diff --git a/src/rag_engine.py b/src/rag_engine.py
index f90523e7..fb568752 100644
--- a/src/rag_engine.py
+++ b/src/rag_engine.py
@@ -335,6 +335,15 @@ class RAGEngine:
    if os.path.exists(cwd_path):
     full_path = cwd_path
    else:
+    # Per Tier 1 addendum 3: log when index_file silently no-ops on a
+    # missing file. This makes misconfigured base_dir visible in the
+    # subprocess log instead of invisible (a real production bug that
+    # made this diagnostic chain 3 sessions long).
+    sys.stderr.write(
+     f"RAG index_file: file not found, base_dir={self.base_dir!r} "
+     f"file_path={file_path!r} cwd={os.getcwd()!r}\n"
+    )
+    sys.stderr.flush()
     return
 
   mtime_result = self._get_file_mtime_result(full_path)