diff --git a/.github/scripts/pr-merge/clear-escalations.mjs b/.github/scripts/pr-merge/clear-escalations.mjs
index 335dcb8aea..9e342c25cf 100755
--- a/.github/scripts/pr-merge/clear-escalations.mjs
+++ b/.github/scripts/pr-merge/clear-escalations.mjs
@@ -142,9 +142,9 @@ export const NATIVE_READS_PER_RUN = 3;
 const REFUSAL_CLEARED = "refusal-cleared";
 // The causes the lanes clear, by READER_HOLDS kind: the only recorded ones
 // this script lifts at the same head, and the only ones a lift tolerates
-// refusing now. Every other code, an unknown one included, is a person's. babysit_risk.PERSON_CODES (#10249) is
-// to become the single source; until then the list is explicit, so it fails
-// closed.
+// refusing now. Every other code, an unknown one included, is a person's. The
+// list is explicit, so it fails closed; lane-causes-person-codes.test.mjs pins
+// it against babysit_risk.PERSON_CODES and PERSON_PREFIXES (#10249).
 // - risk (an entry ending in ":" is a prefix): babysit_risk.py's holding MED
 //   codes files-incomplete (:938) and evidence-unbound:<lane> (:978-998), and
 //   the manifest review (:598). Not the reader's `unreadable: …` score.
diff --git a/.github/scripts/pr-merge/lane-causes-person-codes.test.mjs b/.github/scripts/pr-merge/lane-causes-person-codes.test.mjs
new file mode 100644
index 0000000000..e598f54768
--- /dev/null
+++ b/.github/scripts/pr-merge/lane-causes-person-codes.test.mjs
@@ -0,0 +1,41 @@
+// Expected values come from the invariant in clear-escalations.mjs's LANE_CAUSES
+// comment: the causes the lanes lift are never ones only a person clears. The
+// person set is read from babysit_risk.py (PERSON_CODES, PERSON_PREFIXES), the
+// one list the merge gate reads.
+import assert from "node:assert/strict";
+import { spawnSync } from "node:child_process";
+import { test } from "node:test";
+import { fileURLToPath } from "node:url";
+
+import { LANE_CAUSES } from "./clear-escalations.mjs";
+
+const scripts = fileURLToPath(new URL("../../../plugins/source-control/skills/babysit-prs/scripts", import.meta.url));
+const read = "import json,sys\nsys.path.insert(0, sys.argv[1])\nimport babysit_risk as b\nprint(json.dumps({'codes': sorted(b.PERSON_CODES), 'prefixes': list(b.PERSON_PREFIXES)}))";
+
+function personSets() {
+  const python = spawnSync("python3", ["-I", "-c", read, scripts], { encoding: "utf8" });
+  assert.equal(python.status, 0, python.stderr);
+  return JSON.parse(python.stdout);
+}
+
+test("the person set was read from babysit_risk", () => {
+  const { codes, prefixes } = personSets();
+  assert.ok(codes.length > 0 && codes.every((code) => code !== ""), "PERSON_CODES is empty");
+  assert.ok(prefixes.length > 0 && prefixes.every((prefix) => prefix !== ""), "PERSON_PREFIXES is empty");
+});
+
+test("no LANE_CAUSES.risk entry is a reason only a person clears", () => {
+  const { codes, prefixes } = personSets();
+  assert.ok(LANE_CAUSES.risk.length > 0, "LANE_CAUSES.risk is empty");
+  for (const entry of LANE_CAUSES.risk) {
+    if (entry.endsWith(":")) {
+      // A prefix entry matches every code that starts with it.
+      const overlapping = [...codes, ...prefixes].filter((person) => person.startsWith(entry) || entry.startsWith(person));
+      assert.deepEqual(overlapping, [], `lane prefix ${entry} overlaps person ${overlapping}`);
+    } else {
+      assert.ok(!codes.includes(entry), `lane cause ${entry} is a person code`);
+      const covering = prefixes.filter((prefix) => entry.startsWith(prefix));
+      assert.deepEqual(covering, [], `lane cause ${entry} starts with person prefix ${covering}`);
+    }
+  }
+});