Subscribe post.

Author: bartek-w

src/a2a/client/transports/rest.py

@@ -262,10 +262,11 @@ async def subscribe(
     ) -> AsyncGenerator[StreamResponse]:
         """Reconnects to get task updates."""
         async for event in self._send_stream_request(
-            'GET',
+            'POST',
             f'/tasks/{request.id}:subscribe',
             request.tenant,
             context=context,
+            json=MessageToDict(request),
         ):
             yield event
 

src/a2a/compat/v0_3/rest_adapter.py

@@ -163,6 +163,10 @@ def routes(self) -> dict[tuple[str, str], Callable[[Request], Any]]:
                 self._handle_streaming_request,
                 self.handler.on_subscribe_to_task,
             ),
+            ('/v1/tasks/{id}:subscribe', 'POST'): functools.partial(
+                self._handle_streaming_request,
+                self.handler.on_subscribe_to_task,
+            ),
             ('/v1/tasks/{id}', 'GET'): functools.partial(
                 self._handle_request, self.handler.on_get_task
             ),

src/a2a/compat/v0_3/rest_transport.py

@@ -68,6 +68,8 @@ def __init__(
         self.url = url.removesuffix('/')
         self.httpx_client = httpx_client
         self.agent_card = agent_card
+        self._subscribe_method = 'POST'
+        self._subscribe_retry_attempted = False
 
     async def send_message(
         self,
@@ -273,13 +275,39 @@ async def subscribe(
         *,
         context: ClientCallContext | None = None,
     ) -> AsyncGenerator[StreamResponse]:
-        """Reconnects to get task updates."""
-        async for event in self._send_stream_request(
-            'GET',
-            f'/v1/tasks/{request.id}:subscribe',
-            context=context,
-        ):
-            yield event
+        """Reconnects to get task updates.
+
+        This method implements backward compatibility logic for the subscribe
+        endpoint. It first attempts to use POST, which is the official method
+        for A2A subscribe endpoint. If the server returns 405 Method Not Allowed,
+        it falls back to GET and remembers this preference for future calls
+        on this transport instance. If both fail with 405, it will default back
+        to POST for next calls but will not retry again.
+        """
+        try:
+            async for event in self._send_stream_request(
+                self._subscribe_method,
+                f'/v1/tasks/{request.id}:subscribe',
+                context=context,
+            ):
+                yield event
+        except A2AClientError as e:
+            # Check for 405 Method Not Allowed in the cause (httpx.HTTPStatusError)
+            cause = e.__cause__
+            if (
+                isinstance(cause, httpx.HTTPStatusError)
+                and cause.response.status_code == httpx.codes.METHOD_NOT_ALLOWED
+            ):
+                if self._subscribe_retry_attempted:
+                    self._subscribe_method = 'POST'
+                    raise
+                else:
+                    self._subscribe_method = 'GET'
+                    self._subscribe_retry_attempted = True
+                    async for event in self.subscribe(request, context=context):
+                        yield event
+            else:
+                raise
 
     async def get_extended_agent_card(
         self,
@@ -311,7 +339,11 @@ async def close(self) -> None:
     def _handle_http_error(self, e: httpx.HTTPStatusError) -> NoReturn:
         """Handles HTTP status errors and raises the appropriate A2AError."""
         try:
-            error_data = e.response.json()
+            try:
+                error_data = e.response.json()
+            except (json.JSONDecodeError, ValueError, httpx.ResponseNotRead):
+                error_data = {}
+
             error_type = error_data.get('type')
             message = error_data.get('message', str(e))
 

src/a2a/server/apps/rest/rest_adapter.py

@@ -237,6 +237,10 @@ def routes(self) -> dict[tuple[str, str], Callable[[Request], Any]]:
                 self._handle_streaming_request,
                 self.handler.on_subscribe_to_task,
             ),
+            ('/tasks/{id}:subscribe', 'POST'): functools.partial(
+                self._handle_streaming_request,
+                self.handler.on_subscribe_to_task,
+            ),
             ('/tasks/{id}', 'GET'): functools.partial(
                 self._handle_request, self.handler.on_get_task
             ),

