diff --git a/docs/REVIEW_LOG.md b/docs/REVIEW_LOG.md
index 1996e6b..53110d4 100644
--- a/docs/REVIEW_LOG.md
+++ b/docs/REVIEW_LOG.md
@@ -7791,3 +7791,47 @@ rounds"* fails, the ratchet trips, **the cited-figure-with-duplicate case is now
 the clean tree is silent.
 
 **State:** 21 guards, one merged reader, CLEAN GROUND v0.24.
+
+---
+
+## R-309 — a guard that stopped another guard from running
+
+**The fold exported `scanDocument` so the readers could be tested on strings, and for two
+commits importing it ran the whole guard.** `check-behaviour` imports that module. So:
+
+```
+$ # with the historical 24 restored
+$ node tools/check-behaviour.mjs
+check-figures: FAILED — measured figures are printed with nothing holding them
+  docs/scenarios/CLEAN_GROUND.md:1277 — "**24**" carries no citation marker …
+exit=1
+```
+
+**A figure defect called `process.exit(1)` inside check-behaviour, which reported another
+guard's failure under its own name having run zero of its 108 tests.** The build still went
+red, which is the only reason this was survivable — but it went red in the wrong place, and
+every behavioural test in the repo was silently not running while appearing to.
+
+**This is the sharpest form yet of the fault this log keeps recording.** R-300 found a check
+that passed for a reason unrelated to what it claimed. This is a check that *did not run at
+all* and could not say so, because the thing that stopped it was a different guard's verdict
+arriving through an import. A guard is a claim we are still checking; this one had quietly
+stopped being checked, and the evidence that it had was indistinguishable from a failure.
+
+**Fixed with the idiom already in `step5-split.mjs`** — the executable body sits behind
+`import.meta.main || process.argv[1]?.endsWith(...)`, so importing the module yields the four
+readers and nothing else.
+
+**The test is a spawn, not a source assertion.** The property is *importing has no effect*,
+and only a fresh process can tell you that; asserting the source contains the idiom would
+pass on a file that had grown a second side effect elsewhere. Importing must print nothing
+and exit 0.
+
+**Proved both ways.** Reverting the guard to `if (true)` fails exactly one test — the new one.
+With a figure defect planted, `check-behaviour` now runs all 108 tests green and
+`check-figures` fails in its own slot with its own message.
+
+**It arrived with the fold and it is mine.** It exists because R-307 needed an export for the
+string tests, and it was found because those same tests print their own name — the second
+time in one evening that the string tests paid for themselves in a way nobody designed. The
+general rule for this repo: **a module a guard imports must not be a guard that runs.**
diff --git a/tools/check-behaviour.mjs b/tools/check-behaviour.mjs
index 03a7fa9..4115c2b 100644
--- a/tools/check-behaviour.mjs
+++ b/tools/check-behaviour.mjs
@@ -33,6 +33,7 @@ import { STARTER_AUTHORITY, STARTER_GROUPS, STARTER_CASE, STARTER_TEAM }
   from "./scenario-starter.mjs";
 import { castMarkersIn, castLikeIn } from "./declared-cast.mjs";
 import { scanDocument } from "./check-figures.mjs";
+import { spawnSync } from "node:child_process";
 import { tagsIn, classifiedFiles, playableFiles } from "./check-scenarios.mjs";
 import { beatsIn, beatLikeIn } from "./outcome-coverage.mjs";
 import { step5Split, split as step5Of, THING_RATING } from "./step5-split.mjs";
@@ -999,6 +1000,22 @@ test("a document that cites nothing is held by the ratchet, not the strict regim
   assert.ok(r.unmarked >= 1, "but its bare figures are still counted for the ratchet");
 });
 
+test("importing check-figures runs the readers and NOT the guard", () => {
+  /* THE REGRESSION THIS EXISTS FOR: for one commit the guard's body ran at import, so a
+     figure defect called process.exit(1) inside THIS file — check-behaviour reporting
+     check-figures' failure under its own name, having run none of its tests. Spawned rather
+     than asserted on the source, because the property is "importing has no effect", and only
+     a fresh process can tell you that. */
+  const mod = new URL("./check-figures.mjs", import.meta.url).href;
+  const r = spawnSync(process.execPath,
+    ["--input-type=module", "-e", `await import(${JSON.stringify(mod)});`],
+    { encoding: "utf8" });
+  assert.equal(r.status, 0, "importing check-figures must not exit non-zero");
+  assert.equal((r.stdout + r.stderr).trim(), "",
+    "importing check-figures must print nothing — the guard's body belongs behind the "
+    + "import.meta.main check, or a figure defect stops this file mid-run");
+});
+
 test("the reader reports the line the figure is actually on", () =>
   assert.match(firstFault(doc("one\ntwo\nwipes **8.3%** here")), /x\.md:3/));
 
diff --git a/tools/check-figures.mjs b/tools/check-figures.mjs
index c6cc718..b7853a0 100644
--- a/tools/check-figures.mjs
+++ b/tools/check-figures.mjs
@@ -370,82 +370,92 @@ export function scanDocument(label, src) {
   return { rows, strict, waived, unmarked, optedIn };
 }
 
