feat: Add dynamic system prompt override functionality (strands-agents#108)

Author: Shubhamraut01

src/strands/agent/agent.py

@@ -457,27 +457,28 @@ def _execute_event_loop_cycle(self, callback_handler: Callable, kwargs: dict[str
         Returns:
             The result of the event loop cycle.
         """
-        kwargs.pop("agent", None)
-        kwargs.pop("model", None)
-        kwargs.pop("system_prompt", None)
-        kwargs.pop("tool_execution_handler", None)
-        kwargs.pop("event_loop_metrics", None)
-        kwargs.pop("callback_handler", None)
-        kwargs.pop("tool_handler", None)
-        kwargs.pop("messages", None)
-        kwargs.pop("tool_config", None)
+        # Extract parameters with fallbacks to instance values
+        system_prompt = kwargs.pop("system_prompt", self.system_prompt)
+        model = kwargs.pop("model", self.model)
+        tool_execution_handler = kwargs.pop("tool_execution_handler", self.thread_pool_wrapper)
+        event_loop_metrics = kwargs.pop("event_loop_metrics", self.event_loop_metrics)
+        callback_handler_override = kwargs.pop("callback_handler", callback_handler)
+        tool_handler = kwargs.pop("tool_handler", self.tool_handler)
+        messages = kwargs.pop("messages", self.messages)
+        tool_config = kwargs.pop("tool_config", self.tool_config)
+        kwargs.pop("agent", None)  # Remove agent to avoid conflicts
 
         try:
             # Execute the main event loop cycle
             stop_reason, message, metrics, state = event_loop_cycle(
-                model=self.model,
-                system_prompt=self.system_prompt,
-                messages=self.messages,  # will be modified by event_loop_cycle
-                tool_config=self.tool_config,
-                callback_handler=callback_handler,
-                tool_handler=self.tool_handler,
-                tool_execution_handler=self.thread_pool_wrapper,
-                event_loop_metrics=self.event_loop_metrics,
+                model=model,
+                system_prompt=system_prompt,
+                messages=messages,  # will be modified by event_loop_cycle
+                tool_config=tool_config,
+                callback_handler=callback_handler_override,
+                tool_handler=tool_handler,
+                tool_execution_handler=tool_execution_handler,
+                event_loop_metrics=event_loop_metrics,
                 agent=self,
                 event_loop_parent_span=self.trace_span,
                 **kwargs,
@@ -488,8 +489,8 @@ def _execute_event_loop_cycle(self, callback_handler: Callable, kwargs: dict[str
         except ContextWindowOverflowException as e:
             # Try reducing the context size and retrying
 
-            self.conversation_manager.reduce_context(self.messages, e=e)
-            return self._execute_event_loop_cycle(callback_handler, kwargs)
+            self.conversation_manager.reduce_context(messages, e=e)
+            return self._execute_event_loop_cycle(callback_handler_override, kwargs)
 
     def _record_tool_execution(
         self,

tests/strands/agent/test_agent.py

@@ -337,17 +337,47 @@ def test_agent__call__passes_kwargs(mock_model, system_prompt, callback_handler,
         ],
     ]
 
+    override_system_prompt = "Override system prompt"
+    override_model = unittest.mock.Mock()
+    override_tool_execution_handler = unittest.mock.Mock()
+    override_event_loop_metrics = unittest.mock.Mock()
+    override_callback_handler = unittest.mock.Mock()
+    override_tool_handler = unittest.mock.Mock()
+    override_messages = [{"role": "user", "content": [{"text": "override msg"}]}]
+    override_tool_config = {"test": "config"}
+
     def check_kwargs(some_value, **kwargs):
         assert some_value == "a_value"
         assert kwargs is not None
+        assert kwargs["system_prompt"] == override_system_prompt
+        assert kwargs["model"] == override_model
+        assert kwargs["tool_execution_handler"] == override_tool_execution_handler
+        assert kwargs["event_loop_metrics"] == override_event_loop_metrics
+        assert kwargs["callback_handler"] == override_callback_handler
+        assert kwargs["tool_handler"] == override_tool_handler
+        assert kwargs["messages"] == override_messages
+        assert kwargs["tool_config"] == override_tool_config
+        assert kwargs["agent"] == agent
 
         # Return expected values from event_loop_cycle
         return "stop", {"role": "assistant", "content": [{"text": "Response"}]}, {}, {}
 
     mock_event_loop_cycle.side_effect = check_kwargs
 
-    agent("test message", some_value="a_value")
-    assert mock_event_loop_cycle.call_count == 1
+    agent(
+        "test message",
+        some_value="a_value",
+        system_prompt=override_system_prompt,
+        model=override_model,
+        tool_execution_handler=override_tool_execution_handler,
+        event_loop_metrics=override_event_loop_metrics,
+        callback_handler=override_callback_handler,
+        tool_handler=override_tool_handler,
+        messages=override_messages,
+        tool_config=override_tool_config,
+    )
+
+    mock_event_loop_cycle.assert_called_once()
 
 
 def test_agent__call__retry_with_reduced_context(mock_model, agent, tool):