diff --git a/app/models.py b/app/models.py
index 980030d..665a8cd 100644
--- a/app/models.py
+++ b/app/models.py
@@ -446,15 +446,19 @@ class CsvFormatTemplate(Base):
 
     id: Mapped[int] = mapped_column(_PK, primary_key=True, autoincrement=True)
     fingerprint: Mapped[str] = mapped_column(String(64), unique=True, nullable=False)
-    headers: Mapped[list] = mapped_column(JSON, nullable=False)
-    sample_row: Mapped[list] = mapped_column(JSON, nullable=False)
-    mapping: Mapped[dict] = mapped_column(JSON, nullable=False)
+    headers: Mapped[list[str]] = mapped_column(JSON, nullable=False)
+    sample_row: Mapped[list[str]] = mapped_column(JSON, nullable=False)
+    mapping: Mapped[dict[str, str | None]] = mapped_column(JSON, nullable=False)
     preamble_rows: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
     delimiter: Mapped[str] = mapped_column(String(1), nullable=False, default=",")
     broker_label: Mapped[str | None] = mapped_column(String(128))
     first_seen_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, default=utcnow,
     )
+    # use_count and last_used_at are application-managed: parse_with_llm
+    # increments use_count and sets last_used_at = utcnow() on every cache hit.
+    # No onupdate hook — we don't want unrelated writes (e.g. broker_label edits)
+    # to re-stamp last_used_at.
     use_count: Mapped[int] = mapped_column(Integer, nullable=False, default=1)
     last_used_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, default=utcnow,