Python: Default A2A raw parts without a media type to application/octet-stream

python/packages/a2a/agent_framework_a2a/_agent.py

@@ -1347,7 +1347,8 @@ def _parse_contents_from_a2a(
                     contents.append(
                         Content.from_data(
                             data=part.raw,
-                            media_type=part.media_type or "",
+                            # media_type is optional in A2A; "data:;base64,..." would not parse later.
+                            media_type=part.media_type or "application/octet-stream",
                             additional_properties=part_metadata,
                             raw_representation=part,
                         )

python/packages/a2a/tests/test_a2a_agent.py

@@ -787,6 +787,19 @@ def test_parse_contents_from_a2a_with_hosted_file_uri() -> None:
     assert contents[0].media_type == ""  # Converted None to empty string
 
 
+def test_parse_contents_from_a2a_raw_part_without_media_type_round_trips() -> None:
+    """A raw part without media_type becomes a valid data URI that can be sent back to an A2A agent."""
+
+    agent = A2AAgent(client=MagicMock(), http_client=None)
+
+    contents = agent._parse_contents_from_a2a([Part(raw=b"%PDF-1.7", filename="report.pdf")])  # noqa: SLF001
+
+    assert contents[0].type == "data"
+    assert contents[0].media_type == "application/octet-stream"
+    result = agent._prepare_message_for_a2a(Message(role="user", contents=contents))  # noqa: SLF001
+    assert result.parts[0].raw == b"%PDF-1.7"
+
+
 def test_auth_interceptor_parameter() -> None:
     """Test that auth_interceptor parameter is accepted without errors."""
     # Create a mock auth interceptor