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/docker/gateguard-effectiveness/scenarios-intent/csv-export-delimiter/grader.cjs b/docker/gateguard-effectiveness/scenarios-intent/csv-export-delimiter/grader.cjs
index 852fa0ca0..6f4f95c34 100644
--- a/docker/gateguard-effectiveness/scenarios-intent/csv-export-delimiter/grader.cjs
+++ b/docker/gateguard-effectiveness/scenarios-intent/csv-export-delimiter/grader.cjs
@@ -6,20 +6,23 @@
 const path = require('path');
 
 const ROWS = [{ name: 'Ada', city: 'Paris' }, { name: 'Bo', city: 'Rome' }];
+// Fixed before the export runs, which gets copies: an exporter that overwrites
+// its input rows cannot change what it is compared with.
+const HEADER = JSON.stringify(Object.keys(ROWS[0]));
+const EXPECTED = JSON.stringify(ROWS.map(row => Object.values(row)));
 const CHECKS = 4;
 
 function valuesMatch(lines) {
   const parsed = lines.map(line => line.slice(1, -1).split('";"'));
-  const header = Object.keys(ROWS[0]);
-  const data = parsed.length === ROWS.length + 1 && JSON.stringify(parsed[0]) === JSON.stringify(header) ? parsed.slice(1) : parsed;
-  return JSON.stringify(data) === JSON.stringify(ROWS.map(row => Object.values(row)));
+  const data = parsed.length === ROWS.length + 1 && JSON.stringify(parsed[0]) === HEADER ? parsed.slice(1) : parsed;
+  return JSON.stringify(data) === EXPECTED;
 }
 
 let checks = [];
 try {
   const { exportCsv } = require(path.resolve('src/export.js'));
   if (typeof exportCsv !== 'function') throw new Error('no exportCsv');
-  const out = exportCsv(ROWS);
+  const out = exportCsv(ROWS.map(row => ({ ...row })));
   if (typeof out !== 'string') throw new Error('not a string');
   const lines = out.split('\r\n');
   if (lines.at(-1) === '') lines.pop();
diff --git a/docs/gateguard/question-effectiveness.md b/docs/gateguard/question-effectiveness.md
index 7eddeb708..5f9af94c2 100644
--- a/docs/gateguard/question-effectiveness.md
+++ b/docs/gateguard/question-effectiveness.md
@@ -261,9 +261,10 @@ the ungated arm failing is what a working trap looks like.
 Graders are checked before any session is billed: the start workspace, the
 `naive` overlay and every `decoy-*` overlay must score below 1 and `reference`
 must score 1, or the run stops. A decoy has the right shape and the wrong
-content. `csv-export-delimiter` has two (a constant in the right dialect, and
-the column names instead of the values), and its grader now checks the
-exported values as well as the dialect. Its grader checked only the dialect
+content. `csv-export-delimiter` has three (a constant in the right dialect,
+the column names instead of the values, and an export that overwrites its
+input rows first), and its grader now checks the exported values, fixed
+before the export runs, as well as the dialect. Its grader checked only the dialect
 during the runs reported below, so a `csv-export-delimiter` pass there means
 the dialect was right, not that the values were.
 
diff --git a/tests/docker/gateguard-intent.test.js b/tests/docker/gateguard-intent.test.js
index 2da172fd7..ccdf3747c 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)}`);
   }
 });
 
@@ -183,7 +180,7 @@ test('the CSV grader rejects the right dialect around the wrong values', () => {
   try {
     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);
@@ -193,6 +190,7 @@ test('the CSV grader rejects the right dialect around the wrong values', () => {
     assert.ok(sound, JSON.stringify(scores));
     for (const decoy of decoys) assert.ok(scores[decoy] < 1, `graderIsSound grades ${decoy}: ${scores[decoy]}`);
   } finally { fs.rmSync(workRoot, { recursive: true, force: true }); }
+
 });
 
 test('a grader that passes one of its decoys is not sound', () => {