diff --git a/src/a2a/client/client_factory.py b/src/a2a/client/client_factory.py
index e7dd4868..ff5387ef 100644
--- a/src/a2a/client/client_factory.py
+++ b/src/a2a/client/client_factory.py
@@ -3,7 +3,7 @@
 import logging
 
 from collections.abc import Callable
-from typing import Any
+from typing import Any, cast
 
 import httpx
 
@@ -20,6 +20,8 @@
     AgentInterface,
 )
 from a2a.utils.constants import (
+    PROTOCOL_VERSION_CURRENT,
+    VERSION_HEADER,
     TransportProtocol,
 )
 
@@ -65,6 +67,11 @@ def __init__(
     ):
         if consumers is None:
             consumers = []
+
+        client = config.httpx_client or httpx.AsyncClient()
+        client.headers.setdefault(VERSION_HEADER, PROTOCOL_VERSION_CURRENT)
+        config.httpx_client = client
+
         self._config = config
         self._consumers = consumers
         self._registry: dict[str, TransportProducer] = {}
@@ -72,11 +79,12 @@ def __init__(
 
     def _register_defaults(self, supported: list[str]) -> None:
         # Empty support list implies JSON-RPC only.
+
         if TransportProtocol.JSONRPC in supported or not supported:
             self.register(
                 TransportProtocol.JSONRPC,
                 lambda card, url, config, interceptors: JsonRpcTransport(
-                    config.httpx_client or httpx.AsyncClient(),
+                    cast('httpx.AsyncClient', config.httpx_client),
                     card,
                     url,
                     interceptors,
@@ -87,7 +95,7 @@ def _register_defaults(self, supported: list[str]) -> None:
             self.register(
                 TransportProtocol.HTTP_JSON,
                 lambda card, url, config, interceptors: RestTransport(
-                    config.httpx_client or httpx.AsyncClient(),
+                    cast('httpx.AsyncClient', config.httpx_client),
                     card,
                     url,
                     interceptors,
diff --git a/src/a2a/client/transports/grpc.py b/src/a2a/client/transports/grpc.py
index 3815d722..ffae90d8 100644
--- a/src/a2a/client/transports/grpc.py
+++ b/src/a2a/client/transports/grpc.py
@@ -43,6 +43,7 @@
     Task,
     TaskPushNotificationConfig,
 )
+from a2a.utils.constants import PROTOCOL_VERSION_CURRENT, VERSION_HEADER
 from a2a.utils.telemetry import SpanKind, trace_class
 
 
@@ -303,11 +304,14 @@ async def close(self) -> None:
     def _get_grpc_metadata(
         self,
         extensions: list[str] | None = None,
-    ) -> list[tuple[str, str]] | None:
+    ) -> list[tuple[str, str]]:
         """Creates gRPC metadata for extensions."""
+        metadata = [(VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT)]
+
         extensions_to_use = extensions or self.extensions
         if extensions_to_use:
-            return [
+            metadata.append(
                 (HTTP_EXTENSION_HEADER.lower(), ','.join(extensions_to_use))
-            ]
-        return None
+            )
+
+        return metadata
diff --git a/src/a2a/utils/constants.py b/src/a2a/utils/constants.py
index b90b390d..65d6598f 100644
--- a/src/a2a/utils/constants.py
+++ b/src/a2a/utils/constants.py
@@ -22,3 +22,7 @@ class TransportProtocol(str, Enum):
 
 DEFAULT_MAX_CONTENT_LENGTH = 10 * 1024 * 1024  # 10MB
 JSONRPC_PARSE_ERROR_CODE = -32700
+VERSION_HEADER = 'A2A-Version'
+
+PROTOCOL_VERSION_1_0 = '1.0'
+PROTOCOL_VERSION_CURRENT = PROTOCOL_VERSION_1_0
diff --git a/tests/client/transports/test_grpc_client.py b/tests/client/transports/test_grpc_client.py
index f6615d17..3fd45b6f 100644
--- a/tests/client/transports/test_grpc_client.py
+++ b/tests/client/transports/test_grpc_client.py
@@ -5,6 +5,7 @@
 
