diff --git a/scripts/dev/gateguard-eval.js b/scripts/dev/gateguard-eval.js
index 261595600..70085d107 100644
--- a/scripts/dev/gateguard-eval.js
+++ b/scripts/dev/gateguard-eval.js
@@ -62,6 +62,9 @@ function validateStep(scenario, step, index) {
   if (!step.payload || typeof step.payload !== 'object') fail(`${where}: missing payload`);
   if (typeof step.payload.tool_name !== 'string') fail(`${where}: payload.tool_name must be a string`);
   if (!EXPECTATIONS.has(step.expect)) fail(`${where}: expect must be deny or allow`);
+  if (step.expectOnWindows !== undefined && step.expectOnWindows !== 'deny') {
+    fail(`${where}: expectOnWindows may only strengthen the expectation to deny`);
+  }
   if (typeof step.mustDeny !== 'boolean') fail(`${where}: mustDeny must be a boolean`);
   if (typeof step.redundant !== 'boolean') fail(`${where}: redundant must be a boolean`);
   if (step.mustDeny && step.expect !== 'deny') fail(`${where}: mustDeny requires expect deny`);
@@ -86,7 +89,7 @@ function validateScenario(scenario, file) {
 }
 
 /** Load and validate every scenario file in a corpus directory, sorted by file name. */
-function loadCorpus(dir = DEFAULT_CORPUS_DIR) {
+function loadCorpus(dir = DEFAULT_CORPUS_DIR, platform = process.platform) {
   return fs
     .readdirSync(dir)
     .filter(name => name.endsWith('.json'))
@@ -94,7 +97,12 @@ function loadCorpus(dir = DEFAULT_CORPUS_DIR) {
     .map(name => {
       const scenario = JSON.parse(fs.readFileSync(path.join(dir, name), 'utf8'));
       validateScenario(scenario, name);
-      return { ...scenario, file: name };
+      // Folded Windows filenames cannot prove case-sensitive type coverage.
+      // Normalize the shared corpus so assertions, summaries and SARIF agree.
+      const steps = scenario.steps.map(step => platform === 'win32' && step.expectOnWindows === 'deny'
+        ? { ...step, expect: 'deny', mustDeny: true, redundant: false }
+        : step);
+      return { ...scenario, steps, file: name };
     });
 }
 
diff --git a/tests/fixtures/gateguard-scenarios/21-bypass-filter-forms.json b/tests/fixtures/gateguard-scenarios/21-bypass-filter-forms.json
index d4bdc73e1..a79358630 100644
--- a/tests/fixtures/gateguard-scenarios/21-bypass-filter-forms.json
+++ b/tests/fixtures/gateguard-scenarios/21-bypass-filter-forms.json
@@ -1022,6 +1022,7 @@
     },
     {
       "id": "rg-type-includes-target",
+      "expectOnWindows": "deny",
       "transcript": [
         {
           "type": "user",
diff --git a/tests/scripts/gateguard-eval.test.js b/tests/scripts/gateguard-eval.test.js
index 690c888e8..fed1c19c8 100644
--- a/tests/scripts/gateguard-eval.test.js
+++ b/tests/scripts/gateguard-eval.test.js
@@ -67,6 +67,44 @@ function results(log) {
 
 console.log('\n=== Testing gateguard-eval ===\n');
 
+test('the corpus preserves POSIX type credit and requires denial for folded Windows names', () => {
+  const posix = loadCorpus(undefined, 'linux');
+  const windows = loadCorpus(undefined, 'win32');
+  const typeStep = corpus => corpus.find(scenario => scenario.name === 'bypass-filter-forms')
+    .steps.find(candidate => candidate.id === 'rg-type-includes-target');
+  assert.strictEqual(typeStep(posix).expect, 'allow');
+  assert.strictEqual(typeStep(posix).redundant, true);
+  assert.strictEqual(typeStep(windows).expect, 'deny');
+  assert.strictEqual(typeStep(windows).mustDeny, true);
+  assert.strictEqual(typeStep(windows).redundant, false);
+  posix.forEach((scenario, index) => scenario.steps.forEach((candidate, at) => {
+    if (candidate.mustDeny) assert.deepStrictEqual(windows[index].steps[at], candidate);
+  }));
+  const scenario = windows.find(candidate => candidate.name === 'bypass-filter-forms');
+  const expected = typeStep(windows);
+  const run = decision => [{ scenario: { ...scenario, steps: [expected] }, results: [{
+    decision, kind: decision === 'deny' ? 'deny' : 'credit', latencyMs: 0, reason: ''
+  }] }];
+  assert.strictEqual(summarize(run('deny')).totals.mismatches, 0);
+  assert.strictEqual(summarize(run('deny')).totals.redundantDenials, 0);
+  assert.strictEqual(summarize(run('allow')).totals.mustDenyBypasses, 1);
+});
+
+test('Windows corpus overrides only strengthen denial and reject invalid values on every host', () => {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'gateguard-platform-corpus-'));
+  try {
+    const scenario = loadCorpus()[0];
+    for (const expectOnWindows of ['allow', 'invalid', false]) {
+      fs.writeFileSync(path.join(dir, 'fixture.json'), JSON.stringify({
+        ...scenario, steps: [{ ...scenario.steps[0], expectOnWindows }]
+      }));
+      for (const platform of ['linux', 'win32']) assert.throws(() => loadCorpus(dir, platform), /expectOnWindows/);
+    }
+  } finally {
+    fs.rmSync(dir, { recursive: true, force: true });
+  }
+});
+
 test('--sarif takes a file path and keeps the other options', () => {
   const options = parseArgs(['--json', '--sarif', 'out/gate.sarif', '--baseline', 'main']);
   assert.strictEqual(options.format, 'json');