# 🔥🕊️ Rise of the Phoenix: Event Loop Refactor

src/strands/event_loop/event_loop.py

@@ -28,6 +28,10 @@
 
 logger = logging.getLogger(__name__)
 
+MAX_ATTEMPTS = 6
+INITIAL_DELAY = 4
+MAX_DELAY = 240  # 4 minutes
+
 
 def initialize_state(**kwargs: Any) -> Any:
     """Initialize the request state if not present.
@@ -51,7 +55,7 @@ def event_loop_cycle(
     system_prompt: Optional[str],
     messages: Messages,
     tool_config: Optional[ToolConfig],
-    callback_handler: Any,
+    callback_handler: Callable[..., Any],
     tool_handler: Optional[ToolHandler],
     tool_execution_handler: Optional[ParallelToolExecutorInterface] = None,
     **kwargs: Any,
@@ -130,13 +134,9 @@ def event_loop_cycle(
     stop_reason: StopReason
     usage: Any
     metrics: Metrics
-    max_attempts = 6
-    initial_delay = 4
-    max_delay = 240  # 4 minutes
-    current_delay = initial_delay
 
     # Retry loop for handling throttling exceptions
-    for attempt in range(max_attempts):
+    for attempt in range(MAX_ATTEMPTS):
         model_id = model.config.get("model_id") if hasattr(model, "config") else None
         model_invoke_span = tracer.start_model_invoke_span(
             parent_span=cycle_span,
@@ -177,7 +177,7 @@ def event_loop_cycle(
 
             # Handle throttling errors with exponential backoff
             should_retry, current_delay = handle_throttling_error(
-                e, attempt, max_attempts, current_delay, max_delay, callback_handler, kwargs
+                e, attempt, MAX_ATTEMPTS, INITIAL_DELAY, MAX_DELAY, callback_handler, kwargs
             )
             if should_retry:
                 continue
@@ -204,80 +204,35 @@ def event_loop_cycle(
 
         # If the model is requesting to use tools
         if stop_reason == "tool_use":
-            tool_uses: List[ToolUse] = []
-            tool_results: List[ToolResult] = []
-            invalid_tool_use_ids: List[str] = []
-
-            # Extract and validate tools
-            validate_and_prepare_tools(message, tool_uses, tool_results, invalid_tool_use_ids)
-
-            # Check if tools are available for execution
-            if tool_uses:
-                if tool_handler is None:
-                    raise ValueError("toolUse present but tool handler not set")
-                if tool_config is None:
-                    raise ValueError("toolUse present but tool config not set")
-
-                # Create the tool handler process callable
-                tool_handler_process: Callable[[ToolUse], ToolResult] = partial(
-                    tool_handler.process,
-                    messages=messages,
-                    model=model,
-                    system_prompt=system_prompt,
-                    tool_config=tool_config,
-                    callback_handler=callback_handler,
-                    **kwargs,
+            if not tool_handler:
+                raise EventLoopException(
+                    Exception("Model requested tool use but no tool handler provided"),
+                    kwargs["request_state"],
                 )
 
-                # Execute tools (parallel or sequential)
-                run_tools(
-                    handler=tool_handler_process,
-                    tool_uses=tool_uses,
-                    event_loop_metrics=event_loop_metrics,
-                    request_state=cast(Any, kwargs["request_state"]),
-                    invalid_tool_use_ids=invalid_tool_use_ids,
-                    tool_results=tool_results,
-                    cycle_trace=cycle_trace,
-                    parent_span=cycle_span,
-                    parallel_tool_executor=tool_execution_handler,
+            if tool_config is None:
+                raise EventLoopException(
+                    Exception("Model requested tool use but no tool config provided"),
+                    kwargs["request_state"],
                 )
 
-                # Update state for the next cycle
-                kwargs = prepare_next_cycle(kwargs, event_loop_metrics)
-
-                # Create the tool result message
-                tool_result_message: Message = {
-                    "role": "user",
-                    "content": [{"toolResult": result} for result in tool_results],
-                }
-                messages.append(tool_result_message)
-                callback_handler(message=tool_result_message)
-
-                if cycle_span:
-                    tracer.end_event_loop_cycle_span(
-                        span=cycle_span, message=message, tool_result_message=tool_result_message
-                    )
-
-                # Check if we should stop the event loop
-                if kwargs["request_state"].get("stop_event_loop"):
-                    event_loop_metrics.end_cycle(cycle_start_time, cycle_trace)
-                    return (
-                        stop_reason,
-                        message,
-                        event_loop_metrics,
-                        kwargs["request_state"],
-                    )
-
-                # Recursive call to continue the conversation
-                return recurse_event_loop(
-                    model=model,
-                    system_prompt=system_prompt,
-                    messages=messages,
-                    tool_config=tool_config,
-                    callback_handler=callback_handler,
-                    tool_handler=tool_handler,
-                    **kwargs,
-                )
+            # Handle tool execution
+            return _handle_tool_execution(
+                stop_reason,
+                message,
+                model,
+                system_prompt,
+                messages,
+                tool_config,
+                tool_handler,
+                callback_handler,
+                tool_execution_handler,
+                event_loop_metrics,
+                cycle_trace,
+                cycle_span,
+                cycle_start_time,
+                kwargs,
+            )
 