-const strict = [];
-const loose = {};
-const rows = [];
-const waived = [];
+/* ── running the guard ─────────────────────────────────────────────────────────────────
+   EVERYTHING BELOW RUNS ONLY WHEN THIS FILE IS THE ENTRY POINT, and the reason is not tidiness.
+   `scanDocument` is exported so check-behaviour can test the readers on strings (R-292), and
+   for one commit importing it ALSO ran the whole guard: a figure defect called `process.exit(1)`
+   inside check-behaviour, which then reported check-figures' failure under its own name having
+   run ZERO of its 107 tests. A guard that stops another guard from running while appearing to
+   participate is the worst version of the fault this file exists to catch. Same idiom as
+   step5-split.mjs. */
+if (import.meta.main || process.argv[1]?.endsWith("check-figures.mjs")) {
+  const strict = [];
+  const loose = {};
+  const rows = [];
+  const waived = [];
 
-for (const [label, abs] of playableFiles()) {
-  const r = scanDocument(label, readFileSync(abs, "utf8"));
-  rows.push(...r.rows); strict.push(...r.strict); waived.push(...r.waived);
-  if (!r.optedIn) loose[label] = r.unmarked;
-}
+  for (const [label, abs] of playableFiles()) {
+    const r = scanDocument(label, readFileSync(abs, "utf8"));
+    rows.push(...r.rows); strict.push(...r.strict); waived.push(...r.waived);
+    if (!r.optedIn) loose[label] = r.unmarked;
+  }
 
-/* ── the ratchet, for documents outside the citation regime ────────────────────────────── */
+  /* ── the ratchet, for documents outside the citation regime ────────────────────────────── */
 
-const base = existsSync(BASELINE) ? JSON.parse(readFileSync(BASELINE, "utf8")) : null;
+  const base = existsSync(BASELINE) ? JSON.parse(readFileSync(BASELINE, "utf8")) : null;
 
-if (UPDATE) {
-  writeFileSync(BASELINE, JSON.stringify({
-    note: "Unmarked measurement figures in scenarios that use no citations. May fall, never rise. "
-        + "Generated by tools/check-figures.mjs --update.",
-    uncited: loose
-  }, null, 1) + "\n");
-  console.log(`check-figures: recorded ${Object.keys(loose).length} document(s) outside the citation regime`);
-  process.exit(0);
-}
+  if (UPDATE) {
+    writeFileSync(BASELINE, JSON.stringify({
+      note: "Unmarked measurement figures in scenarios that use no citations. May fall, never rise. "
+          + "Generated by tools/check-figures.mjs --update.",
+      uncited: loose
+    }, null, 1) + "\n");
+    console.log(`check-figures: recorded ${Object.keys(loose).length} document(s) outside the citation regime`);
+    process.exit(0);
+  }
 
-const problems = [...strict];
+  const problems = [...strict];
 
