feat: Enable hooks for MultiAgents (#760)

It's been a customer ask and we don't have a pressing need to keep it restricted.

The primary concern is that because agent's state is manipulated between invocations (state is reset) hooks designed for a single agent may not work for multi-agents. With documentation, we can guide folks to be aware of what happens rather than restricting it outright.

---------

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

Author: zastrowm

src/strands/multiagent/graph.py

@@ -201,10 +201,6 @@ def _validate_node_executor(
         if executor._session_manager is not None:
             raise ValueError("Session persistence is not supported for Graph agents yet.")
 
-        # Check for callbacks
-        if executor.hooks.has_callbacks():
-            raise ValueError("Agent callbacks are not supported for Graph agents yet.")
-
 
 class GraphBuilder:
     """Builder pattern for constructing graphs."""

src/strands/multiagent/swarm.py

@@ -318,10 +318,6 @@ def _validate_swarm(self, nodes: list[Agent]) -> None:
             if node._session_manager is not None:
                 raise ValueError("Session persistence is not supported for Swarm agents yet.")
 
-            # Check for callbacks
-            if node.hooks.has_callbacks():
-                raise ValueError("Agent callbacks are not supported for Swarm agents yet.")
-
     def _inject_swarm_tools(self) -> None:
         """Add swarm coordination tools to each agent."""
         # Create tool functions with proper closures

tests/fixtures/mock_hook_provider.py

@@ -1,13 +1,44 @@
-from typing import Iterator, Tuple, Type
+from typing import Iterator, Literal, Tuple, Type
 
-from strands.hooks import HookEvent, HookProvider, HookRegistry
+from strands import Agent
+from strands.experimental.hooks import (
+    AfterModelInvocationEvent,
+    AfterToolInvocationEvent,
+    BeforeModelInvocationEvent,
+    BeforeToolInvocationEvent,
+)
+from strands.hooks import (
+    AfterInvocationEvent,
+    AgentInitializedEvent,
+    BeforeInvocationEvent,
+    HookEvent,
+    HookProvider,
+    HookRegistry,
+    MessageAddedEvent,
+)
 
 
 class MockHookProvider(HookProvider):
-    def __init__(self, event_types: list[Type]):
+    def __init__(self, event_types: list[Type] | Literal["all"]):
+        if event_types == "all":
+            event_types = [
+                AgentInitializedEvent,
+                BeforeInvocationEvent,
+                AfterInvocationEvent,
+                AfterToolInvocationEvent,
+                BeforeToolInvocationEvent,
+                BeforeModelInvocationEvent,
+                AfterModelInvocationEvent,
+                MessageAddedEvent,
+            ]
+
         self.events_received = []
         self.events_types = event_types
 
+    @property
+    def event_types_received(self):
+        return [type(event) for event in self.events_received]
+
     def get_events(self) -> Tuple[int, Iterator[HookEvent]]:
         return len(self.events_received), iter(self.events_received)
 
@@ -17,3 +48,11 @@ def register_hooks(self, registry: HookRegistry) -> None:
 
     def add_event(self, event: HookEvent) -> None:
         self.events_received.append(event)
+
+    def extract_for(self, agent: Agent) -> "MockHookProvider":
+        """Extracts a hook provider for the given agent, including the events that were fired for that agent.
+
+        Convenience method when sharing a hook provider between multiple agents."""
+        child_provider = MockHookProvider(self.events_types)
+        child_provider.events_received = [event for event in self.events_received if event.agent == agent]
+        return child_provider

tests/strands/multiagent/test_graph.py

@@ -873,15 +873,6 @@ class TestHookProvider(HookProvider):
         def register_hooks(self, registry, **kwargs):
             registry.add_callback(AgentInitializedEvent, lambda e: None)
 
-    agent_with_hooks = create_mock_agent("agent_with_hooks")
-    agent_with_hooks._session_manager = None
-    agent_with_hooks.hooks = HookRegistry()
-    agent_with_hooks.hooks.add_hook(TestHookProvider())
-
-    builder = GraphBuilder()
-    with pytest.raises(ValueError, match="Agent callbacks are not supported for Graph agents yet"):
-        builder.add_node(agent_with_hooks)
-
     # Test validation in Graph constructor (when nodes are passed directly)
     # Test with session manager in Graph constructor
     node_with_session = GraphNode("node_with_session", agent_with_session)
@@ -892,15 +883,6 @@ def register_hooks(self, registry, **kwargs):
             entry_points=set(),
         )
 
