feat: relax decision option validation — allow cross-references to any node in tree

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/tests/test_ai_tree_validator.py

@@ -122,7 +122,7 @@ class TestReferenceIntegrity:
         tree = _make_valid_tree()
         tree["options"][0]["next_node_id"] = "nonexistent"
         errors = validate_generated_tree(tree)
-        assert any("non-existent child" in e for e in errors)
+        assert any("does not exist" in e for e in errors)
 
     def test_action_next_node_id_references_nonexistent_node(self):
         """Action next_node_id pointing to a node that doesn't exist anywhere in the tree."""
@@ -188,6 +188,31 @@ class TestDeadEndDetection:
         assert any("dead end" in e for e in errors)
 
 
+class TestCrossReferenceSupport:
+    def test_option_referencing_non_child_node_in_tree_is_valid(self):
+        """A decision option can reference any node in the tree, not just direct children."""
+        tree = _make_valid_tree()
+        # Make root option point to a grandchild (not a direct child) — cross-reference
+        tree["options"][0]["next_node_id"] = "fix-errors"  # grandchild of root
+        errors = validate_generated_tree(tree)
+        assert not any("non-existent child" in e for e in errors)
+        assert not any("does not exist" in e for e in errors)
+
+    def test_option_referencing_nonexistent_node_still_fails(self):
+        """Cross-references must still point to nodes that exist in the tree."""
+        tree = _make_valid_tree()
+        tree["options"][0]["next_node_id"] = "totally-fake-id"
+        errors = validate_generated_tree(tree)
+        assert any("does not exist" in e for e in errors)
+
+    def test_action_next_node_id_to_ancestor_is_valid(self):
+        """Action node can loop back to an ancestor node."""
+        tree = _make_valid_tree()
+        tree["children"][1]["next_node_id"] = "root"
+        errors = validate_generated_tree(tree)
+        assert not any("does not exist" in e for e in errors)
+
+
 class TestCountTreeStats:
     def test_stats_correct(self):
         tree = _make_valid_tree()