fix: add SSE parameter and content alias for v0.3 compat streaming

src/a2a/compat/v0_3/rest_transport.py

@@ -139,7 +139,7 @@ async def send_message_streaming(
 
         async for event in self._send_stream_request(
             'POST',
-            '/v1/message:stream',
+            '/v1/message:stream?alt=sse',
             context=context,
             json=MessageToDict(req_proto, preserving_proto_field_name=True),
         ):
@@ -291,7 +291,7 @@ async def subscribe(
         try:
             async for event in self._send_stream_request(
                 subscribe_method,
-                f'/v1/tasks/{request.id}:subscribe',
+                f'/v1/tasks/{request.id}:subscribe?alt=sse',
                 context=context,
             ):
                 yield event

src/a2a/compat/v0_3/types.py

@@ -6,7 +6,7 @@
 from enum import Enum
 from typing import Any, Literal
 
-from pydantic import Field, RootModel
+from pydantic import AliasChoices, Field, RootModel
 
 from a2a._base import A2ABaseModel
 
@@ -1458,7 +1458,7 @@ class Message(A2ABaseModel):
     """
     Optional metadata for extensions. The key is an extension-specific identifier.
     """
-    parts: list[Part]
+    parts: list[Part] = Field(validation_alias=AliasChoices('parts', 'content'))
     """
     An array of content parts that form the message body. A message can be
     composed of multiple parts of different types (e.g., text and files).

tests/compat/v0_3/test_conversions.py

@@ -203,6 +203,27 @@ def test_message_conversion():
     assert v03_restored == v03_msg
 
 
+def test_message_content_alias():
+    """Test that Message accepts 'content' as alias for 'parts' (v0.3 compatibility)."""
+    # Test with 'content' key (legacy v0.3 wire format)
+    legacy_json = {
+        'messageId': 'msg-123',
+        'role': 'user',
+        'content': [{'kind': 'text', 'text': 'hello'}],
+    }
+    msg = types_v03.Message.model_validate(legacy_json)
+    assert msg.parts == [types_v03.Part(root=types_v03.TextPart(text='hello'))]
+
+    # Test with 'parts' key (modern format)
+    modern_json = {
+        'messageId': 'msg-456',
+        'role': 'agent',
+        'parts': [{'kind': 'text', 'text': 'response'}],
+    }
+    msg2 = types_v03.Message.model_validate(modern_json)
+    assert msg2.parts == [types_v03.Part(root=types_v03.TextPart(text='response'))]
+
+
 def test_message_conversion_minimal():
     v03_msg = types_v03.Message(
         message_id='m1',

tests/compat/v0_3/test_rest_transport.py

@@ -233,6 +233,35 @@ async def mock_send_stream_request(*args, **kwargs):
     assert events[1] == StreamResponse(message=Message(message_id='msg-123'))
 
 
+@pytest.mark.asyncio
+async def test_compat_rest_transport_stream_url_has_alt_sse_param(transport):
+    """Test that streaming endpoints include ?alt=sse parameter."""
+    captured_paths = []
+
+    async def mock_send_stream_request(method, path, context=None, json=None):
+        captured_paths.append(path)
+        task = Task(id='task-123')
+        task.status.message.role = Role.ROLE_AGENT
+        yield StreamResponse(task=task)
+
+    transport._send_stream_request = mock_send_stream_request
+
+    # Test send_message_streaming
+    req = SendMessageRequest(
+        message=Message(message_id='msg-1', role=Role.ROLE_USER)
+    )
+    _ = [event async for event in transport.send_message_streaming(req)]
+
+    assert captured_paths[-1] == '/v1/message:stream?alt=sse'
+
+    # Test subscribe
+    captured_paths.clear()
+    sub_req = SubscribeToTaskRequest(id='task-123')
+    _ = [event async for event in transport.subscribe(sub_req)]
+
+    assert captured_paths[-1] == '/v1/tasks/task-123:subscribe?alt=sse'
+
+
 def create_405_error():
     mock_response = MagicMock(spec=httpx.Response)
     mock_response.status_code = 405