     ) -> None:
         """Sets the grpc errors appropriately in the context."""
         code = _ERROR_CODE_MAP.get(type(error))
+
+        status_code = (
+            code.value[0] if code else grpc.StatusCode.UNKNOWN.value[0]
+        )
+        error_msg = error.message if hasattr(error, 'message') else str(error)
+        status = status_pb2.Status(code=status_code, message=error_msg)
+
+        if code:
+            reason = A2A_ERROR_REASONS.get(type(error), 'UNKNOWN_ERROR')
+
+            error_info = error_details_pb2.ErrorInfo(
+                reason=reason,
+                domain='a2a-protocol.org',
+            )
+
+            detail = any_pb2.Any()
+            detail.Pack(error_info)
+            status.details.append(detail)
+
+        trailing_metadata = context.trailing_metadata() or ()
+
+        context.set_trailing_metadata(
+            cast(
+                'tuple[tuple[str, str | bytes], ...]',
+                (
+                    *trailing_metadata,
+                    ('grpc-status-details-bin', status.SerializeToString()),
+                ),
+            )
+        )
+
         if code:
             await context.abort(
                 code,
-                f'{type(error).__name__}: {error.message}',
+                status.message,
             )
         else:
             await context.abort(

src/a2a/utils/errors.py

@@ -82,11 +82,26 @@ class MethodNotFoundError(A2AError):
     message = 'Method not found'
 
 
+class ExtensionSupportRequiredError(A2AError):
+    """Exception raised when extension support is required but not present."""
+
+    message = 'Extension support required'
+
+
+class VersionNotSupportedError(A2AError):
+    """Exception raised when the requested version is not supported."""
+
+    message = 'Version not supported'
+
+
 # For backward compatibility if needed, or just aliases for clean refactor
 # We remove the Pydantic models here.
 
 __all__ = [
+    'A2A_ERROR_REASONS',
+    'A2A_REASON_TO_ERROR',
     'JSON_RPC_ERROR_CODE_MAP',
+    'ExtensionSupportRequiredError',
     'InternalError',
     'InvalidAgentResponseError',
     'InvalidParamsError',
@@ -96,6 +111,7 @@ class MethodNotFoundError(A2AError):
     'TaskNotCancelableError',
     'TaskNotFoundError',
     'UnsupportedOperationError',
+    'VersionNotSupportedError',
 ]
 
 
@@ -112,3 +128,22 @@ class MethodNotFoundError(A2AError):
     MethodNotFoundError: -32601,
     InternalError: -32603,
 }
+
+
+A2A_ERROR_REASONS = {
+    TaskNotFoundError: 'TASK_NOT_FOUND',
+    TaskNotCancelableError: 'TASK_NOT_CANCELABLE',
+    PushNotificationNotSupportedError: 'PUSH_NOTIFICATION_NOT_SUPPORTED',
+    UnsupportedOperationError: 'UNSUPPORTED_OPERATION',
+    ContentTypeNotSupportedError: 'CONTENT_TYPE_NOT_SUPPORTED',
+    InvalidAgentResponseError: 'INVALID_AGENT_RESPONSE',
+    AuthenticatedExtendedCardNotConfiguredError: 'EXTENDED_AGENT_CARD_NOT_CONFIGURED',
+    ExtensionSupportRequiredError: 'EXTENSION_SUPPORT_REQUIRED',
+    VersionNotSupportedError: 'VERSION_NOT_SUPPORTED',
+    InvalidParamsError: 'INVALID_PARAMS',
+    InvalidRequestError: 'INVALID_REQUEST',
+    MethodNotFoundError: 'METHOD_NOT_FOUND',
+    InternalError: 'INTERNAL_ERROR',
+}
+
+A2A_REASON_TO_ERROR = {reason: cls for cls, reason in A2A_ERROR_REASONS.items()}

tests/client/transports/test_grpc_client.py

@@ -3,10 +3,14 @@
 import grpc
 import pytest
 
+from google.protobuf import any_pb2
+from google.rpc import error_details_pb2, status_pb2
+
 from a2a.client.middleware import ClientCallContext
 from a2a.client.transports.grpc import GrpcTransport
 from a2a.extensions.common import HTTP_EXTENSION_HEADER
 from a2a.utils.constants import VERSION_HEADER, PROTOCOL_VERSION_CURRENT
+from a2a.utils.errors import A2A_ERROR_REASONS
 from a2a.types import a2a_pb2
 from a2a.types.a2a_pb2 import (
     AgentCapabilities,
@@ -257,16 +261,15 @@ async def test_send_message_with_timeout_context(
 
 @pytest.mark.parametrize('error_cls', list(JSON_RPC_ERROR_CODE_MAP.keys()))
 @pytest.mark.asyncio
-async def test_grpc_mapped_errors(
+async def test_grpc_mapped_errors_legacy(
     grpc_transport: GrpcTransport,
     mock_grpc_stub: AsyncMock,
     sample_message_send_params: SendMessageRequest,
     error_cls,
 ) -> None:
-    """Test handling of mapped gRPC error responses."""
+    """Test handling of legacy gRPC error responses."""
     error_details = f'{error_cls.__name__}: Mapped Error'
 
