fix(merge): coerce malformed tags before adding "tested" (#113)

Codex flagged that prod_node.setdefault("tags", []) returns the existing
value when the key is present, so a raw LLM batch with tags=None or
tags="some string" would crash the whole merge on the next "tested" not
in tags membership check.

The TypeScript autoFixGraph normalizer that handles this case runs
downstream of merge-batch-graphs.py, not before it, so the Python side
has to defend itself. Coerce non-list tags to a fresh [] before the
membership/append.

Regression test exercises None / comma-string / single-string / int /
dict inputs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

understand-anything-plugin/skills/understand/merge-batch-graphs.py

@@ -479,7 +479,14 @@ def link_tests(
                 "description": "Path-based pairing (deterministic)",
             })
             added += 1
-            tags = prod_node.setdefault("tags", [])
+            # `tags` may be missing, None, or a malformed string from raw LLM
+            # batch JSON — the TypeScript autoFixGraph layer that normalizes it
+            # only runs downstream of this script. Coerce to a fresh list so a
+            # bad batch can't crash the whole merge.
+            tags = prod_node.get("tags")
+            if not isinstance(tags, list):
+                tags = []
+                prod_node["tags"] = tags
             if "tested" not in tags:
                 tags.append("tested")
                 tagged += 1

understand-anything-plugin/skills/understand/test_merge_batch_graphs.py

@@ -443,6 +443,28 @@ class LinkTestsTests(unittest.TestCase):
         self.assertEqual(edges[0]["target"], "file:src/foo.test.ts")
         self.assertIn("tested", prod["tags"])
 
+    def test_malformed_tags_is_replaced_not_crashed(self) -> None:
+        # Raw LLM batch JSON can ship `tags` as None, a string, or other
+        # non-list values — the TypeScript autoFixGraph normalizer runs
+        # downstream of this script. The linker must coerce instead of crash.
+        for bad_tags in (None, "tested,foo", "single", 0, {"k": "v"}):
+            with self.subTest(bad_tags=bad_tags):
+                prod = {
+                    "id": "file:src/foo.ts",
+                    "type": "file",
+                    "name": "foo.ts",
+                    "filePath": "src/foo.ts",
+                    "tags": bad_tags,
+                }
+                test = _file_node("src/foo.test.ts")
+                nodes_by_id = {prod["id"]: prod, test["id"]: test}
+                edges: list[dict[str, Any]] = []
+
+                added, dropped, tagged = mbg.link_tests(nodes_by_id, edges)
+
+                self.assertEqual((added, dropped, tagged), (1, 0, 1))
+                self.assertEqual(prod["tags"], ["tested"])
+
 
 # ── merge_and_normalize integration ───────────────────────────────────────