         # End the cycle and return results
         event_loop_metrics.end_cycle(cycle_start_time, cycle_trace)
@@ -377,3 +332,105 @@ def prepare_next_cycle(kwargs: Dict[str, Any], event_loop_metrics: EventLoopMetr
     kwargs["event_loop_parent_cycle_id"] = kwargs["event_loop_cycle_id"]
 
     return kwargs
+
+
+def _handle_tool_execution(
+    stop_reason: StopReason,
+    message: Message,
+    model: Model,
+    system_prompt: Optional[str],
+    messages: Messages,
+    tool_config: ToolConfig,
+    tool_handler: ToolHandler,
+    callback_handler: Callable[..., Any],
+    tool_execution_handler: Optional[ParallelToolExecutorInterface],
+    event_loop_metrics: EventLoopMetrics,
+    cycle_trace: Trace,
+    cycle_span: Any,
+    cycle_start_time: float,
+    kwargs: Dict[str, Any],
+) -> Tuple[StopReason, Message, EventLoopMetrics, Dict[str, Any]]:
+    tool_uses: List[ToolUse] = []
+    tool_results: List[ToolResult] = []
+    invalid_tool_use_ids: List[str] = []
+
+    """
+    Handles the execution of tools requested by the model during an event loop cycle.
+
+    Args:
+        stop_reason (StopReason): The reason the model stopped generating.
+        message (Message): The message from the model that may contain tool use requests.
+        model (Model): The model provider instance.
+        system_prompt (Optional[str]): The system prompt instructions for the model.
+        messages (Messages): The conversation history messages.
+        tool_config (ToolConfig): Configuration for available tools.
+        tool_handler (ToolHandler): Handler for tool execution.
+        callback_handler (Callable[..., Any]): Callback for processing events as they happen.
+        tool_execution_handler (Optional[ParallelToolExecutorInterface]): Optional handler for parallel tool execution.
+        event_loop_metrics (EventLoopMetrics): Metrics tracking object for the event loop.
+        cycle_trace (Trace): Trace object for the current event loop cycle.
+        cycle_span (Any): Span object for tracing the cycle (type may vary).
+        cycle_start_time (float): Start time of the current cycle.
+        kwargs (Dict[str, Any]): Additional keyword arguments, including request state.
+
+    Returns:
+        Tuple[StopReason, Message, EventLoopMetrics, Dict[str, Any]]: 
+            - The stop reason,
+            - The updated message,
+            - The updated event loop metrics,
+            - The updated request state.
+    """
+    validate_and_prepare_tools(message, tool_uses, tool_results, invalid_tool_use_ids)
+
+    if not tool_uses:
+        return stop_reason, message, event_loop_metrics, kwargs["request_state"]
+
+    tool_handler_process = partial(
+        tool_handler.process,
+        messages=messages,
+        model=model,
+        system_prompt=system_prompt,
+        tool_config=tool_config,
+        callback_handler=callback_handler,
+        **kwargs,
+    )
+
+    run_tools(
+        handler=tool_handler_process,
+        tool_uses=tool_uses,
+        event_loop_metrics=event_loop_metrics,
+        request_state=cast(Any, kwargs["request_state"]),
+        invalid_tool_use_ids=invalid_tool_use_ids,
+        tool_results=tool_results,
+        cycle_trace=cycle_trace,
+        parent_span=cycle_span,
+        parallel_tool_executor=tool_execution_handler,
+    )
+
+    kwargs = prepare_next_cycle(kwargs, event_loop_metrics)
+
+    tool_result_message: Message = {
+        "role": "user",
+        "content": [{"toolResult": result} for result in tool_results],
+    }
+
+    messages.append(tool_result_message)
+    callback_handler(message=tool_result_message)
+
+    if cycle_span:
+        tracer = get_tracer()
+        tracer.end_event_loop_cycle_span(span=cycle_span, message=message, tool_result_message=tool_result_message)
+
+    if kwargs["request_state"].get("stop_event_loop", False):
+        event_loop_metrics.end_cycle(cycle_start_time, cycle_trace)
+        return stop_reason, message, event_loop_metrics, kwargs["request_state"]
+
+    return recurse_event_loop(
+        model=model,
+        system_prompt=system_prompt,
+        messages=messages,
+        tool_config=tool_config,
+        callback_handler=callback_handler,
+        tool_handler=tool_handler,
+        **kwargs,
+    )