-    # We must trigger it from a standard transport method call, for example `send_message`.
     mock_grpc_stub.SendMessage.side_effect = grpc.aio.AioRpcError(
         code=grpc.StatusCode.INTERNAL,
         initial_metadata=grpc.aio.Metadata(),
@@ -278,6 +281,46 @@ async def test_grpc_mapped_errors(
         await grpc_transport.send_message(sample_message_send_params)
 
 
+@pytest.mark.parametrize('error_cls', list(JSON_RPC_ERROR_CODE_MAP.keys()))
+@pytest.mark.asyncio
+async def test_grpc_mapped_errors_rich(
+    grpc_transport: GrpcTransport,
+    mock_grpc_stub: AsyncMock,
+    sample_message_send_params: SendMessageRequest,
+    error_cls,
+) -> None:
+    """Test handling of rich gRPC error responses with Status metadata."""
+
+    reason = A2A_ERROR_REASONS.get(error_cls, 'UNKNOWN_ERROR')
+
+    error_info = error_details_pb2.ErrorInfo(
+        reason=reason,
+        domain='a2a-protocol.org',
+    )
+
+    error_details = f'{error_cls.__name__}: Mapped Error'
+    status = status_pb2.Status(
+        code=grpc.StatusCode.INTERNAL.value[0], message=error_details
+    )
+    detail = any_pb2.Any()
+    detail.Pack(error_info)
+    status.details.append(detail)
+
+    mock_grpc_stub.SendMessage.side_effect = grpc.aio.AioRpcError(
+        code=grpc.StatusCode.INTERNAL,
+        initial_metadata=grpc.aio.Metadata(),
+        trailing_metadata=grpc.aio.Metadata(
+            ('grpc-status-details-bin', status.SerializeToString()),
+        ),
+        details='A generic error message',
+    )
+
+    with pytest.raises(error_cls) as excinfo:
+        await grpc_transport.send_message(sample_message_send_params)
+
+    assert str(excinfo.value) == error_details
+
+
 @pytest.mark.asyncio
 async def test_send_message_message_response(
     grpc_transport: GrpcTransport,

tests/server/request_handlers/test_grpc_handler.py

@@ -5,6 +5,7 @@
 import grpc.aio
 import pytest
 
+from google.rpc import error_details_pb2, status_pb2
 from a2a import types
 from a2a.extensions.common import HTTP_EXTENSION_HEADER
 from a2a.server.context import ServerCallContext
@@ -99,7 +100,7 @@ async def test_send_message_server_error(
     await grpc_handler.SendMessage(request_proto, mock_grpc_context)
 
     mock_grpc_context.abort.assert_awaited_once_with(
-        grpc.StatusCode.INVALID_ARGUMENT, 'InvalidParamsError: Bad params'
+        grpc.StatusCode.INVALID_ARGUMENT, 'Bad params'
     )
 
 
@@ -138,7 +139,7 @@ async def test_get_task_not_found(
     await grpc_handler.GetTask(request_proto, mock_grpc_context)
 
     mock_grpc_context.abort.assert_awaited_once_with(
-        grpc.StatusCode.NOT_FOUND, 'TaskNotFoundError: Task not found'
+        grpc.StatusCode.NOT_FOUND, 'Task not found'
     )
 
 
@@ -157,7 +158,7 @@ async def test_cancel_task_server_error(
 
     mock_grpc_context.abort.assert_awaited_once_with(
         grpc.StatusCode.UNIMPLEMENTED,
-        'TaskNotCancelableError: Task cannot be canceled',
+        'Task cannot be canceled',
     )
 
 
@@ -379,7 +380,44 @@ async def test_abort_context_error_mapping(  # noqa: PLR0913
     mock_grpc_context.abort.assert_awaited_once()
     call_args, _ = mock_grpc_context.abort.call_args
     assert call_args[0] == grpc_status_code
-    assert error_message_part in call_args[1]
+
+    # We shouldn't rely on the legacy ExceptionName: message string format
+    # But for backward compatability fallback it shouldn't fail
+    mock_grpc_context.set_trailing_metadata.assert_called_once()
+    metadata = mock_grpc_context.set_trailing_metadata.call_args[0][0]
+
+    assert any(key == 'grpc-status-details-bin' for key, _ in metadata)
+
+
+@pytest.mark.asyncio
+async def test_abort_context_rich_error_format(
+    grpc_handler: GrpcHandler,
+    mock_request_handler: AsyncMock,
+    mock_grpc_context: AsyncMock,
+) -> None:
+
+    error = types.TaskNotFoundError('Could not find the task')
+    mock_request_handler.on_get_task.side_effect = error
+    request_proto = a2a_pb2.GetTaskRequest(id='any')
+    await grpc_handler.GetTask(request_proto, mock_grpc_context)
+
+    mock_grpc_context.set_trailing_metadata.assert_called_once()
+    metadata = mock_grpc_context.set_trailing_metadata.call_args[0][0]
+
+    bin_values = [v for k, v in metadata if k == 'grpc-status-details-bin']
+    assert len(bin_values) == 1
+
+    status = status_pb2.Status.FromString(bin_values[0])
+    assert status.code == grpc.StatusCode.NOT_FOUND.value[0]
+    assert status.message == 'Could not find the task'
+
+    assert len(status.details) == 1
+
+    error_info = error_details_pb2.ErrorInfo()
+    status.details[0].Unpack(error_info)
+
+    assert error_info.reason == 'TASK_NOT_FOUND'
+    assert error_info.domain == 'a2a-protocol.org'
 
 
 @pytest.mark.asyncio