diff --git a/graphify/build.py b/graphify/build.py
index 470f2c6ba6..45590f05fc 100644
--- a/graphify/build.py
+++ b/graphify/build.py
@@ -1619,7 +1619,10 @@ def build_merge(
     dedup_llm_backend: str | None = None,
     root: str | Path | None = None,
 ) -> nx.Graph:
-    """Load existing graph.json, merge new chunks into it, and save back.
+    """Merge new chunks with an existing graph.json, when present.
+
+    Returns the merged graph without writing it to ``graph_path``. The caller
+    is responsible for persistence.
 
     Re-extracted files REPLACE their prior contribution per tier (#2333/#2336):
     a source_file present in new_chunks has its existing nodes/edges dropped