fix(graph): only evaluate outbound edges from completed nodes (#1846)

Co-authored-by: giulio-leone <giulio.leone@users.noreply.github.com>

Author: giulio-leone

src/strands/multiagent/graph.py

@@ -827,9 +827,16 @@ async def _handle_node_timeout(self, node: GraphNode, event_queue: asyncio.Queue
         return timeout_exception
 
     def _find_newly_ready_nodes(self, completed_batch: list["GraphNode"]) -> list["GraphNode"]:
-        """Find nodes that became ready after the last execution."""
+        """Find nodes that became ready after the last execution.
+
+        Only evaluates destination nodes of outbound edges from the completed batch,
+        instead of iterating over all nodes in the graph.
+        """
+        # Collect unique candidate nodes reachable from the completed batch
+        candidates = {edge.to_node for edge in self.edges if edge.from_node in completed_batch}
+
         newly_ready = []
-        for _node_id, node in self.nodes.items():
+        for node in candidates:
             if self._is_node_ready_with_conditions(node, completed_batch):
                 newly_ready.append(node)
         return newly_ready

tests/strands/multiagent/test_graph.py

@@ -2405,3 +2405,38 @@ async def stream_async(self, prompt=None, **kwargs):
     assert result.completed_nodes == 2
     assert "custom_node" in result.results
     assert "regular_node" in result.results
+
+
+def test_find_newly_ready_nodes_only_evaluates_outbound_edges():
+    """Verify _find_newly_ready_nodes only checks destinations of outbound edges from completed batch.
+
+    Previously, it iterated over ALL nodes, which could cause nodes to fire
+    before their actual dependencies completed.
+
+    See: https://github.com/strands-agents/sdk-python/issues/685
+    """
+    # Build a graph: A -> B -> C, D -> E (independent chain)
+    node_a = GraphNode(node_id="A", executor=create_mock_agent("A"))
+    node_b = GraphNode(node_id="B", executor=create_mock_agent("B"))
+    node_c = GraphNode(node_id="C", executor=create_mock_agent("C"))
+    node_d = GraphNode(node_id="D", executor=create_mock_agent("D"))
+    node_e = GraphNode(node_id="E", executor=create_mock_agent("E"))
+
+    graph = Graph.__new__(Graph)
+    graph.nodes = {"A": node_a, "B": node_b, "C": node_c, "D": node_d, "E": node_e}
+    graph.edges = [
+        GraphEdge(from_node=node_a, to_node=node_b),
+        GraphEdge(from_node=node_b, to_node=node_c),
+        GraphEdge(from_node=node_d, to_node=node_e),
+    ]
+    graph.state = GraphState()
+
+    # When A completes, only B should be ready (not E)
+    ready = graph._find_newly_ready_nodes([node_a])
+    ready_ids = {n.node_id for n in ready}
+    assert ready_ids == {"B"}, f"Expected only B, got {ready_ids}"
+
+    # When D completes, only E should be ready (not B or C)
+    ready = graph._find_newly_ready_nodes([node_d])
+    ready_ids = {n.node_id for n in ready}
+    assert ready_ids == {"E"}, f"Expected only E, got {ready_ids}"