feat: redact content from a message in a session

src/strands/agent/agent.py

@@ -543,6 +543,19 @@ async def _run_loop(
             # Execute the event loop cycle with retry logic for context limits
             events = self._execute_event_loop_cycle(invocation_state)
             async for event in events:
+                # Signal from the model provider that the message sent by the user should be redacted,
+                # likely due to a guardrail.
+                if (
+                    event.get("callback")
+                    and event["callback"].get("event")
+                    and event["callback"]["event"].get("redactContent")
+                    and event["callback"]["event"]["redactContent"].get("redactUserContentMessage")
+                ):
+                    self.messages[-1]["content"] = [
+                        {"text": event["callback"]["event"]["redactContent"]["redactUserContentMessage"]}
+                    ]
+                    if self._session_manager:
+                        self._session_manager.redact_latest_message(self.messages[-1], self)
                 yield event
 
         finally:

src/strands/event_loop/streaming.py

@@ -221,17 +221,13 @@ def handle_message_stop(event: MessageStopEvent) -> StopReason:
     return event["stopReason"]
 
 
-def handle_redact_content(event: RedactContentEvent, messages: Messages, state: dict[str, Any]) -> None:
+def handle_redact_content(event: RedactContentEvent, state: dict[str, Any]) -> None:
     """Handles redacting content from the input or output.
 
     Args:
         event: Redact Content Event.
-        messages: Agent messages.
         state: The current state of message processing.
     """
-    if event.get("redactUserContentMessage") is not None:
-        messages[-1]["content"] = [{"text": event["redactUserContentMessage"]}]  # type: ignore
-
     if event.get("redactAssistantContentMessage") is not None:
         state["message"]["content"] = [{"text": event["redactAssistantContentMessage"]}]
 
@@ -251,15 +247,11 @@ def extract_usage_metrics(event: MetadataEvent) -> tuple[Usage, Metrics]:
     return usage, metrics
 
 
-async def process_stream(
-    chunks: AsyncIterable[StreamEvent],
-    messages: Messages,
-) -> AsyncGenerator[dict[str, Any], None]:
+async def process_stream(chunks: AsyncIterable[StreamEvent]) -> AsyncGenerator[dict[str, Any], None]:
     """Processes the response stream from the API, constructing the final message and extracting usage metrics.
 
     Args:
         chunks: The chunks of the response stream from the model.
-        messages: The agents messages.
 
     Returns:
         The reason for stopping, the constructed message, and the usage metrics.
@@ -295,7 +287,7 @@ async def process_stream(
         elif "metadata" in chunk:
             usage, metrics = extract_usage_metrics(chunk["metadata"])
         elif "redactContent" in chunk:
-            handle_redact_content(chunk["redactContent"], messages, state)
+            handle_redact_content(chunk["redactContent"], state)
 
     yield {"stop": (stop_reason, state["message"], usage, metrics)}
 
@@ -323,5 +315,5 @@ async def stream_messages(
 
     chunks = model.stream(messages, tool_specs if tool_specs else None, system_prompt)
 
-    async for event in process_stream(chunks, messages):
+    async for event in process_stream(chunks):
         yield event

src/strands/models/anthropic.py

@@ -407,7 +407,7 @@ async def structured_output(
         tool_spec = convert_pydantic_to_tool_spec(output_model)
 
         response = self.stream(messages=prompt, tool_specs=[tool_spec], **kwargs)
-        async for event in process_stream(response, prompt):
+        async for event in process_stream(response):
             yield event
 
         stop_reason, messages, _, _ = event["stop"]

src/strands/models/bedrock.py

@@ -577,7 +577,7 @@ async def structured_output(
         tool_spec = convert_pydantic_to_tool_spec(output_model)
 
         response = self.stream(messages=prompt, tool_specs=[tool_spec], **kwargs)
-        async for event in streaming.process_stream(response, prompt):
+        async for event in streaming.process_stream(response):
             yield event
 
         stop_reason, messages, _, _ = event["stop"]

src/strands/session/repository_session_manager.py

@@ -1,6 +1,7 @@
 """Repository session manager implementation."""
 
 import logging
+from typing import Optional
 
 from ..agent.agent import Agent
 from ..agent.state import AgentState
@@ -50,15 +51,30 @@ def __init__(
         # Keep track of the initialized agent id's so that two agents in a session cannot share an id
         self._initialized_agent_ids: set[str] = set()
 
+        # Keep track of the latest message stored in the session in case we need to redact its content.
+        self._latest_message: Optional[SessionMessage] = None
+
     def append_message(self, message: Message, agent: Agent) -> None:
         """Append a message to the agent's session.
 
