diff --git a/src/google/adk/a2a/utils/agent_card_builder.py b/src/google/adk/a2a/utils/agent_card_builder.py
index 4c00b22a457..8d5131c7438 100644
--- a/src/google/adk/a2a/utils/agent_card_builder.py
+++ b/src/google/adk/a2a/utils/agent_card_builder.py
@@ -69,7 +69,7 @@ def __init__(
 
     self._agent = agent
     self._rpc_url = rpc_url or 'http://localhost:80/a2a'
-    self._capabilities = capabilities or AgentCapabilities()
+    self._capabilities = capabilities or AgentCapabilities(streaming=True)
     self._doc_url = doc_url
     self._provider = provider
     self._security_schemes = security_schemes
diff --git a/tests/unittests/a2a/utils/test_agent_card_builder.py b/tests/unittests/a2a/utils/test_agent_card_builder.py
index 76c107d3f15..aa4f436dc4b 100644
--- a/tests/unittests/a2a/utils/test_agent_card_builder.py
+++ b/tests/unittests/a2a/utils/test_agent_card_builder.py
@@ -65,6 +65,10 @@ def test_init_with_valid_agent(self):
     assert builder._agent == mock_agent
     assert builder._rpc_url == "http://localhost:80/a2a"
     assert isinstance(builder._capabilities, AgentCapabilities)
+    # The card is served alongside a DefaultRequestHandler that always
+    # implements message/stream, so the default capabilities must advertise
+    # streaming support rather than leaving peers to assume it is missing.
+    assert builder._capabilities.streaming is True
     assert builder._doc_url is None
     assert builder._provider is None
     assert builder._security_schemes is None