 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.types import a2a_pb2
 from a2a.types.a2a_pb2 import (
     AgentCapabilities,
@@ -217,10 +218,11 @@ async def test_send_message_task_response(
     mock_grpc_stub.SendMessage.assert_awaited_once()
     _, kwargs = mock_grpc_stub.SendMessage.call_args
     assert kwargs['metadata'] == [
+        (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
         (
             HTTP_EXTENSION_HEADER.lower(),
             'https://example.com/test-ext/v3',
-        )
+        ),
     ]
     assert response.HasField('task')
     assert response.task.id == sample_task.id
@@ -266,10 +268,11 @@ async def test_send_message_message_response(
     mock_grpc_stub.SendMessage.assert_awaited_once()
     _, kwargs = mock_grpc_stub.SendMessage.call_args
     assert kwargs['metadata'] == [
+        (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
         (
             HTTP_EXTENSION_HEADER.lower(),
             'https://example.com/test-ext/v1,https://example.com/test-ext/v2',
-        )
+        ),
     ]
     assert response.HasField('message')
     assert response.message.message_id == sample_message.message_id
@@ -315,10 +318,11 @@ async def test_send_message_streaming(  # noqa: PLR0913
     mock_grpc_stub.SendStreamingMessage.assert_called_once()
     _, kwargs = mock_grpc_stub.SendStreamingMessage.call_args
     assert kwargs['metadata'] == [
+        (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
         (
             HTTP_EXTENSION_HEADER.lower(),
             'https://example.com/test-ext/v1,https://example.com/test-ext/v2',
-        )
+        ),
     ]
     # Responses are StreamResponse proto objects
     assert responses[0].HasField('message')
@@ -350,10 +354,11 @@ async def test_get_task(
     mock_grpc_stub.GetTask.assert_awaited_once_with(
         a2a_pb2.GetTaskRequest(id=f'{sample_task.id}', history_length=None),
         metadata=[
+            (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
             (
                 HTTP_EXTENSION_HEADER.lower(),
                 'https://example.com/test-ext/v1,https://example.com/test-ext/v2',
-            )
+            ),
         ],
     )
     assert response.id == sample_task.id
@@ -378,10 +383,11 @@ async def test_list_tasks(
     mock_grpc_stub.ListTasks.assert_awaited_once_with(
         params,
         metadata=[
+            (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
             (
                 HTTP_EXTENSION_HEADER.lower(),
                 'https://example.com/test-ext/v1,https://example.com/test-ext/v2',
-            )
+            ),
         ],
     )
     assert result.total_size == 2
@@ -405,10 +411,11 @@ async def test_get_task_with_history(
             id=f'{sample_task.id}', history_length=history_len
         ),
         metadata=[
+            (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
             (
                 HTTP_EXTENSION_HEADER.lower(),
                 'https://example.com/test-ext/v1,https://example.com/test-ext/v2',
-            )
+            ),
         ],
     )
 