         Args:
             message: Message to add to the agent in the session
             agent: Agent to append the message to
         """
-        session_message = SessionMessage.from_message(message)
-        self.session_repository.create_message(self.session_id, agent.agent_id, session_message)
+        self._latest_message = SessionMessage.from_message(message)
+        self.session_repository.create_message(self.session_id, agent.agent_id, self._latest_message)
+
+    def redact_latest_message(self, redact_message: Message, agent: Agent) -> None:
+        """Redact the latest message appended to the session.
+
+        Args:
+            redact_message: New message to use that contains the redact content
+            agent: Agent to apply the message redaction to
+        """
+        if self._latest_message is None:
+            raise SessionException("No message to redact.")
+        self._latest_message.redact_message = redact_message
+        return self.session_repository.update_message(self.session_id, agent.agent_id, self._latest_message)
 
     def sync_agent(self, agent: Agent) -> None:
         """Serialize and update the agent into the session repository.

src/strands/session/session_manager.py

@@ -26,6 +26,15 @@ def register_hooks(self, registry: HookRegistry, **kwargs: Any) -> None:
         registry.add_callback(MessageAddedEvent, lambda event: self.append_message(event.message, event.agent))
         registry.add_callback(MessageAddedEvent, lambda event: self.sync_agent(event.agent))
 
+    @abstractmethod
+    def redact_latest_message(self, redact_message: Message, agent: "Agent") -> None:
+        """Redact the message most recently appended to the agent in the session.
+
+        Args:
+            redact_message: New message to use that contains the redact content
+            agent: Agent to apply the message redaction to
+        """
+
     @abstractmethod
     def append_message(self, message: Message, agent: "Agent") -> None:
         """Append a message to the agent's session.

src/strands/types/session.py

@@ -5,7 +5,7 @@
 from dataclasses import dataclass, field
 from datetime import datetime, timezone
 from enum import Enum
-from typing import Any, Dict, cast
+from typing import Any, Dict, Optional, cast
 from uuid import uuid4
 
 from ..agent.agent import Agent
@@ -54,9 +54,18 @@ def decode_bytes_values(obj: Any) -> Any:
 
 @dataclass
 class SessionMessage:
-    """Message within a SessionAgent."""
+    """Message within a SessionAgent.
+
+    Attributes:
+        message: Message content
+        redact_message: If the original message is redacted, this is the new content to use
+        message_id: Unique id for a message
+        created_at: ISO format timestamp for when this message was created
+        updated_at: ISO format timestamp for when this message was last updated
+    """
 
     message: Message
+    redact_message: Optional[Message] = None
     message_id: str = field(default_factory=lambda: str(uuid4()))
     created_at: str = field(default_factory=lambda: datetime.now(timezone.utc).isoformat())
     updated_at: str = field(default_factory=lambda: datetime.now(timezone.utc).isoformat())
@@ -73,8 +82,14 @@ def from_message(cls, message: Message) -> "SessionMessage":
         )
 
     def to_message(self) -> Message:
-        """Convert SessionMessage back to a Message, decoding any bytes values."""
-        return cast(Message, decode_bytes_values(self.message))
+        """Convert SessionMessage back to a Message, decoding any bytes values.
+
+        If the message was redacted, return the redact content instead.
+        """
+        if self.redact_message is not None:
+            return cast(Message, decode_bytes_values(self.redact_message))
+        else:
+            return cast(Message, decode_bytes_values(self.message))
 
     @classmethod
     def from_dict(cls, env: dict[str, Any]) -> "SessionMessage":

tests/fixtures/mocked_model_provider.py

@@ -1,5 +1,5 @@
 import json
-from typing import Any, AsyncGenerator, Iterable, Optional, Type, TypeVar
+from typing import Any, AsyncGenerator, Iterable, Optional, Type, TypedDict, TypeVar, Union
 
 from pydantic import BaseModel
 
@@ -12,6 +12,11 @@
 T = TypeVar("T", bound=BaseModel)
 
 
+class RedactionMessage(TypedDict):
+    redactedUserContent: str
+    redactedAssistantContent: str
+
+
 class MockedModelProvider(Model):
     """A mock implementation of the Model interface for testing purposes.
 
@@ -20,7 +25,7 @@ class MockedModelProvider(Model):
     to stream mock responses as events.
     """
 
-    def __init__(self, agent_responses: Messages):
+    def __init__(self, agent_responses: list[Union[Message, RedactionMessage]]):
         self.agent_responses = agent_responses
         self.index = 0
 
