fix(a2a): stamp A2A-Version on the rebound client with a2a-sdk 1.3

src/google/adk/a2a/_compat.py

@@ -923,18 +923,19 @@ def rebind_client_factory_httpx(factory: Any, httpx_client: Any) -> Any:
 
   new_factory = copy.copy(factory)
   new_factory._config = config
+  # a2a-sdk 1.3 dropped ``ClientFactory._httpx_client``; it now reads the
+  # client from ``_config`` when it builds a transport, so the version header
+  # has to be stamped on the rebound client whether that attribute exists.
   if hasattr(new_factory, "_httpx_client"):
     new_factory._httpx_client = httpx_client
-    if hasattr(httpx_client, "headers"):
-      try:
-        from a2a.utils.constants import PROTOCOL_VERSION_CURRENT
-        from a2a.utils.constants import VERSION_HEADER
+  if IS_A2A_V1 and hasattr(httpx_client, "headers"):
+    try:
+      from a2a.utils.constants import PROTOCOL_VERSION_CURRENT
+      from a2a.utils.constants import VERSION_HEADER
 
-        httpx_client.headers.setdefault(
-            VERSION_HEADER, PROTOCOL_VERSION_CURRENT
-        )
-      except ImportError:
-        pass
+      httpx_client.headers.setdefault(VERSION_HEADER, PROTOCOL_VERSION_CURRENT)
+    except ImportError:
+      pass
   if hasattr(new_factory, "_registry") and new_factory._registry is not None:
     new_factory._registry = copy.copy(factory._registry)
   if hasattr(new_factory, "_register_defaults"):

tests/unittests/a2a/agent/test_remote_a2a_agent.py

@@ -640,7 +640,7 @@ def create(self, *args, **kwargs):
       )
       if _compat.IS_A2A_V1:
         assert (
-            agent._a2a_client_factory._httpx_client.headers[VERSION_HEADER]
+            agent._httpx_client.headers[VERSION_HEADER]
             == PROTOCOL_VERSION_CURRENT
         )
       assert factory._config.httpx_client is None

tests/unittests/a2a/test_compat.py

@@ -359,6 +359,24 @@ async def test_rebind_client_factory_httpx_defaults_protocol_bindings():
     await new_client.aclose()
 
 
+@pytest.mark.asyncio
+async def test_rebind_client_factory_httpx_sets_version_header_on_new_client():
+  if not _compat.IS_A2A_V1:
+    pytest.skip('the A2A-Version header is 1.x only')
+  from a2a.utils.constants import PROTOCOL_VERSION_CURRENT
+  from a2a.utils.constants import VERSION_HEADER
+
+  new_client = httpx.AsyncClient()
+  factory = ClientFactory(ClientConfig(httpx_client=None))
+  rebound = _compat.rebind_client_factory_httpx(factory, new_client)
+
+  try:
+    assert rebound._config.httpx_client is new_client
+    assert new_client.headers[VERSION_HEADER] == PROTOCOL_VERSION_CURRENT
+  finally:
+    await new_client.aclose()
+
+
 @pytest.mark.asyncio
 async def test_rebind_client_factory_httpx_preserves_protocol_bindings():
   old_client, new_client = httpx.AsyncClient(), httpx.AsyncClient()