Python: skip empty and whitespace-only instructions in prepend

python/packages/core/agent_framework/_types.py

@@ -2009,6 +2009,13 @@ def prepend_instructions_to_messages(
     if isinstance(instructions, str):
         instructions = [instructions]
 
+    # An empty instruction list (or all-empty strings) adds nothing; without
+    # this a caller that passes an unset options default of "" gets a
+    # contentless system message injected ahead of the real conversation.
+    instructions = [part for part in instructions if part.strip()]
+    if not instructions:
+        return messages
+
     # Skip instructions that are already present as the leading messages with the
     # same role and text.  This prevents duplicate system messages when
     # instructions are injected by multiple layers (e.g. Agent + chat client).

python/packages/core/tests/test_types.py

@@ -0,0 +1,21 @@
+
+
+class TestPrependInstructionsEmpty:
+    def test_empty_string_instructions_add_no_message(self) -> None:
+        """An unset "" instruction must not inject a contentless system message."""
+        from agent_framework import Message, prepend_instructions_to_messages
+
+        messages = [Message(role="user", contents=["hi"])]
+        out = prepend_instructions_to_messages(messages, "")
+        assert [(m.role, [getattr(c, "text", c) for c in m.contents]) for m in out] == [("user", ["hi"])]
+
+    def test_whitespace_only_instructions_add_no_message(self) -> None:
+        from agent_framework import prepend_instructions_to_messages
+
+        assert prepend_instructions_to_messages([], ["", "   "]) == []
+
+    def test_real_instructions_still_prepend_verbatim(self) -> None:
+        from agent_framework import Message, prepend_instructions_to_messages
+
+        out = prepend_instructions_to_messages([], ["  real  "])
+        assert [(m.role, [getattr(c, "text", c) for c in m.contents]) for m in out] == [("system", ["  real  "])]