diff --git a/AGENTS.md b/AGENTS.md
index 10b37df..87aaf17 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -133,6 +133,11 @@ Concretely:
 - If the reported count moves between runs, find out why before writing anything
   down. A drop means either an environment difference or a failure, and the two
   look identical from the summary line.
+- **The total is the same whether or not the disc is present.** A skipped disc
+  test returns early and still counts as passed, so `cargo test --workspace` reads
+  278 either way. The only signal is the wall clock: the disc binary takes ~36s
+  with `GC_DISC` set and ~4s without it. If a run finishes suspiciously fast, the
+  disc tests did not run.
 - Never quote a number you computed rather than read.
 
 This is the same defect as rules 3 and 3a, one level up: a quantity that should