-    # Test with callbacks in Graph constructor
-    node_with_hooks = GraphNode("node_with_hooks", agent_with_hooks)
-    with pytest.raises(ValueError, match="Agent callbacks are not supported for Graph agents yet"):
-        Graph(
-            nodes={"node_with_hooks": node_with_hooks},
-            edges=set(),
-            entry_points=set(),
-        )
-
 
 @pytest.mark.asyncio
 async def test_controlled_cyclic_execution():

tests/strands/multiagent/test_swarm.py

@@ -5,8 +5,7 @@
 
 from strands.agent import Agent, AgentResult
 from strands.agent.state import AgentState
-from strands.hooks import AgentInitializedEvent
-from strands.hooks.registry import HookProvider, HookRegistry
+from strands.hooks.registry import HookRegistry
 from strands.multiagent.base import Status
 from strands.multiagent.swarm import SharedContext, Swarm, SwarmNode, SwarmResult, SwarmState
 from strands.session.session_manager import SessionManager
@@ -470,16 +469,3 @@ def test_swarm_validate_unsupported_features():
 
     with pytest.raises(ValueError, match="Session persistence is not supported for Swarm agents yet"):
         Swarm([agent_with_session])
-
-    # Test with callbacks (should fail)
-    class TestHookProvider(HookProvider):
-        def register_hooks(self, registry, **kwargs):
-            registry.add_callback(AgentInitializedEvent, lambda e: None)
-
-    agent_with_hooks = create_mock_agent("agent_with_hooks")
-    agent_with_hooks._session_manager = None
-    agent_with_hooks.hooks = HookRegistry()
-    agent_with_hooks.hooks.add_hook(TestHookProvider())
-
-    with pytest.raises(ValueError, match="Agent callbacks are not supported for Swarm agents yet"):
-        Swarm([agent_with_hooks])

tests_integ/test_multiagent_graph.py

@@ -1,8 +1,11 @@
 import pytest
 
 from strands import Agent, tool
+from strands.experimental.hooks import AfterModelInvocationEvent, BeforeModelInvocationEvent
+from strands.hooks import AfterInvocationEvent, AgentInitializedEvent, BeforeInvocationEvent, MessageAddedEvent
 from strands.multiagent.graph import GraphBuilder
 from strands.types.content import ContentBlock
+from tests.fixtures.mock_hook_provider import MockHookProvider
 
 
 @tool
@@ -18,49 +21,59 @@ def multiply_numbers(x: int, y: int) -> int:
 
 
 @pytest.fixture
-def math_agent():
+def hook_provider():
+    return MockHookProvider("all")
+
+
+@pytest.fixture
+def math_agent(hook_provider):
     """Create an agent specialized in mathematical operations."""
     return Agent(
         model="us.amazon.nova-pro-v1:0",
         system_prompt="You are a mathematical assistant. Always provide clear, step-by-step calculations.",
+        hooks=[hook_provider],
         tools=[calculate_sum, multiply_numbers],
     )
 
 
 @pytest.fixture
-def analysis_agent():
+def analysis_agent(hook_provider):
     """Create an agent specialized in data analysis."""
     return Agent(
         model="us.amazon.nova-pro-v1:0",
+        hooks=[hook_provider],
         system_prompt="You are a data analysis expert. Provide insights and interpretations of numerical results.",
     )
 
 
 @pytest.fixture
-def summary_agent():
+def summary_agent(hook_provider):
     """Create an agent specialized in summarization."""
     return Agent(
         model="us.amazon.nova-lite-v1:0",
+        hooks=[hook_provider],
         system_prompt="You are a summarization expert. Create concise, clear summaries of complex information.",
     )
 
 
 @pytest.fixture
-def validation_agent():
+def validation_agent(hook_provider):
     """Create an agent specialized in validation."""
     return Agent(
         model="us.amazon.nova-pro-v1:0",
+        hooks=[hook_provider],
         system_prompt="You are a validation expert. Check results for accuracy and completeness.",
     )
 
 
 @pytest.fixture
-def image_analysis_agent():
+def image_analysis_agent(hook_provider):
     """Create an agent specialized in image analysis."""
     return Agent(
+        hooks=[hook_provider],
         system_prompt=(
             "You are an image analysis expert. Describe what you see in images and provide detailed analysis."
-        )
+        ),
     )
 
