refactor: Forbid routed edges starting from START in graph validation

Modifies graph validation to forbid edges starting from the START node from having routes defined. Edges originating from START must be unconditional.

Co-authored-by: Shangjie Chen <deanchen@google.com>
PiperOrigin-RevId: 944703369

Author: DeanChensj

src/google/adk/workflow/utils/_graph_validation.py

@@ -129,6 +129,16 @@ def _validate_duplicate_edges(edges: list[Edge]) -> None:
     seen_edges.add(edge_tuple)
 
 
+def _validate_start_edges(edges: list[Edge]) -> None:
+  """Checks that edges from START do not have routes."""
+  for edge in edges:
+    if edge.from_node.name == START.name and edge.route is not None:
+      raise ValueError(
+          "Graph validation failed. Edges from START must not have routes"
+          f" (edge to {edge.to_node.name} has route {edge.route})."
+      )
+
+
 def _validate_default_routes(edges: list[Edge]) -> None:
   """Checks constraints on DEFAULT_ROUTE."""
   default_route_edges: dict[str, str] = {}
@@ -202,6 +212,7 @@ def validate_graph(nodes: list[BaseNode], edges: list[Edge]) -> set[str]:
   """Validates the workflow graph and returns terminal node names."""
   node_names = _validate_duplicate_node_names(nodes)
   _validate_start_node(node_names)
+  _validate_start_edges(edges)
   _validate_connectivity(edges, node_names)
   _validate_duplicate_edges(edges)
   _validate_default_routes(edges)

tests/unittests/workflow/utils/test_graph_validation.py

@@ -100,30 +100,47 @@ def test_duplicate_edges_fail_validation(
 ) -> None:
   """Tests that duplicate edges fail validation, regardless of routes."""
   node_a = TestingNode(name='NodeA')
+  node_b = TestingNode(name='NodeB')
   graph = Graph(
       edges=[
+          Edge(from_node=START, to_node=node_a),
           Edge(
-              from_node=START,
-              to_node=node_a,
+              from_node=node_a,
+              to_node=node_b,
               route=routes[0],
           ),
           Edge(
-              from_node=START,
-              to_node=node_a,
+              from_node=node_a,
+              to_node=node_b,
               route=routes[1],
           ),
       ],
   )
   with pytest.raises(
       ValueError,
       match=(
-          r'Graph validation failed\. Duplicate edge found: from=__START__,'
-          r' to=NodeA'
+          r'Graph validation failed\. Duplicate edge found: from=NodeA,'
+          r' to=NodeB'
       ),
   ):
     validate_graph(graph.nodes, graph.edges)
 
 
+def test_routed_start_edge_fails_validation() -> None:
+  """Tests that routed edges from START node fail validation."""
+  node_a = TestingNode(name='NodeA')
+  graph = Graph(
+      edges=[
+          Edge(from_node=START, to_node=node_a, route='route1'),
+      ],
+  )
+  with pytest.raises(
+      ValueError,
+      match=r'Graph validation failed\. Edges from START must not have routes',
+  ):
+    validate_graph(graph.nodes, graph.edges)
+
+
 def test_start_node_with_incoming_edge() -> None:
   """Tests graph with incoming edge to START node fails validation."""
   node_a = TestingNode(name='NodeA')