fix(models): allow setter on system_prompt and system_prompt_content (#1171)

Author: dbschmigelski

src/strands/agent/agent.py

@@ -287,8 +287,8 @@ def __init__(
         """
         self.model = BedrockModel() if not model else BedrockModel(model_id=model) if isinstance(model, str) else model
         self.messages = messages if messages is not None else []
-        # initializing self.system_prompt for backwards compatibility
-        self.system_prompt, self._system_prompt_content = self._initialize_system_prompt(system_prompt)
+        # initializing self._system_prompt for backwards compatibility
+        self._system_prompt, self._system_prompt_content = self._initialize_system_prompt(system_prompt)
         self._default_structured_output_model = structured_output_model
         self.agent_id = _identifier.validate(agent_id or _DEFAULT_AGENT_ID, _identifier.Identifier.AGENT)
         self.name = name or _DEFAULT_AGENT_NAME
@@ -365,6 +365,35 @@ def __init__(
                 self.hooks.add_hook(hook)
         self.hooks.invoke_callbacks(AgentInitializedEvent(agent=self))
 
+    @property
+    def system_prompt(self) -> str | None:
+        """Get the system prompt as a string for backwards compatibility.
+
+        Returns the system prompt as a concatenated string when it contains text content,
+        or None if no text content is present. This maintains backwards compatibility
+        with existing code that expects system_prompt to be a string.
+
+        Returns:
+            The system prompt as a string, or None if no text content exists.
+        """
+        return self._system_prompt
+
+    @system_prompt.setter
+    def system_prompt(self, value: str | list[SystemContentBlock] | None) -> None:
+        """Set the system prompt and update internal content representation.
+
+        Accepts either a string or list of SystemContentBlock objects.
+        When set, both the backwards-compatible string representation and the internal
+        content block representation are updated to maintain consistency.
+
+        Args:
+            value: System prompt as string, list of SystemContentBlock objects, or None.
+                  - str: Simple text prompt (most common use case)
+                  - list[SystemContentBlock]: Content blocks with features like caching
+                  - None: Clear the system prompt
+        """
+        self._system_prompt, self._system_prompt_content = self._initialize_system_prompt(value)
+
     @property
     def tool(self) -> ToolCaller:
         """Call tool as a function.

tests/strands/agent/test_agent.py

@@ -1221,6 +1221,37 @@ async def test_stream_async_multi_modal_input(mock_model, agent, agenerator, ali
     assert tru_message == exp_message
 
 
+def test_system_prompt_setter_string():
+    """Test that setting system_prompt with string updates both internal fields."""
+    agent = Agent(system_prompt="initial prompt")
+
+    agent.system_prompt = "updated prompt"
+
+    assert agent.system_prompt == "updated prompt"
+    assert agent._system_prompt_content == [{"text": "updated prompt"}]
+
+
+def test_system_prompt_setter_list():
+    """Test that setting system_prompt with list updates both internal fields."""
+    agent = Agent()
+
+    content_blocks = [{"text": "You are helpful"}, {"cache_control": {"type": "ephemeral"}}]
+    agent.system_prompt = content_blocks
+
+    assert agent.system_prompt == "You are helpful"
+    assert agent._system_prompt_content == content_blocks
+
+
+def test_system_prompt_setter_none():
+    """Test that setting system_prompt to None clears both internal fields."""
+    agent = Agent(system_prompt="initial prompt")
+
+    agent.system_prompt = None
+
+    assert agent.system_prompt is None
+    assert agent._system_prompt_content is None
+
+
 @pytest.mark.asyncio
 async def test_stream_async_passes_invocation_state(agent, mock_model, mock_event_loop_cycle, agenerator, alist):
     mock_model.mock_stream.side_effect = [