 
@@ -149,7 +162,7 @@ def proceed_to_second_summary(state):
 
 
 @pytest.mark.asyncio
-async def test_graph_execution_with_image(image_analysis_agent, summary_agent, yellow_img):
+async def test_graph_execution_with_image(image_analysis_agent, summary_agent, yellow_img, hook_provider):
     """Test graph execution with multi-modal image input."""
     builder = GraphBuilder()
 
@@ -186,3 +199,16 @@ async def test_graph_execution_with_image(image_analysis_agent, summary_agent, y
     # Verify both nodes completed
     assert "image_analyzer" in result.results
     assert "summarizer" in result.results
+
+    expected_hook_events = [
+        AgentInitializedEvent,
+        BeforeInvocationEvent,
+        MessageAddedEvent,
+        BeforeModelInvocationEvent,
+        AfterModelInvocationEvent,
+        MessageAddedEvent,
+        AfterInvocationEvent,
+    ]
+
+    assert hook_provider.extract_for(image_analysis_agent).event_types_received == expected_hook_events
+    assert hook_provider.extract_for(summary_agent).event_types_received == expected_hook_events

tests_integ/test_multiagent_swarm.py

@@ -1,8 +1,16 @@
 import pytest
 
 from strands import Agent, tool
+from strands.experimental.hooks import (
+    AfterModelInvocationEvent,
+    AfterToolInvocationEvent,
+    BeforeModelInvocationEvent,
+    BeforeToolInvocationEvent,
+)
+from strands.hooks import AfterInvocationEvent, BeforeInvocationEvent, MessageAddedEvent
 from strands.multiagent.swarm import Swarm
 from strands.types.content import ContentBlock
+from tests.fixtures.mock_hook_provider import MockHookProvider
 
 
 @tool
@@ -22,44 +30,52 @@ def calculate(expression: str) -> str:
 
 
 @pytest.fixture
-def researcher_agent():
+def hook_provider():
+    return MockHookProvider("all")
+
+
+@pytest.fixture
+def researcher_agent(hook_provider):
     """Create an agent specialized in research."""
     return Agent(
         name="researcher",
         system_prompt=(
             "You are a research specialist who excels at finding information. When you need to perform calculations or"
             " format documents, hand off to the appropriate specialist."
         ),
+        hooks=[hook_provider],
         tools=[web_search],
     )
 
 
 @pytest.fixture
-def analyst_agent():
+def analyst_agent(hook_provider):
     """Create an agent specialized in data analysis."""
     return Agent(
         name="analyst",
         system_prompt=(
             "You are a data analyst who excels at calculations and numerical analysis. When you need"
             " research or document formatting, hand off to the appropriate specialist."
         ),
+        hooks=[hook_provider],
         tools=[calculate],
     )
 
 
 @pytest.fixture
-def writer_agent():
+def writer_agent(hook_provider):
     """Create an agent specialized in writing and formatting."""
     return Agent(
         name="writer",
+        hooks=[hook_provider],
         system_prompt=(
             "You are a professional writer who excels at formatting and presenting information. When you need research"
             " or calculations, hand off to the appropriate specialist."
         ),
     )
 
 
-def test_swarm_execution_with_string(researcher_agent, analyst_agent, writer_agent):
+def test_swarm_execution_with_string(researcher_agent, analyst_agent, writer_agent, hook_provider):
     """Test swarm execution with string input."""
     # Create the swarm
     swarm = Swarm([researcher_agent, analyst_agent, writer_agent])
@@ -82,6 +98,16 @@ def test_swarm_execution_with_string(researcher_agent, analyst_agent, writer_age
     # Verify agent history - at least one agent should have been used
     assert len(result.node_history) > 0
 
+    # Just ensure that hooks are emitted; actual content is not verified
+    researcher_hooks = hook_provider.extract_for(researcher_agent).event_types_received
+    assert BeforeInvocationEvent in researcher_hooks
+    assert MessageAddedEvent in researcher_hooks
+    assert BeforeModelInvocationEvent in researcher_hooks
+    assert BeforeToolInvocationEvent in researcher_hooks
+    assert AfterToolInvocationEvent in researcher_hooks
+    assert AfterModelInvocationEvent in researcher_hooks
+    assert AfterInvocationEvent in researcher_hooks
+
 
 @pytest.mark.asyncio
 async def test_swarm_execution_with_image(researcher_agent, analyst_agent, writer_agent, yellow_img):