feat: Implement typed events internally (#745)

Step 1/N for implementing typed-events; first just preserve the existing behaviors with no changes to the public api.

A follow-up change will update how we invoke callbacks and pass invocation_state around, while this one just adds typed classes for events internally.

---------

Co-authored-by: Mackenzie Zastrow <zastrowm@users.noreply.github.com>

Author: zastrowm

src/strands/agent/agent.py

@@ -50,6 +50,7 @@
 from ..tools.executors._executor import ToolExecutor
 from ..tools.registry import ToolRegistry
 from ..tools.watcher import ToolWatcher
+from ..types._events import InitEventLoopEvent
 from ..types.agent import AgentInput
 from ..types.content import ContentBlock, Message, Messages
 from ..types.exceptions import ContextWindowOverflowException
@@ -604,7 +605,7 @@ async def _run_loop(
         self.hooks.invoke_callbacks(BeforeInvocationEvent(agent=self))
 
         try:
-            yield {"callback": {"init_event_loop": True, **invocation_state}}
+            yield InitEventLoopEvent(invocation_state)
 
             for message in messages:
                 self._append_message(message)

src/strands/event_loop/event_loop.py

@@ -25,6 +25,15 @@
 from ..telemetry.metrics import Trace
 from ..telemetry.tracer import get_tracer
 from ..tools._validator import validate_and_prepare_tools
+from ..types._events import (
+    EventLoopStopEvent,
+    EventLoopThrottleEvent,
+    ForceStopEvent,
+    ModelMessageEvent,
+    StartEvent,
+    StartEventLoopEvent,
+    ToolResultMessageEvent,
+)
 from ..types.content import Message
 from ..types.exceptions import (
     ContextWindowOverflowException,
@@ -91,8 +100,8 @@ async def event_loop_cycle(agent: "Agent", invocation_state: dict[str, Any]) ->
     cycle_start_time, cycle_trace = agent.event_loop_metrics.start_cycle(attributes=attributes)
     invocation_state["event_loop_cycle_trace"] = cycle_trace
 
-    yield {"callback": {"start": True}}
-    yield {"callback": {"start_event_loop": True}}
+    yield StartEvent()
+    yield StartEventLoopEvent()
 
     # Create tracer span for this event loop cycle
     tracer = get_tracer()
@@ -175,7 +184,7 @@ async def event_loop_cycle(agent: "Agent", invocation_state: dict[str, Any]) ->
 
                 if isinstance(e, ModelThrottledException):
                     if attempt + 1 == MAX_ATTEMPTS:
-                        yield {"callback": {"force_stop": True, "force_stop_reason": str(e)}}
+                        yield ForceStopEvent(reason=e)
                         raise e
 
                     logger.debug(
@@ -189,7 +198,7 @@ async def event_loop_cycle(agent: "Agent", invocation_state: dict[str, Any]) ->
                     time.sleep(current_delay)
                     current_delay = min(current_delay * 2, MAX_DELAY)
 
-                    yield {"callback": {"event_loop_throttled_delay": current_delay, **invocation_state}}
+                    yield EventLoopThrottleEvent(delay=current_delay, invocation_state=invocation_state)
                 else:
                     raise e
 
@@ -201,7 +210,7 @@ async def event_loop_cycle(agent: "Agent", invocation_state: dict[str, Any]) ->
         # Add the response message to the conversation
         agent.messages.append(message)
         agent.hooks.invoke_callbacks(MessageAddedEvent(agent=agent, message=message))
-        yield {"callback": {"message": message}}
+        yield ModelMessageEvent(message=message)
 
         # Update metrics
         agent.event_loop_metrics.update_usage(usage)
@@ -235,8 +244,8 @@ async def event_loop_cycle(agent: "Agent", invocation_state: dict[str, Any]) ->
                 cycle_start_time=cycle_start_time,
                 invocation_state=invocation_state,
             )
-            async for event in events:
-                yield event
+            async for typed_event in events:
+                yield typed_event
 
             return
 
@@ -264,11 +273,11 @@ async def event_loop_cycle(agent: "Agent", invocation_state: dict[str, Any]) ->
             tracer.end_span_with_error(cycle_span, str(e), e)
 
