diff --git a/tools/check-lethality.mjs b/tools/check-lethality.mjs
index ad87df5..3eb0afe 100644
--- a/tools/check-lethality.mjs
+++ b/tools/check-lethality.mjs
@@ -174,9 +174,27 @@ if (VERBOSE) {
   }
 }
 
-if (added.length) console.log(`  note: ${added.length} new creature(s) not in the baseline — ${added.join(", ")}`);
 if (removed.length) console.log(`  note: ${removed.length} creature(s) gone from content — ${removed.join(", ")}`);
 
+/* A creature with no entry in the baseline is UNMEASURED, and it used to be a note.
+   The success line underneath said "every one fighting exactly as recorded" while N of
+   them had no record to fight differently from — the claim was false by exactly the
+   number in the note above it, and the note is the part a green build does not get read
+   for. It is the same shape as the seam defect this guard was on the wrong side of
+   earlier: 47 creatures measured, OK printed for 184.
+
+   Recording a new creature is one `--update` in the commit that adds it, which this
+   guard's own failure message has always instructed. That is a cheap price for a
+   success line that means what it says. */
+if (added.length) {
+  console.error(`check-lethality: FAILED — ${added.length} creature(s) have no recorded fight, `
+    + `so nothing here says how they behave:`);
+  for (const k of added) console.error(`  ${k}`);
+  console.error(`\n  Run with --update in the same commit that adds them, and say in the message `
+    + `what they are. A creature nobody has measured is not a creature this guard covers.`);
+  process.exit(1);
+}
+
 if (problems.length) {
   console.error(`check-lethality: FAILED — ${problems.length} of ${Object.keys(current).length} `
     + `creature(s) fight differently than recorded`);
@@ -187,5 +205,4 @@ if (problems.length) {
 }
 
 console.log(`check-lethality: OK — ${Object.keys(current).length} creatures, every one fighting `
-  + `exactly as recorded against the frozen party`
-  + `${added.length ? `, ${added.length} new` : ""}`);
+  + `exactly as recorded against the frozen party`);