@@ -433,7 +440,8 @@ async def test_cancel_task(
     mock_grpc_stub.CancelTask.assert_awaited_once_with(
         a2a_pb2.CancelTaskRequest(id=f'{sample_task.id}'),
         metadata=[
-            (HTTP_EXTENSION_HEADER.lower(), 'https://example.com/test-ext/v3')
+            (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
+            (HTTP_EXTENSION_HEADER.lower(), 'https://example.com/test-ext/v3'),
         ],
     )
     assert response.status.state == TaskState.TASK_STATE_CANCELED
@@ -462,10 +470,11 @@ async def test_create_task_push_notification_config_with_valid_task(
     mock_grpc_stub.CreateTaskPushNotificationConfig.assert_awaited_once_with(
         request,
         metadata=[
+            (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
             (
                 HTTP_EXTENSION_HEADER.lower(),
                 'https://example.com/test-ext/v1,https://example.com/test-ext/v2',
-            )
+            ),
         ],
     )
     assert response.task_id == sample_task_push_notification_config.task_id
@@ -524,10 +533,11 @@ async def test_get_task_push_notification_config_with_valid_task(
             id=config_id,
         ),
         metadata=[
+            (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
             (
                 HTTP_EXTENSION_HEADER.lower(),
                 'https://example.com/test-ext/v1,https://example.com/test-ext/v2',
-            )
+            ),
         ],
     )
     assert response.task_id == sample_task_push_notification_config.task_id
@@ -577,10 +587,11 @@ async def test_list_task_push_notification_configs(
     mock_grpc_stub.ListTaskPushNotificationConfigs.assert_awaited_once_with(
         a2a_pb2.ListTaskPushNotificationConfigsRequest(task_id='task-1'),
         metadata=[
+            (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
             (
                 HTTP_EXTENSION_HEADER.lower(),
                 'https://example.com/test-ext/v1,https://example.com/test-ext/v2',
-            )
+            ),
         ],
     )
     assert len(response.configs) == 1
@@ -609,10 +620,11 @@ async def test_delete_task_push_notification_config(
             id='config-1',
         ),
         metadata=[
+            (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
             (
                 HTTP_EXTENSION_HEADER.lower(),
                 'https://example.com/test-ext/v1,https://example.com/test-ext/v2',
-            )
+            ),
         ],
     )
 
@@ -623,32 +635,47 @@ async def test_delete_task_push_notification_config(
         (
             None,
             None,
-            None,
+            [(VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT)],
         ),  # Case 1: No initial, No input
         (
             ['ext1'],
             None,
-            [(HTTP_EXTENSION_HEADER.lower(), 'ext1')],
+            [
+                (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
+                (HTTP_EXTENSION_HEADER.lower(), 'ext1'),
+            ],
         ),  # Case 2: Initial, No input
         (
             None,
             ['ext2'],
-            [(HTTP_EXTENSION_HEADER.lower(), 'ext2')],
+            [
+                (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
+                (HTTP_EXTENSION_HEADER.lower(), 'ext2'),
+            ],
         ),  # Case 3: No initial, Input
         (
             ['ext1'],
             ['ext2'],
-            [(HTTP_EXTENSION_HEADER.lower(), 'ext2')],
+            [
+                (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
+                (HTTP_EXTENSION_HEADER.lower(), 'ext2'),
+            ],
         ),  # Case 4: Initial, Input (override)
         (
             ['ext1'],
             ['ext2', 'ext3'],
-            [(HTTP_EXTENSION_HEADER.lower(), 'ext2,ext3')],
+            [
+                (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
+                (HTTP_EXTENSION_HEADER.lower(), 'ext2,ext3'),
+            ],
         ),  # Case 5: Initial, Multiple inputs (override)
         (
             ['ext1', 'ext2'],
             ['ext3'],
-            [(HTTP_EXTENSION_HEADER.lower(), 'ext3')],
+            [
+                (VERSION_HEADER.lower(), PROTOCOL_VERSION_CURRENT),
+                (HTTP_EXTENSION_HEADER.lower(), 'ext3'),
+            ],
         ),  # Case 6: Multiple initial, Single input (override)
     ],
 )
diff --git a/tests/utils/test_constants.py b/tests/utils/test_constants.py
index 4208268d..1c427b3f 100644
--- a/tests/utils/test_constants.py
+++ b/tests/utils/test_constants.py
@@ -13,3 +13,14 @@ def test_agent_card_constants():
 def test_default_rpc_url():
     """Test default RPC URL constant."""
     assert constants.DEFAULT_RPC_URL == '/'
+
+
+def test_version_header():
+    """Test version header constant."""
+    assert constants.VERSION_HEADER == 'A2A-Version'
+
+
+def test_protocol_versions():
+    """Test protocol version constants."""
+    assert constants.PROTOCOL_VERSION_1_0 == '1.0'
+    assert constants.PROTOCOL_VERSION_CURRENT == '1.0'