diff --git a/chat/state/memory.py b/chat/state/memory.py
index 42a7e95..a9d62df 100644
--- a/chat/state/memory.py
+++ b/chat/state/memory.py
@@ -112,6 +112,25 @@ SIGNIFICANCE_RANK_BIAS = 0.5
 RRF_CONST = 60
 
 
+def _max_event_id(conn: Connection, owner_id: str) -> int:
+    """Return the largest ``memories.id`` for ``owner_id`` (1 if none exist).
+
+    Used as the recency-boost denominator by both ``_composite_rerank`` and
+    ``_rrf_fuse_and_rerank`` (T104). The row id is a monotonic recency proxy
+    — newer memories have larger ids — so dividing by the per-owner max keeps
+    the boost in [0, 1] regardless of how many memories the owner has.
+
+    Returns 1 (not 0) when the owner has no rows so callers can divide by
+    the result without a guard. The "no memories" case never actually hits
+    this helper because the FTS query above would have returned no rows,
+    but the safe default keeps the helper trivially reusable.
+    """
+    row = conn.execute(
+        "SELECT MAX(id) FROM memories WHERE owner_id = ?", (owner_id,)
+    ).fetchone()
+    return row[0] if row and row[0] else 1
+
+
 def search_memories(
     conn: Connection,
     owner_id: str,
@@ -163,6 +182,14 @@ def search_memories(
 
     When ``query_vector`` is None: FTS-only behaviour unchanged — all
     Phase 1-3.5 callers see the same row shape and ordering as before.
+
+    **Row-shape contract (T104):** every returned dict carries an
+    ``fts_rank`` key. For FTS hits this is the BM25 score (a negative float,
+    lower-is-better). For *vector-only* hits surfaced by the fused path —
+    rows that matched the query embedding but did NOT match FTS — the
+    ``fts_rank`` value is ``None``. Downstream consumers must accept
+    ``None`` here; do not assume ``fts_rank`` is always numeric. The
+    ``composite_score`` is always a float on every returned row.
     """
     if witness_role not in _VALID_WITNESS_ROLES:
         raise ValueError(
@@ -227,10 +254,7 @@ def _composite_rerank(
     Extracted from ``search_memories`` so the no-vector path stays a single
     call and the fused path can re-use the same boost formulae after RRF.
     """
-    max_id_row = conn.execute(
-        "SELECT MAX(id) FROM memories WHERE owner_id = ?", (owner_id,)
-    ).fetchone()
-    max_id = max_id_row[0] if max_id_row and max_id_row[0] else 1
+    max_id = _max_event_id(conn, owner_id)
 
     result_cols = cols + ["fts_rank"]
     enriched: list[dict] = []
@@ -343,10 +367,7 @@ def _rrf_fuse_and_rerank(
 
     # Final composite re-rank: significance + recency boosts on top of the
     # negated fusion score so the sort direction matches the FTS-only path.
-    max_id_row = conn.execute(
-        "SELECT MAX(id) FROM memories WHERE owner_id = ?", (owner_id,)
-    ).fetchone()
-    max_id = max_id_row[0] if max_id_row and max_id_row[0] else 1
+    max_id = _max_event_id(conn, owner_id)
 
     result_cols = cols + ["fts_rank"]
     enriched: list[dict] = []