         # Handle any other exceptions
-        yield {"callback": {"force_stop": True, "force_stop_reason": str(e)}}
+        yield ForceStopEvent(reason=e)
         logger.exception("cycle failed")
         raise EventLoopException(e, invocation_state["request_state"]) from e
 
-    yield {"stop": (stop_reason, message, agent.event_loop_metrics, invocation_state["request_state"])}
+    yield EventLoopStopEvent(stop_reason, message, agent.event_loop_metrics, invocation_state["request_state"])
 
 
 async def recurse_event_loop(agent: "Agent", invocation_state: dict[str, Any]) -> AsyncGenerator[dict[str, Any], None]:
@@ -295,7 +304,7 @@ async def recurse_event_loop(agent: "Agent", invocation_state: dict[str, Any]) -
     recursive_trace = Trace("Recursive call", parent_id=cycle_trace.id)
     cycle_trace.add_child(recursive_trace)
 
-    yield {"callback": {"start": True}}
+    yield StartEvent()
 
     events = event_loop_cycle(agent=agent, invocation_state=invocation_state)
     async for event in events:
@@ -339,7 +348,7 @@ async def _handle_tool_execution(
     validate_and_prepare_tools(message, tool_uses, tool_results, invalid_tool_use_ids)
     tool_uses = [tool_use for tool_use in tool_uses if tool_use.get("toolUseId") not in invalid_tool_use_ids]
     if not tool_uses:
-        yield {"stop": (stop_reason, message, agent.event_loop_metrics, invocation_state["request_state"])}
+        yield EventLoopStopEvent(stop_reason, message, agent.event_loop_metrics, invocation_state["request_state"])
         return
 
     tool_events = agent.tool_executor._execute(
@@ -358,15 +367,15 @@ async def _handle_tool_execution(
 
     agent.messages.append(tool_result_message)
     agent.hooks.invoke_callbacks(MessageAddedEvent(agent=agent, message=tool_result_message))
-    yield {"callback": {"message": tool_result_message}}
+    yield ToolResultMessageEvent(message=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 invocation_state["request_state"].get("stop_event_loop", False):
         agent.event_loop_metrics.end_cycle(cycle_start_time, cycle_trace)
-        yield {"stop": (stop_reason, message, agent.event_loop_metrics, invocation_state["request_state"])}
+        yield EventLoopStopEvent(stop_reason, message, agent.event_loop_metrics, invocation_state["request_state"])
         return
 
     events = recurse_event_loop(agent=agent, invocation_state=invocation_state)

src/strands/event_loop/streaming.py

@@ -5,6 +5,16 @@
 from typing import Any, AsyncGenerator, AsyncIterable, Optional
 
 from ..models.model import Model
+from ..types._events import (
+    ModelStopReason,
+    ModelStreamChunkEvent,
+    ModelStreamEvent,
+    ReasoningSignatureStreamEvent,
+    ReasoningTextStreamEvent,
+    TextStreamEvent,
+    ToolUseStreamEvent,
+    TypedEvent,
+)
 from ..types.content import ContentBlock, Message, Messages
 from ..types.streaming import (
     ContentBlockDeltaEvent,
@@ -105,7 +115,7 @@ def handle_content_block_start(event: ContentBlockStartEvent) -> dict[str, Any]:
 
 def handle_content_block_delta(
     event: ContentBlockDeltaEvent, state: dict[str, Any]
-) -> tuple[dict[str, Any], dict[str, Any]]:
+) -> tuple[dict[str, Any], ModelStreamEvent]:
     """Handles content block delta updates by appending text, tool input, or reasoning content to the state.
 