@@ -54,27 +59,36 @@ async def stream(
 
         self.index += 1
 
-    def map_agent_message_to_events(self, agent_message: Message) -> Iterable[dict[str, Any]]:
+    def map_agent_message_to_events(self, agent_message: Union[Message, RedactionMessage]) -> Iterable[dict[str, Any]]:
         stop_reason: StopReason = "end_turn"
         yield {"messageStart": {"role": "assistant"}}
-        for content in agent_message["content"]:
-            if "text" in content:
-                yield {"contentBlockStart": {"start": {}}}
-                yield {"contentBlockDelta": {"delta": {"text": content["text"]}}}
-                yield {"contentBlockStop": {}}
-            if "toolUse" in content:
-                stop_reason = "tool_use"
-                yield {
-                    "contentBlockStart": {
-                        "start": {
-                            "toolUse": {
-                                "name": content["toolUse"]["name"],
-                                "toolUseId": content["toolUse"]["toolUseId"],
+        if agent_message.get("redactedAssistantContent"):
+            yield {"redactContent": {"redactUserContentMessage": agent_message["redactedUserContent"]}}
+            yield {"contentBlockStart": {"start": {}}}
+            yield {"contentBlockDelta": {"delta": {"text": agent_message["redactedAssistantContent"]}}}
+            yield {"contentBlockStop": {}}
+            stop_reason = "guardrail_intervened"
+        else:
+            for content in agent_message["content"]:
+                if "text" in content:
+                    yield {"contentBlockStart": {"start": {}}}
+                    yield {"contentBlockDelta": {"delta": {"text": content["text"]}}}
+                    yield {"contentBlockStop": {}}
+                if "toolUse" in content:
+                    stop_reason = "tool_use"
+                    yield {
+                        "contentBlockStart": {
+                            "start": {
+                                "toolUse": {
+                                    "name": content["toolUse"]["name"],
+                                    "toolUseId": content["toolUse"]["toolUseId"],
+                                }
                             }
                         }
                     }
-                }
-                yield {"contentBlockDelta": {"delta": {"toolUse": {"input": json.dumps(content["toolUse"]["input"])}}}}
-                yield {"contentBlockStop": {}}
+                    yield {
+                        "contentBlockDelta": {"delta": {"toolUse": {"input": json.dumps(content["toolUse"]["input"])}}}
+                    }
+                    yield {"contentBlockStop": {}}
 
         yield {"messageStop": {"stopReason": stop_reason}}

tests/strands/agent/test_agent.py

@@ -4,6 +4,7 @@
 import os
 import textwrap
 import unittest.mock
+from uuid import uuid4
 
 import pytest
 from pydantic import BaseModel
@@ -1425,3 +1426,61 @@ def test_agent_restored_from_session_management():
     agent = Agent(session_manager=session_manager)
 
     assert agent.state.get("foo") == "bar"
+
+
+def test_agent_redacts_input_on_triggered_guardrail():
+    mocked_model = MockedModelProvider(
+        [{"redactedUserContent": "BLOCKED!", "redactedAssistantContent": "INPUT BLOCKED!"}]
+    )
+
+    agent = Agent(
+        model=mocked_model,
+        system_prompt="You are a helpful assistant.",
+        callback_handler=None,
+    )
+
+    response1 = agent("CACTUS")
+
+    assert response1.stop_reason == "guardrail_intervened"
+    assert agent.messages[0]["content"][0]["text"] == "BLOCKED!"
+
+
+def test_agent_restored_from_session_management_with_redacted_input():
+    mocked_model = MockedModelProvider(
+        [{"redactedUserContent": "BLOCKED!", "redactedAssistantContent": "INPUT BLOCKED!"}]
+    )
+
+    test_session_id = str(uuid4())
+    mocked_session_repository = MockedSessionRepository()
+    session_manager = RepositorySessionManager(session_id=test_session_id, session_repository=mocked_session_repository)
+
+    agent = Agent(
+        model=mocked_model,
+        system_prompt="You are a helpful assistant.",
+        callback_handler=None,
+        session_manager=session_manager,
+    )
+
+    assert mocked_session_repository.read_agent(test_session_id, agent.agent_id) is not None
+
+    response1 = agent("CACTUS")
+
+    assert response1.stop_reason == "guardrail_intervened"
+    assert agent.messages[0]["content"][0]["text"] == "BLOCKED!"
+    user_input_session_message = mocked_session_repository.list_messages(test_session_id, agent.agent_id)[0]
+    # Assert persisted message is equal to the redacted message in the agent
+    assert user_input_session_message.to_message() == agent.messages[0]
+
+    # Restore an agent from the session, confirm input is still redacted
+    session_manager_2 = RepositorySessionManager(
+        session_id=test_session_id, session_repository=mocked_session_repository
+    )
+    agent_2 = Agent(
+        model=mocked_model,
+        system_prompt="You are a helpful assistant.",
+        callback_handler=None,
+        session_manager=session_manager_2,
+    )
+
+    # Assert that the restored agent redacted message is equal to the original agent
+    assert agent.messages[0] == agent_2.messages[0]

tests/strands/event_loop/test_streaming.py

@@ -528,8 +528,7 @@ def test_extract_usage_metrics():
 )
 @pytest.mark.asyncio
 async def test_process_stream(response, exp_events, agenerator, alist):
-    messages = [{"role": "user", "content": [{"text": "Some input!"}]}]
-    stream = strands.event_loop.streaming.process_stream(agenerator(response), messages)
+    stream = strands.event_loop.streaming.process_stream(agenerator(response))
 
     tru_events = await alist(stream)
     assert tru_events == exp_events