diff --git a/FABRIC.md b/FABRIC.md
index d696eab..3fab816 100644
--- a/FABRIC.md
+++ b/FABRIC.md
@@ -2057,7 +2057,13 @@ document and committing that amendment as its own item.*
   mass of its pinned patrons (derived) or a constant (tuned). *Refs:* §22.5 #1.
 
 - [ ] **1.3 — What triggers a capacity transfer.** Hera arbitrates; on what signal, and how
-  often. Should read the density gradient, not a schedule. *Refs:* §22.5 #2.
+  often. Should read the density gradient, not a schedule.
+  **Constraint, not optional:** arbitration mutates patron mass, so §18.5's invariant binds
+  it directly — *anything that influences patron state advances on tick count; wall-clock
+  time may be recorded for diagnostics and must never be an input to a decision.* Pacing
+  arbitration off a wall-clock interval would reintroduce exactly the defect item 2.1
+  exists to remove, in a new place. Whatever 1.3 decides must be expressible in ticks.
+  *Refs:* §22.5 #2, **§18.5**, §22.4.
 
 - [ ] **1.4 — The heat/capacity timescale ratio.** The ordering is fixed (capacity slower);
   the ratio is not. *Refs:* §22.4, §22.5 #3.