     Args:
@@ -117,43 +127,41 @@ def handle_content_block_delta(
     """
     delta_content = event["delta"]
 
-    callback_event = {}
+    typed_event: ModelStreamEvent = ModelStreamEvent({})
 
     if "toolUse" in delta_content:
         if "input" not in state["current_tool_use"]:
             state["current_tool_use"]["input"] = ""
 
         state["current_tool_use"]["input"] += delta_content["toolUse"]["input"]
-        callback_event["callback"] = {"delta": delta_content, "current_tool_use": state["current_tool_use"]}
+        typed_event = ToolUseStreamEvent(delta_content, state["current_tool_use"])
 
     elif "text" in delta_content:
         state["text"] += delta_content["text"]
-        callback_event["callback"] = {"data": delta_content["text"], "delta": delta_content}
+        typed_event = TextStreamEvent(text=delta_content["text"], delta=delta_content)
 
     elif "reasoningContent" in delta_content:
         if "text" in delta_content["reasoningContent"]:
             if "reasoningText" not in state:
                 state["reasoningText"] = ""
 
             state["reasoningText"] += delta_content["reasoningContent"]["text"]
-            callback_event["callback"] = {
-                "reasoningText": delta_content["reasoningContent"]["text"],
-                "delta": delta_content,
-                "reasoning": True,
-            }
+            typed_event = ReasoningTextStreamEvent(
+                reasoning_text=delta_content["reasoningContent"]["text"],
+                delta=delta_content,
+            )
 
         elif "signature" in delta_content["reasoningContent"]:
             if "signature" not in state:
                 state["signature"] = ""
 
             state["signature"] += delta_content["reasoningContent"]["signature"]
-            callback_event["callback"] = {
-                "reasoning_signature": delta_content["reasoningContent"]["signature"],
-                "delta": delta_content,
-                "reasoning": True,
-            }
+            typed_event = ReasoningSignatureStreamEvent(
+                reasoning_signature=delta_content["reasoningContent"]["signature"],
+                delta=delta_content,
+            )
 
-    return state, callback_event
+    return state, typed_event
 
 
 def handle_content_block_stop(state: dict[str, Any]) -> dict[str, Any]:
@@ -251,7 +259,7 @@ def extract_usage_metrics(event: MetadataEvent) -> tuple[Usage, Metrics]:
     return usage, metrics
 
 
-async def process_stream(chunks: AsyncIterable[StreamEvent]) -> AsyncGenerator[dict[str, Any], None]:
+async def process_stream(chunks: AsyncIterable[StreamEvent]) -> AsyncGenerator[TypedEvent, None]:
     """Processes the response stream from the API, constructing the final message and extracting usage metrics.
 
     Args:
@@ -274,14 +282,14 @@ async def process_stream(chunks: AsyncIterable[StreamEvent]) -> AsyncGenerator[d
     metrics: Metrics = Metrics(latencyMs=0)
 
     async for chunk in chunks:
-        yield {"callback": {"event": chunk}}
+        yield ModelStreamChunkEvent(chunk=chunk)
         if "messageStart" in chunk:
             state["message"] = handle_message_start(chunk["messageStart"], state["message"])
         elif "contentBlockStart" in chunk:
             state["current_tool_use"] = handle_content_block_start(chunk["contentBlockStart"])
         elif "contentBlockDelta" in chunk:
-            state, callback_event = handle_content_block_delta(chunk["contentBlockDelta"], state)
-            yield callback_event
+            state, typed_event = handle_content_block_delta(chunk["contentBlockDelta"], state)
+            yield typed_event
         elif "contentBlockStop" in chunk:
             state = handle_content_block_stop(state)
         elif "messageStop" in chunk:
@@ -291,15 +299,15 @@ async def process_stream(chunks: AsyncIterable[StreamEvent]) -> AsyncGenerator[d
         elif "redactContent" in chunk:
             handle_redact_content(chunk["redactContent"], state)
 
-    yield {"stop": (stop_reason, state["message"], usage, metrics)}
+    yield ModelStopReason(stop_reason=stop_reason, message=state["message"], usage=usage, metrics=metrics)
 
 
 async def stream_messages(
     model: Model,
     system_prompt: Optional[str],
     messages: Messages,
     tool_specs: list[ToolSpec],
-) -> AsyncGenerator[dict[str, Any], None]:
+) -> AsyncGenerator[TypedEvent, None]:
     """Streams messages to the model and processes the response.
 
     Args: