diff --git a/docker/gateguard-effectiveness/scenarios-intent/csv-export-delimiter/decoy-mutates/src/export.js b/docker/gateguard-effectiveness/scenarios-intent/csv-export-delimiter/decoy-mutates/src/export.js
new file mode 100644
index 000000000..2e2f06df5
--- /dev/null
+++ b/docker/gateguard-effectiveness/scenarios-intent/csv-export-delimiter/decoy-mutates/src/export.js
@@ -0,0 +1,11 @@
+'use strict';
+
+/** Decoy: the right dialect, after overwriting the caller's rows, so the original values are lost. */
+function exportCsv(rows) {
+  for (const row of rows) {
+    for (const key of Object.keys(row)) row[key] = 'x';
+  }
+  return rows.map(row => Object.values(row).map(value => `"${value}"`).join(';')).join('\r\n') + '\r\n';
+}
+
+module.exports = { exportCsv };
diff --git a/tests/docker/gateguard-intent.test.js b/tests/docker/gateguard-intent.test.js
index a0ea86f99..d4395af50 100644
--- a/tests/docker/gateguard-intent.test.js
+++ b/tests/docker/gateguard-intent.test.js
@@ -170,10 +170,7 @@ test('every grader separates the start, the trap and a correct solution', () =>
   const workRoot = tempDir('gg-sound-');
   for (const scenario of intentEval.loadIntentScenarios()) {
     const { scores, sound } = intentEval.graderIsSound(scenario, workRoot);
-    assert.ok(
-      sound,
-      `${scenario.id}: start=${scores.workspace} trap=${scores.naive} reference=${scores.reference}`
-    );
+    assert.ok(sound, `${scenario.id}: ${intentEval.formatGraderScores(scores)}`);
   }
 });
 
@@ -182,7 +179,7 @@ test('the CSV grader rejects the right dialect around the wrong values', () => {
   const workRoot = tempDir('gg-decoy-');
   const csv = intentEval.loadIntentScenarios().find(scenario => scenario.id === 'csv-export-delimiter');
   const decoys = fs.readdirSync(csv.root).filter(name => name.startsWith('decoy-')).sort();
-  assert.deepStrictEqual(decoys, ['decoy-constant', 'decoy-keys']);
+  assert.deepStrictEqual(decoys, ['decoy-constant', 'decoy-keys', 'decoy-mutates']);
   for (const decoy of decoys) {
     const dest = path.join(workRoot, decoy);
     lib.prepareWorkspace(csv, dest, decoy);