src/a2a/server/request_handlers/rest_handler.py

@@ -159,9 +159,15 @@ async def on_subscribe_to_task(
         Yields:
             JSON serialized objects containing streaming events
         """
-        task_id = request.path_params['id']
+        params = SubscribeToTaskRequest()
+        if request.method == 'POST':
+            body = await request.body()
+            if body:
+                Parse(body, params)
+
+        params.id = request.path_params['id']
         async for event in self.request_handler.on_subscribe_to_task(
-            SubscribeToTaskRequest(id=task_id), context
+            params, context
         ):
             yield MessageToDict(proto_utils.to_stream_response(event))
 

tests/client/transports/test_rest_client.py

@@ -716,8 +716,15 @@ async def empty_aiter():
         async for _ in method(request=request_obj):
             pass
 
-        # 4. Verify the URL
+        # 4. Verify the URL and method
         mock_aconnect_sse.assert_called_once()
-        args, _ = mock_aconnect_sse.call_args
+        args, kwargs = mock_aconnect_sse.call_args
+        # method is 2nd positional argument
+        if method_name == 'subscribe':
+            assert args[1] == 'POST'
+            assert kwargs.get('json') == json_format.MessageToDict(request_obj)
+        else:
+            assert args[1] == 'POST'
+
         # url is 3rd positional argument in aconnect_sse(client, method, url, ...)
         assert args[2] == f'http://agent.example.com/api{expected_path}'

tests/compat/v0_3/test_rest_handler.py

@@ -186,6 +186,44 @@ async def mock_stream(*args, **kwargs):
     ]
 
 
+@pytest.mark.anyio
+async def test_on_subscribe_to_task_post(
+    rest_handler, mock_request, mock_context
+):
+    mock_request.path_params = {'id': 'task-1'}
+    mock_request.method = 'POST'
+    request_body = {'name': 'tasks/task-1'}
+    mock_request.body = AsyncMock(
+        return_value=json.dumps(request_body).encode('utf-8')
+    )
+
+    async def mock_stream(*args, **kwargs):
+        yield types_v03.SendStreamingMessageSuccessResponse(
+            id='req-1',
+            result=types_v03.Message(
+                message_id='msg-2',
+                role='agent',
+                parts=[types_v03.TextPart(text='Update')],
+            ),
+        )
+
+    rest_handler.handler03.on_subscribe_to_task = MagicMock(
+        side_effect=mock_stream
+    )
+
+    results = [
+        chunk
+        async for chunk in rest_handler.on_subscribe_to_task(
+            mock_request, mock_context
+        )
+    ]
+
+    assert len(results) == 1
+    rest_handler.handler03.on_subscribe_to_task.assert_called_once()
+    called_req = rest_handler.handler03.on_subscribe_to_task.call_args[0][0]
+    assert called_req.params.id == 'task-1'
+
+
 @pytest.mark.anyio
 async def test_get_push_notification(rest_handler, mock_request, mock_context):
     mock_request.path_params = {'id': 'task-1', 'push_id': 'push-1'}

tests/compat/v0_3/test_rest_transport.py

@@ -1,4 +1,5 @@
 import json
+
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import httpx
@@ -232,14 +233,48 @@ async def mock_send_stream_request(*args, **kwargs):
     assert events[1] == StreamResponse(message=Message(message_id='msg-123'))
 
 
+def create_405_error():
+    mock_response = MagicMock(spec=httpx.Response)
+    mock_response.status_code = 405
+    mock_response.json.return_value = {
+        'type': 'MethodNotAllowed',
+        'message': 'Method Not Allowed',
+    }
+    mock_request = MagicMock(spec=httpx.Request)
+    mock_request.url = 'http://example.com/v1/tasks/task-123:subscribe'
+
+    status_error = httpx.HTTPStatusError(
+        '405 Method Not Allowed', request=mock_request, response=mock_response
+    )
+    raise A2AClientError('HTTP Error 405') from status_error
+
+
+def create_500_error():
+    mock_response = MagicMock(spec=httpx.Response)
+    mock_response.status_code = 500
+    mock_response.json.return_value = {
+        'type': 'InternalError',
+        'message': 'Internal Error',
+    }
+    mock_request = MagicMock(spec=httpx.Request)
+
+    status_error = httpx.HTTPStatusError(
+        '500 Internal Error', request=mock_request, response=mock_response
+    )
+    raise A2AClientError('HTTP Error 500') from status_error
+
+
 @pytest.mark.asyncio
-async def test_compat_rest_transport_subscribe(transport):
-    async def mock_send_stream_request(*args, **kwargs):
+async def test_compat_rest_transport_subscribe_post_works_no_retry(transport):
+    """Scenario: POST works, no retry."""
+
+    async def mock_stream(method, path, context=None):
+        assert method == 'POST'
         task = Task(id='task-123')
         task.status.message.role = Role.ROLE_AGENT
         yield StreamResponse(task=task)
 
-    transport._send_stream_request = mock_send_stream_request
+    transport._send_stream_request = mock_stream
 
     req = SubscribeToTaskRequest(id='task-123')
     events = [event async for event in transport.subscribe(req)]
@@ -248,6 +283,101 @@ async def mock_send_stream_request(*args, **kwargs):
     expected_task = Task(id='task-123')
     expected_task.status.message.role = Role.ROLE_AGENT
     assert events[0] == StreamResponse(task=expected_task)
+    assert transport._subscribe_method == 'POST'
+    assert transport._subscribe_retry_attempted is False
+
+
+@pytest.mark.asyncio
+async def test_compat_rest_transport_subscribe_post_405_retry_get_success(
+    transport,
+):
+    """Scenario: POST returns 405, automatic retry GET. Second call uses GET directly."""
+    call_count = 0
+
+    async def mock_stream(method, path, context=None):
+        nonlocal call_count
+        call_count += 1
+        if method == 'POST':
+            create_405_error()
+        if method == 'GET':
+            task = Task(id='task-123')
+            task.status.message.role = Role.ROLE_AGENT
+            yield StreamResponse(task=task)
+
+    transport._send_stream_request = mock_stream
+
+    req = SubscribeToTaskRequest(id='task-123')
+    events = [event async for event in transport.subscribe(req)]
+
+    assert len(events) == 1
+    assert call_count == 2
+    assert transport._subscribe_method == 'GET'
+    assert transport._subscribe_retry_attempted is True
+
+    # Second call should use GET directly
+    call_count = 0
+    events = [event async for event in transport.subscribe(req)]
+    assert len(events) == 1
+    assert call_count == 1  # Only GET called
+    assert transport._subscribe_method == 'GET'
+
+
+@pytest.mark.asyncio
+async def test_compat_rest_transport_subscribe_post_405_get_405_fails(
+    transport,
+):
+    """Scenario: POST return 405, retry GET, return 405 - error. Second call is just POST."""
+    call_count = 0
+
+    async def mock_stream(method, path, context=None):
+        nonlocal call_count
+        call_count += 1
+        # To make it an async generator even when it raises
+        if False:
+            yield
+        create_405_error()
+
+    transport._send_stream_request = mock_stream
+
+    req = SubscribeToTaskRequest(id='task-123')
+    with pytest.raises(A2AClientError) as exc_info:
+        [event async for event in transport.subscribe(req)]
+
+    assert '405' in str(exc_info.value)
+    assert call_count == 2  # Tried POST then GET
+    assert transport._subscribe_method == 'POST'
+    assert transport._subscribe_retry_attempted is True
+
+    # Second call should try POST directly and fail without retry
+    call_count = 0
+    with pytest.raises(A2AClientError):
+        [event async for event in transport.subscribe(req)]
+    assert call_count == 1
+    assert transport._subscribe_method == 'POST'
+
+
+@pytest.mark.asyncio
+async def test_compat_rest_transport_subscribe_post_500_no_retry(transport):
+    """Scenario: POST return 500, no automatic retry."""
+    call_count = 0
+
+    async def mock_stream(method, path, context=None):
+        nonlocal call_count
+        call_count += 1
+        if False:
+            yield
+        create_500_error()
+
+    transport._send_stream_request = mock_stream
+
+    req = SubscribeToTaskRequest(id='task-123')
+    with pytest.raises(A2AClientError) as exc_info:
+        [event async for event in transport.subscribe(req)]
+
+    assert '500' in str(exc_info.value)
+    assert call_count == 1  # No retry on 500
+    assert transport._subscribe_method == 'POST'
+    assert transport._subscribe_retry_attempted is False
 
 
 def test_compat_rest_transport_handle_http_error(transport):