-if (!base) {
-  problems.push(`  no ratchet at tools/figures-baseline.json — run: node tools/check-figures.mjs --update`);
-} else {
-  for (const [label, n] of Object.entries(loose)) {
-    const was = base.uncited?.[label];
-    if (was === undefined) {
-      problems.push(`  ${label} uses no citations and is not in the ratchet. Re-record with --update, `
-        + `or give it its first citation marker and it joins the strict regime.`);
-    } else if (n > was) {
-      problems.push(`  ${label}: unmarked measurement figures went ${was} -> ${n}. This document cites `
-        + `nothing, so the ratchet is all that holds it: the count may fall, never rise.`);
+  if (!base) {
+    problems.push(`  no ratchet at tools/figures-baseline.json — run: node tools/check-figures.mjs --update`);
+  } else {
+    for (const [label, n] of Object.entries(loose)) {
+      const was = base.uncited?.[label];
+      if (was === undefined) {
+        problems.push(`  ${label} uses no citations and is not in the ratchet. Re-record with --update, `
+          + `or give it its first citation marker and it joins the strict regime.`);
+      } else if (n > was) {
+        problems.push(`  ${label}: unmarked measurement figures went ${was} -> ${n}. This document cites `
+          + `nothing, so the ratchet is all that holds it: the count may fall, never rise.`);
+      }
+    }
+    /* A document that LEAVES the loose set has gained its first citation, which is good news
+       and must not be reported as a missing row. A document that vanishes entirely has been
+       deleted or reclassified, and that is worth saying out loud. */
+    for (const label of Object.keys(base.uncited ?? {})) {
+      if (!(label in loose) && !rows.some(r => r.label === label)) {
+        problems.push(`  ${label} is in the ratchet but no longer in the corpus. If it was renamed or `
+          + `deleted, re-record with --update.`);
+      }
     }
   }
-  /* A document that LEAVES the loose set has gained its first citation, which is good news
-     and must not be reported as a missing row. A document that vanishes entirely has been
-     deleted or reclassified, and that is worth saying out loud. */
-  for (const label of Object.keys(base.uncited ?? {})) {
-    if (!(label in loose) && !rows.some(r => r.label === label)) {
-      problems.push(`  ${label} is in the ratchet but no longer in the corpus. If it was renamed or `
-        + `deleted, re-record with --update.`);
+
+  if (LIST) {
+    for (const r of rows) {
+      console.log(`  ${r.marked ? "cited " : "BARE  "} ${r.label}:${r.line}  [${r.kind}]  ${r.text}`);
     }
+    problems.forEach(p => console.log(p));
+    process.exit(0);
   }
-}
-
-if (LIST) {
-  for (const r of rows) {
-    console.log(`  ${r.marked ? "cited " : "BARE  "} ${r.label}:${r.line}  [${r.kind}]  ${r.text}`);
+  if (problems.length) {
+    console.error("check-figures: FAILED — measured figures are printed with nothing holding them");
+    problems.forEach(p => console.error(p));
+    console.error(`  check-cited resolves the markers that exist; it cannot see a figure that has none. `
+      + `That is how "a median 24 rounds" survived ten passes against a baseline holding 21.`);
+    process.exit(1);
+  }
+  /* WAIVERS ARE PRINTED ON EVERY GREEN BUILD. A figure nobody checks is the defect this guard
+     exists for, and a waived figure is one we have decided to stop checking — so the decision
+     stays on screen rather than becoming the default. Six of them is a short list; sixty would
+     be a finding. */
+  if (waived.length) {
+    console.log(`check-figures: ${waived.length} figure(s) waived, each on the record:`);
+    for (const w of waived) console.log(`    ${w.label}:${w.line}  ${w.text} — ${w.reason}`);
   }
-  problems.forEach(p => console.log(p));
-  process.exit(0);
-}
-if (problems.length) {
-  console.error("check-figures: FAILED — measured figures are printed with nothing holding them");
-  problems.forEach(p => console.error(p));
-  console.error(`  check-cited resolves the markers that exist; it cannot see a figure that has none. `
-    + `That is how "a median 24 rounds" survived ten passes against a baseline holding 21.`);
-  process.exit(1);
-}
-/* WAIVERS ARE PRINTED ON EVERY GREEN BUILD. A figure nobody checks is the defect this guard
-   exists for, and a waived figure is one we have decided to stop checking — so the decision
-   stays on screen rather than becoming the default. Six of them is a short list; sixty would
-   be a finding. */
-if (waived.length) {
-  console.log(`check-figures: ${waived.length} figure(s) waived, each on the record:`);
-  for (const w of waived) console.log(`    ${w.label}:${w.line}  ${w.text} — ${w.reason}`);
-}
 
-const marked = rows.filter(r => r.marked).length;
-const looseTotal = Object.values(loose).reduce((a, b) => a + b, 0);
-console.log(`check-figures: OK — ${rows.length} measurement-shaped figures across ${new Set(rows.map(r => r.label)).size} `
-  + `scenarios, ${marked} marked and ${waived.length} waived; ${looseTotal} bare in `
-  + `${Object.keys(loose).length} document(s) that cite nothing, held at the ratchet`);
+  const marked = rows.filter(r => r.marked).length;
+  const looseTotal = Object.values(loose).reduce((a, b) => a + b, 0);
+  console.log(`check-figures: OK — ${rows.length} measurement-shaped figures across ${new Set(rows.map(r => r.label)).size} `
+    + `scenarios, ${marked} marked and ${waived.length} waived; ${looseTotal} bare in `
+    + `${Object.keys(loose).length} document(s) that cite nothing, held at the ratchet`);
+}