strands-agents · Unshure · Jun 10, 2025 · May 25, 2025
diff --git a/src/strands/agent/__init__.py b/src/strands/agent/__init__.py
@@ -8,12 +8,18 @@
 
 from .agent import Agent
 from .agent_result import AgentResult
-from .conversation_manager import ConversationManager, NullConversationManager, SlidingWindowConversationManager
+from .conversation_manager import (
+    ConversationManager,
+    NullConversationManager,
+    SlidingWindowConversationManager,
+    SummarizingConversationManager,
+)
 
 __all__ = [
     "Agent",
     "AgentResult",
     "ConversationManager",
     "NullConversationManager",
     "SlidingWindowConversationManager",
+    "SummarizingConversationManager",
 ]
diff --git a/src/strands/agent/conversation_manager/__init__.py b/src/strands/agent/conversation_manager/__init__.py
@@ -6,6 +6,8 @@
 - NullConversationManager: A no-op implementation that does not modify conversation history
 - SlidingWindowConversationManager: An implementation that maintains a sliding window of messages to control context
   size while preserving conversation coherence
+- SummarizingConversationManager: An implementation that summarizes older context instead
+  of simply trimming it
 
 Conversation managers help control memory usage and context length while maintaining relevant conversation state, which
 is critical for effective agent interactions.
@@ -14,5 +16,11 @@
 from .conversation_manager import ConversationManager
 from .null_conversation_manager import NullConversationManager
 from .sliding_window_conversation_manager import SlidingWindowConversationManager
+from .summarizing_conversation_manager import SummarizingConversationManager
 
