diff --git a/src/a2a/extensions/common.py b/src/a2a/extensions/common.py
index cba3517e..21c36fb6 100644
--- a/src/a2a/extensions/common.py
+++ b/src/a2a/extensions/common.py
@@ -3,7 +3,8 @@
 from a2a.types import AgentCard, AgentExtension
 
 
-HTTP_EXTENSION_HEADER = 'X-A2A-Extensions'
+HTTP_EXTENSION_HEADER = 'A2A-Extensions'
+HTTP_EXTENSION_HEADER_DEPRECATED = 'X-A2A-Extensions'
 
 
 def get_requested_extensions(values: list[str]) -> set[str]:
@@ -33,7 +34,7 @@ def update_extension_header(
     http_kwargs: dict[str, Any] | None,
     extensions: list[str] | None,
 ) -> dict[str, Any]:
-    """Update the X-A2A-Extensions header with active extensions."""
+    """Update the A2A-Extensions header with active extensions."""
     http_kwargs = http_kwargs or {}
     if extensions is not None:
         headers = http_kwargs.setdefault('headers', {})
diff --git a/src/a2a/server/apps/jsonrpc/jsonrpc_app.py b/src/a2a/server/apps/jsonrpc/jsonrpc_app.py
index 3e7c2854..4ad892d1 100644
--- a/src/a2a/server/apps/jsonrpc/jsonrpc_app.py
+++ b/src/a2a/server/apps/jsonrpc/jsonrpc_app.py
@@ -13,6 +13,7 @@
 from a2a.auth.user import User as A2AUser
 from a2a.extensions.common import (
     HTTP_EXTENSION_HEADER,
+    HTTP_EXTENSION_HEADER_DEPRECATED,
     get_requested_extensions,
 )
 from a2a.server.context import ServerCallContext
@@ -136,12 +137,13 @@ def build(self, request: Request) -> ServerCallContext:
             user = StarletteUserProxy(request.user)
             state['auth'] = request.auth
         state['headers'] = dict(request.headers)
+        extension_values = request.headers.getlist(
+            HTTP_EXTENSION_HEADER
+        ) or request.headers.getlist(HTTP_EXTENSION_HEADER_DEPRECATED)
         return ServerCallContext(
             user=user,
             state=state,
-            requested_extensions=get_requested_extensions(
-                request.headers.getlist(HTTP_EXTENSION_HEADER)
-            ),
+            requested_extensions=get_requested_extensions(extension_values),
         )
 
 
diff --git a/src/a2a/server/request_handlers/grpc_handler.py b/src/a2a/server/request_handlers/grpc_handler.py
index e2ec69a1..43cf23ab 100644
--- a/src/a2a/server/request_handlers/grpc_handler.py
+++ b/src/a2a/server/request_handlers/grpc_handler.py
@@ -26,6 +26,7 @@
 from a2a.auth.user import UnauthenticatedUser
 from a2a.extensions.common import (
     HTTP_EXTENSION_HEADER,
+    HTTP_EXTENSION_HEADER_DEPRECATED,
     get_requested_extensions,
 )
 from a2a.grpc import a2a_pb2
@@ -72,12 +73,13 @@ def build(self, context: grpc.aio.ServicerContext) -> ServerCallContext:
         state = {}
         with contextlib.suppress(Exception):
             state['grpc_context'] = context
+        extension_values = _get_metadata_value(
+            context, HTTP_EXTENSION_HEADER
+        ) or _get_metadata_value(context, HTTP_EXTENSION_HEADER_DEPRECATED)
         return ServerCallContext(
             user=user,
             state=state,
-            requested_extensions=get_requested_extensions(
-                _get_metadata_value(context, HTTP_EXTENSION_HEADER)
-            ),
+            requested_extensions=get_requested_extensions(extension_values),
         )
 
 
diff --git a/tests/client/transports/test_jsonrpc_client.py b/tests/client/transports/test_jsonrpc_client.py
index edbcd6c7..f05000c1 100644
--- a/tests/client/transports/test_jsonrpc_client.py
+++ b/tests/client/transports/test_jsonrpc_client.py
@@ -847,7 +847,7 @@ async def test_send_message_streaming_with_new_extensions(
         mock_httpx_client: AsyncMock,
         mock_agent_card: MagicMock,
     ):
-        """Test X-A2A-Extensions header in send_message_streaming."""
+        """Test A2A-Extensions header in send_message_streaming."""
         new_extensions = ['https://example.com/test-ext/v2']
         extensions = ['https://example.com/test-ext/v1']
         client = JsonRpcTransport(
diff --git a/tests/client/transports/test_rest_client.py b/tests/client/transports/test_rest_client.py
index cd68b443..d54c1145 100644
--- a/tests/client/transports/test_rest_client.py
+++ b/tests/client/transports/test_rest_client.py
@@ -96,7 +96,7 @@ async def test_send_message_streaming_with_new_extensions(
         mock_httpx_client: AsyncMock,
         mock_agent_card: MagicMock,
     ):
-        """Test X-A2A-Extensions header in send_message_streaming."""
+        """Test A2A-Extensions header in send_message_streaming."""
         new_extensions = ['https://example.com/test-ext/v2']
         extensions = ['https://example.com/test-ext/v1']
         client = RestTransport(
diff --git a/tests/integration/test_client_server_integration.py b/tests/integration/test_client_server_integration.py
index e0a564ee..a8407991 100644
--- a/tests/integration/test_client_server_integration.py
+++ b/tests/integration/test_client_server_integration.py
@@ -1,4 +1,5 @@
 import asyncio
+
 from collections.abc import AsyncGenerator
 from typing import NamedTuple
 from unittest.mock import ANY, AsyncMock, patch
@@ -7,6 +8,7 @@
 import httpx
 import pytest
 import pytest_asyncio
+
 from grpc.aio import Channel
 
 from a2a.client import ClientConfig
@@ -38,6 +40,7 @@
     TransportProtocol,
 )
 
+
 # --- Test Constants ---
 
 TASK_FROM_STREAM = Task(
@@ -819,9 +822,9 @@ async def test_base_client_sends_message_with_extensions(
         call_args, _ = mock_send_request.call_args
         kwargs = call_args[1]
         headers = kwargs.get('headers', {})
-        assert 'X-A2A-Extensions' in headers
+        assert 'A2A-Extensions' in headers
         assert (
-            headers['X-A2A-Extensions']
+            headers['A2A-Extensions']
             == 'https://example.com/test-ext/v1,https://example.com/test-ext/v2'
         )