fix: Fix silent dead end when conditional routes are unmatched

If a node has conditional (routed) edges but the emitted route does not
match any of them, and no default route is configured, the branch would
end silently. This change adds a warning diagnostic in get_next_pending_nodes
to log when a branch ends due to an unmatched route.

Co-authored-by: Shangjie Chen <deanchen@google.com>
PiperOrigin-RevId: 930749225

Author: DeanChensj

src/google/adk/workflow/_graph.py

@@ -18,6 +18,9 @@
 
 from collections import Counter
 from collections.abc import Callable
+import logging
+
+logger = logging.getLogger("google_adk." + __name__)
 from collections.abc import Set
 from typing import Annotated
 from typing import Any
@@ -345,6 +348,7 @@ def get_next_pending_nodes(
     next_pending_nodes: list[str] = []
     matched_specific_route = False
     default_route_node: str | None = None
+    has_routing_edges = False
 
     for edge in self.edges:
       if edge.from_node.name == node_name:
@@ -353,6 +357,7 @@ def get_next_pending_nodes(
           next_pending_nodes.append(edge.to_node.name)
           continue
 
+        has_routing_edges = True
         if edge.route == DEFAULT_ROUTE:
           default_route_node = edge.to_node.name
           continue
@@ -376,6 +381,14 @@ def get_next_pending_nodes(
     if not matched_specific_route and default_route_node:
       next_pending_nodes.append(default_route_node)
 
+    if has_routing_edges and not next_pending_nodes:
+      logger.warning(
+          "Node '%s' has conditional/DEFAULT edges but none were matched by the"
+          " emitted route(s): %s. The branch will end.",
+          node_name,
+          routes_to_match,
+      )
+
     return next_pending_nodes
 
   def _detect_unconditional_cycles(self, node_names: Set[str]) -> None:

tests/unittests/workflow/test_graph.py

@@ -14,6 +14,8 @@
 
 """Tests for Graph validation."""
 
+import logging
+
 from google.adk.workflow import Edge
 from google.adk.workflow import FunctionNode
 from google.adk.workflow import START
@@ -791,3 +793,26 @@ def test_chat_agent_wiring_validation_only_runs_on_llm_agent() -> None:
       ],
   )
   graph.validate_graph()  # Should not raise because node_b is a TestingNode, not LlmAgent
+
+
+def test_get_next_pending_nodes_unmatched_route_warning(caplog) -> None:
+  """Tests that a warning is logged when a route is unmatched and there's no DEFAULT_ROUTE."""
+  node_a = TestingNode(name='NodeA')
+  node_c = TestingNode(name='NodeC')
+
+  graph = Graph(
+      edges=[
+          Edge(from_node=node_a, to_node=node_c, route='route1'),
+      ],
+  )
+
+  with caplog.at_level(logging.WARNING):
+    next_nodes = graph.get_next_pending_nodes(
+        'NodeA', routes_to_match='unknown_route'
+    )
+
+  assert not next_nodes
+  assert any(
+      'has conditional/DEFAULT edges but none were matched' in record.message
+      for record in caplog.records
+  )