feat(hooks): add BeforeReduceContextEvent and AfterReduceContextEvent

Fires BeforeReduceContextEvent before reduce_context() and
AfterReduceContextEvent after it returns, giving observability plugins
a way to detect and measure context compaction without polling or
inspecting internals.

Closes #2048

Author: Zelys-DFKH

src/strands/agent/agent.py

@@ -35,8 +35,10 @@
 from ..handlers.callback_handler import PrintingCallbackHandler, null_callback_handler
 from ..hooks import (
     AfterInvocationEvent,
+    AfterReduceContextEvent,
     AgentInitializedEvent,
     BeforeInvocationEvent,
+    BeforeReduceContextEvent,
     HookCallback,
     HookProvider,
     HookRegistry,
@@ -963,9 +965,13 @@ async def _execute_event_loop_cycle(
                 yield event
 
         except ContextWindowOverflowException as e:
+            await self.hooks.invoke_callbacks_async(BeforeReduceContextEvent(agent=self, exception=e))
+
             # Try reducing the context size and retrying
             self.conversation_manager.reduce_context(self, e=e)
 
+            await self.hooks.invoke_callbacks_async(AfterReduceContextEvent(agent=self))
+
             # Sync agent after reduce_context to keep conversation_manager_state up to date in the session
             if self._session_manager:
                 self._session_manager.sync_agent(self)

src/strands/hooks/__init__.py

@@ -35,12 +35,14 @@ def log_end(self, event: AfterInvocationEvent) -> None:
     # Multiagent hook events
     AfterMultiAgentInvocationEvent,
     AfterNodeCallEvent,
+    AfterReduceContextEvent,
     AfterToolCallEvent,
     AgentInitializedEvent,
     BeforeInvocationEvent,
     BeforeModelCallEvent,
     BeforeMultiAgentInvocationEvent,
     BeforeNodeCallEvent,
+    BeforeReduceContextEvent,
     BeforeToolCallEvent,
     MessageAddedEvent,
     MultiAgentInitializedEvent,
@@ -55,6 +57,8 @@ def log_end(self, event: AfterInvocationEvent) -> None:
     "BeforeModelCallEvent",
     "AfterModelCallEvent",
     "AfterInvocationEvent",
+    "BeforeReduceContextEvent",
+    "AfterReduceContextEvent",
     "MessageAddedEvent",
     "HookEvent",
     "HookProvider",

src/strands/hooks/events.py

@@ -303,6 +303,39 @@ def should_reverse_callbacks(self) -> bool:
         return True
 
 
+@dataclass
+class BeforeReduceContextEvent(HookEvent):
+    """Event triggered before the conversation manager reduces context.
+
+    This event is fired just before the agent calls reduce_context() in response
+    to a context window overflow. Hook providers can use this event for logging,
+    observability, or displaying progress indicators during long-running sessions.
+
+    Attributes:
+        exception: The ContextWindowOverflowException that triggered the context reduction.
+    """
+
+    exception: Exception
+
+
+@dataclass
+class AfterReduceContextEvent(HookEvent):
+    """Event triggered after the conversation manager has reduced context.
+
+    This event is fired immediately after reduce_context() returns, before the
+    agent retries the model call. Hook providers can use this event to log the
+    outcome of the reduction or update observability dashboards.
+
+    Note: This event uses reverse callback ordering, meaning callbacks registered
+    later will be invoked first during cleanup.
+    """
+
+    @property
+    def should_reverse_callbacks(self) -> bool:
+        """True to invoke callbacks in reverse order."""
+        return True
+
+
 # Multiagent hook events start here
 @dataclass
 class MultiAgentInitializedEvent(BaseHookEvent):

tests/strands/agent/test_agent.py

@@ -21,7 +21,13 @@
 from strands.agent.conversation_manager.sliding_window_conversation_manager import SlidingWindowConversationManager
 from strands.agent.state import AgentState
 from strands.handlers.callback_handler import PrintingCallbackHandler, null_callback_handler
-from strands.hooks import BeforeInvocationEvent, BeforeModelCallEvent, BeforeToolCallEvent
+from strands.hooks import (
+    AfterReduceContextEvent,
+    BeforeInvocationEvent,
+    BeforeModelCallEvent,
+    BeforeReduceContextEvent,
+    BeforeToolCallEvent,
+)
 from strands.interrupt import Interrupt
 from strands.models.bedrock import DEFAULT_BEDROCK_MODEL_ID, BedrockModel
 from strands.session.repository_session_manager import RepositorySessionManager
@@ -2756,3 +2762,34 @@ def test_as_tool_defaults_description_when_agent_has_none():
     tool = agent.as_tool()
 
     assert tool.tool_spec["description"] == "Use the researcher agent as a tool by providing a natural language input"
+
+
+@pytest.mark.asyncio
+async def test_stream_async_fires_before_and_after_reduce_context_hook_events(mock_model, agent, agenerator, alist):
+    """BeforeReduceContextEvent and AfterReduceContextEvent are fired around reduce_context."""
+    overflow_exc = ContextWindowOverflowException(RuntimeError("Input is too long for requested model"))
+
+    mock_model.mock_stream.side_effect = [
+        overflow_exc,
+        agenerator(
+            [
+                {"contentBlockStart": {"contentBlockIndex": 0, "start": {}}},
+                {"contentBlockDelta": {"contentBlockIndex": 0, "delta": {"text": "OK"}}},
+                {"contentBlockStop": {"contentBlockIndex": 0}},
+                {"messageStop": {"stopReason": "end_turn"}},
+            ]
+        ),
+    ]
+
+    before_events = []
+    after_events = []
+    agent.add_hook(lambda e: before_events.append(e), BeforeReduceContextEvent)
+    agent.add_hook(lambda e: after_events.append(e), AfterReduceContextEvent)
+
+    agent.conversation_manager.reduce_context = unittest.mock.Mock()
+
+    await alist(agent.stream_async("hello"))
+
+    assert len(before_events) == 1
+    assert before_events[0].exception is overflow_exc
+    assert len(after_events) == 1