fix: do not crash on SSE comment line

src/a2a/client/transports/jsonrpc.py

@@ -176,6 +176,8 @@ async def send_message_streaming(
             try:
                 event_source.response.raise_for_status()
                 async for sse in event_source.aiter_sse():
+                    if not sse.data:
+                        continue
                     response = SendStreamingMessageResponse.model_validate(
                         json.loads(sse.data)
                     )

src/a2a/client/transports/rest.py

@@ -154,23 +154,25 @@
             try:
                 event_source.response.raise_for_status()
                 async for sse in event_source.aiter_sse():
+                    if not sse.data:
+                        continue
                     event = a2a_pb2.StreamResponse()
                     Parse(sse.data, event)
                     yield proto_utils.FromProto.stream_response(event)
             except httpx.HTTPStatusError as e:
                 raise A2AClientHTTPError(e.response.status_code, str(e)) from e
             except SSEError as e:
                 raise A2AClientHTTPError(
                     400, f'Invalid SSE response or protocol error: {e}'
                 ) from e
             except json.JSONDecodeError as e:
                 raise A2AClientJSONError(str(e)) from e
             except httpx.RequestError as e:
                 raise A2AClientHTTPError(
                     503, f'Network communication error: {e}'
                 ) from e
 
     async def _send_request(self, request: httpx.Request) -> dict[str, Any]:
         try:
             response = await self.httpx_client.send(request)
             response.raise_for_status()

tests/client/transports/test_jsonrpc_client.py

@@ -6,6 +6,7 @@
 
 import httpx
 import pytest
+import respx
 
 from httpx_sse import EventSource, SSEError, ServerSentEvent
 
@@ -466,6 +467,63 @@ async def test_send_message_streaming_success(
             == mock_stream_response_2.result.model_dump()
         )
 
+    # Repro of https://github.com/a2aproject/a2a-python/issues/540
+    @pytest.mark.asyncio
+    @respx.mock
+    async def test_send_message_streaming_comment_success(
+        self,
+        mock_agent_card: MagicMock,
+    ):
+        async with httpx.AsyncClient() as client:
+            transport = JsonRpcTransport(
+                httpx_client=client, agent_card=mock_agent_card
+            )
+            params = MessageSendParams(
+                message=create_text_message_object(content='Hello stream')
+            )
+            mock_stream_response_1 = SendMessageSuccessResponse(
+                id='stream_id_123',
+                jsonrpc='2.0',
+                result=create_text_message_object(
+                    content='First part', role=Role.agent
+                ),
+            )
+            mock_stream_response_2 = SendMessageSuccessResponse(
+                id='stream_id_123',
+                jsonrpc='2.0',
+                result=create_text_message_object(
+                    content='Second part', role=Role.agent
+                ),
+            )
+
+            sse_content = (
+                'id: stream_id_1\n'
+                f'data: {mock_stream_response_1.model_dump_json()}\n\n'
+                ': keep-alive\n\n'
+                'id: stream_id_2\n'
+                f'data: {mock_stream_response_2.model_dump_json()}\n\n'
+                ': keep-alive\n\n'
+            )
+
+            respx.post(mock_agent_card.url).mock(
+                return_value=httpx.Response(
+                    200,
+                    headers={'Content-Type': 'text/event-stream'},
+                    content=sse_content,
+                )
+            )
+
+            results = [
+                item
+                async for item in transport.send_message_streaming(
+                    request=params
+                )
+            ]
+
+            assert len(results) == 2
+            assert results[0] == mock_stream_response_1.result
+            assert results[1] == mock_stream_response_2.result
+
     @pytest.mark.asyncio
     async def test_send_request_http_status_error(
         self, mock_httpx_client: AsyncMock, mock_agent_card: MagicMock

tests/client/transports/test_rest_client.py

@@ -3,18 +3,23 @@
 
 import httpx
 import pytest
+import respx
 
+from google.protobuf.json_format import MessageToJson
 from httpx_sse import EventSource, ServerSentEvent
 
 from a2a.client import create_text_message_object
 from a2a.client.errors import A2AClientHTTPError
 from a2a.client.transports.rest import RestTransport
 from a2a.extensions.common import HTTP_EXTENSION_HEADER
+from a2a.grpc import a2a_pb2
 from a2a.types import (
     AgentCapabilities,
     AgentCard,
     MessageSendParams,
+    Role,
 )
+from a2a.utils import proto_utils
 
 
 @pytest.fixture
@@ -50,7 +55,7 @@ class TestRestTransportExtensions:
     async def test_send_message_with_default_extensions(
         self, mock_httpx_client: AsyncMock, mock_agent_card: MagicMock
     ):
-        """Test that send_message adds extensions to headers."""
+        """Test that SSE comments are ignored."""
         extensions = [
             'https://example.com/test-ext/v1',
             'https://example.com/test-ext/v2',
@@ -88,6 +93,64 @@ async def test_send_message_with_default_extensions(
             },
         )
 
+    # Repro of https://github.com/a2aproject/a2a-python/issues/540
+    @pytest.mark.asyncio
+    @respx.mock
+    async def test_send_message_streaming_comment_success(
+        self,
+        mock_agent_card: MagicMock,
+    ):
+        """Test successful streaming in RestTransport."""
+        async with httpx.AsyncClient() as client:
+            transport = RestTransport(
+                httpx_client=client, agent_card=mock_agent_card
+            )
+            params = MessageSendParams(
+                message=create_text_message_object(content='Hello stream')
+            )
+
+            mock_stream_response_1 = a2a_pb2.StreamResponse(
+                msg=proto_utils.ToProto.message(
+                    create_text_message_object(
+                        content='First part', role=Role.agent
+                    )
+                )
+            )
+            mock_stream_response_2 = a2a_pb2.StreamResponse(
+                msg=proto_utils.ToProto.message(
+                    create_text_message_object(
+                        content='Second part', role=Role.agent
+                    )
+                )
+            )
+
+            sse_content = (
+                'id: stream_id_1\n'
+                f'data: {MessageToJson(mock_stream_response_1, indent=None)}\n\n'
+                ': keep-alive\n\n'
+                'id: stream_id_2\n'
+                f'data: {MessageToJson(mock_stream_response_2, indent=None)}\n\n'
+                ': keep-alive\n\n'
+            )
+
+            respx.post(
+                f'{mock_agent_card.url.rstrip("/")}/v1/message:stream'
+            ).mock(
+                return_value=httpx.Response(
+                    200,
+                    headers={'Content-Type': 'text/event-stream'},
+                    content=sse_content,
+                )
+            )
+
+            results = []
+            async for item in transport.send_message_streaming(request=params):
+                results.append(item)
+
+            assert len(results) == 2
+            assert results[0].parts[0].root.text == 'First part'
+            assert results[1].parts[0].root.text == 'Second part'
+
     @pytest.mark.asyncio
     @patch('a2a.client.transports.rest.aconnect_sse')
     async def test_send_message_streaming_with_new_extensions(