-__all__ = ["ConversationManager", "NullConversationManager", "SlidingWindowConversationManager"]
+__all__ = [
+    "ConversationManager",
+    "NullConversationManager",
+    "SlidingWindowConversationManager",
+    "SummarizingConversationManager",
+]
diff --git a/src/strands/agent/conversation_manager/summarizing_conversation_manager.py b/src/strands/agent/conversation_manager/summarizing_conversation_manager.py
@@ -0,0 +1,222 @@
+"""Summarizing conversation history management with configurable options."""
+
+import logging
+from typing import TYPE_CHECKING, List, Optional
+
+from ...types.content import Message
+from ...types.exceptions import ContextWindowOverflowException
+from .conversation_manager import ConversationManager
+
+if TYPE_CHECKING:
+    from ..agent import Agent
+
+
+logger = logging.getLogger(__name__)
+
+
+DEFAULT_SUMMARIZATION_PROMPT = """You are a conversation summarizer. Provide a concise summary of the conversation \
+history.
+
+Format Requirements:
+- You MUST create a structured and concise summary in bullet-point format.
+- You MUST NOT respond conversationally.
+- You MUST NOT address the user directly.
+
+Task:
+Your task is to create a structured summary document:
+- It MUST contain bullet points with key topics and questions covered
+- It MUST contain bullet points for all significant tools executed and their results
+- It MUST contain bullet points for any code or technical information shared
+- It MUST contain a section of key insights gained
+- It MUST format the summary in the third person
+
+Example format:
+
+## Conversation Summary
+* Topic 1: Key information
+* Topic 2: Key information
+*
+## Tools Executed
+* Tool X: Result Y"""
+
+
+class SummarizingConversationManager(ConversationManager):
+    """Implements a summarizing window manager.
+
+    This manager provides a configurable option to summarize older context instead of
+    simply trimming it, helping preserve important information while staying within
+    context limits.
+    """
+
+    def __init__(
+        self,
+        summary_ratio: float = 0.3,
+        preserve_recent_messages: int = 10,
+        summarization_agent: Optional["Agent"] = None,
+        summarization_system_prompt: Optional[str] = None,
+    ):
+        """Initialize the summarizing conversation manager.
+
+        Args:
+            summary_ratio: Ratio of messages to summarize vs keep when context overflow occurs.
+                Value between 0.1 and 0.8. Defaults to 0.3 (summarize 30% of oldest messages).
+            preserve_recent_messages: Minimum number of recent messages to always keep.
+                Defaults to 10 messages.
+            summarization_agent: Optional agent to use for summarization instead of the parent agent.
+                If provided, this agent can use tools as part of the summarization process.
+            summarization_system_prompt: Optional system prompt override for summarization.
+                If None, uses the default summarization prompt.
+        """
+        if summarization_agent is not None and summarization_system_prompt is not None:
+            raise ValueError(
+                "Cannot provide both summarization_agent and summarization_system_prompt. "
+                "Agents come with their own system prompt."
+            )
+
+        self.summary_ratio = max(0.1, min(0.8, summary_ratio))
+        self.preserve_recent_messages = preserve_recent_messages
+        self.summarization_agent = summarization_agent
+        self.summarization_system_prompt = summarization_system_prompt
+
+    def apply_management(self, agent: "Agent") -> None:
+        """Apply management strategy to conversation history.
+
+        For the summarizing conversation manager, no proactive management is performed.
+        Summarization only occurs when there's a context overflow that triggers reduce_context.
+
+        Args:
+            agent: The agent whose conversation history will be managed.
+                The agent's messages list is modified in-place.
+        """
+        # No proactive management - summarization only happens on context overflow
+        pass
+
+    def reduce_context(self, agent: "Agent", e: Optional[Exception] = None) -> None:
+        """Reduce context using summarization.
+
+        Args:
+            agent: The agent whose conversation history will be reduced.
+                The agent's messages list is modified in-place.
+            e: The exception that triggered the context reduction, if any.
+
+        Raises:
+            ContextWindowOverflowException: If the context cannot be summarized.
+        """
+        try:
+            # Calculate how many messages to summarize
+            messages_to_summarize_count = max(1, int(len(agent.messages) * self.summary_ratio))
+
+            # Ensure we don't summarize recent messages
+            messages_to_summarize_count = min(
+                messages_to_summarize_count, len(agent.messages) - self.preserve_recent_messages
+            )
+
+            if messages_to_summarize_count <= 0:
+                raise ContextWindowOverflowException("Cannot summarize: insufficient messages for summarization")
+
+            # Adjust split point to avoid breaking ToolUse/ToolResult pairs
+            messages_to_summarize_count = self._adjust_split_point_for_tool_pairs(
+                agent.messages, messages_to_summarize_count
+            )
+
+            if messages_to_summarize_count <= 0:
+                raise ContextWindowOverflowException("Cannot summarize: insufficient messages for summarization")
+
+            # Extract messages to summarize
+            messages_to_summarize = agent.messages[:messages_to_summarize_count]
+            remaining_messages = agent.messages[messages_to_summarize_count:]
+
+            # Generate summary
+            summary_message = self._generate_summary(messages_to_summarize, agent)
+
+            # Replace the summarized messages with the summary
+            agent.messages[:] = [summary_message] + remaining_messages
+
+        except Exception as summarization_error:
+            logger.error("Summarization failed: %s", summarization_error)
+            raise summarization_error from e
+
+    def _generate_summary(self, messages: List[Message], agent: "Agent") -> Message:
+        """Generate a summary of the provided messages.
+
+        Args:
+            messages: The messages to summarize.
+            agent: The agent instance to use for summarization.
+
+        Returns:
+            A message containing the conversation summary.
+
+        Raises:
+            Exception: If summary generation fails.
+        """
+        # Choose which agent to use for summarization
+        summarization_agent = self.summarization_agent if self.summarization_agent is not None else agent
+
+        # Save original system prompt and messages to restore later
+        original_system_prompt = summarization_agent.system_prompt
+        original_messages = summarization_agent.messages.copy()
+
+        try:
+            # Only override system prompt if no agent was provided during initialization
+            if self.summarization_agent is None:
+                # Use custom system prompt if provided, otherwise use default
+                system_prompt = (
+                    self.summarization_system_prompt
+                    if self.summarization_system_prompt is not None
+                    else DEFAULT_SUMMARIZATION_PROMPT
+                )
+                # Temporarily set the system prompt for summarization
+                summarization_agent.system_prompt = system_prompt
+            summarization_agent.messages = messages
+
+            # Use the agent to generate summary with rich content (can use tools if needed)
+            result = summarization_agent("Please summarize this conversation.")
+
+            return result.message
+
+        finally:
+            # Restore original agent state
+            summarization_agent.system_prompt = original_system_prompt
+            summarization_agent.messages = original_messages
+
+    def _adjust_split_point_for_tool_pairs(self, messages: List[Message], split_point: int) -> int:
+        """Adjust the split point to avoid breaking ToolUse/ToolResult pairs.
+
+        Uses the same logic as SlidingWindowConversationManager for consistency.
+
+        Args:
+            messages: The full list of messages.
+            split_point: The initially calculated split point.
+
+        Returns:
+            The adjusted split point that doesn't break ToolUse/ToolResult pairs.
+
+        Raises:
+            ContextWindowOverflowException: If no valid split point can be found.
+        """
+        if split_point > len(messages):
+            raise ContextWindowOverflowException("Split point exceeds message array length")
+
+        if split_point == len(messages):
+            return split_point
+
+        # Find the next valid split_point
+        while split_point < len(messages):
+            if (
+                # Oldest message cannot be a toolResult because it needs a toolUse preceding it
+                any("toolResult" in content for content in messages[split_point]["content"])
+                or (
+                    # Oldest message can be a toolUse only if a toolResult immediately follows it.
+                    any("toolUse" in content for content in messages[split_point]["content"])
+                    and split_point + 1 < len(messages)
+                    and not any("toolResult" in content for content in messages[split_point + 1]["content"])
+                )
+            ):
+                split_point += 1
+            else:
+                break
+        else:
+            # If we didn't find a valid split_point, then we throw
+            raise ContextWindowOverflowException("Unable to trim conversation context!")
+
+        return split_point