diff --git a/src/__tests__/test-file-size-ratchet.test.ts b/scripts/__tests__/test-file-size-ratchet.test.ts
similarity index 99%
rename from src/__tests__/test-file-size-ratchet.test.ts
rename to scripts/__tests__/test-file-size-ratchet.test.ts
index ec134c7ab5..c2dc784c3a 100644
--- a/src/__tests__/test-file-size-ratchet.test.ts
+++ b/scripts/__tests__/test-file-size-ratchet.test.ts
@@ -1,8 +1,8 @@
 import fs from 'node:fs';
 import path from 'node:path';
 import { expect, test } from 'vitest';
-import { walkFiles } from '../../scripts/lib/walk-files.ts';
-import { runCmdSync } from '../utils/exec.ts';
+import { walkFiles } from '../lib/walk-files.ts';
+import { runCmdSync } from '../../src/utils/exec.ts';
 
 /**
  * Test-file size ratchet (AGENTS.md "Scope & shape": past 1,000 lines is architecture debt,
diff --git a/scripts/mutation/modules.test.ts b/scripts/mutation/modules.test.ts
index 898c74ddc7..fd113ace37 100644
--- a/scripts/mutation/modules.test.ts
+++ b/scripts/mutation/modules.test.ts
@@ -6,6 +6,7 @@ import { fileURLToPath } from 'node:url';
 import {
   affectedModules,
   ALL_MODULE_IDS,
+  isKernelTestFile,
   KERNEL_MODULES,
   moduleForFile,
   mutateGlobs,
@@ -75,3 +76,16 @@ test('the shard matrix slices only the modules that declare shards', () => {
     );
   }
 });
+
+test('a kernel test is reachable only under root or package src', () => {
+  // Load-bearing for where `test-file-size-ratchet.test.ts` lives. It measures the
+  // repository's own files and git history, neither of which Stryker's sandbox copy can
+  // answer, so it sits in `scripts/__tests__/` (explicitly included by `unit-core`) and
+  // this predicate is what keeps it out of every mutation lane. Widening the pattern to
+  // scripts/ would silently pull it back in and fail every dry run.
+  assert.ok(isKernelTestFile('src/daemon/__tests__/ref-frame.test.ts'));
+  assert.ok(isKernelTestFile('packages/selectors/src/parse.test.ts'));
+  assert.ok(!isKernelTestFile('scripts/__tests__/test-file-size-ratchet.test.ts'));
+  assert.ok(!isKernelTestFile('test/integration/daemon.test.ts'));
+  assert.ok(!isKernelTestFile('src/daemon/ref-frame.ts'));
+});
diff --git a/test/integration/provider-scenarios/android-settings-contract.ts b/test/integration/provider-scenarios/android-settings-contract.ts
index c0f4b20048..ad7d0abc68 100644
--- a/test/integration/provider-scenarios/android-settings-contract.ts
+++ b/test/integration/provider-scenarios/android-settings-contract.ts
@@ -7,7 +7,7 @@ type AndroidSettingsWorld = Awaited<ReturnType<typeof createAndroidSettingsWorld
  * The adb evidence the Android settings flow must leave behind: the appearance, location,
  * fingerprint, permission, animation-scale, and network mutations each named with the exact
  * argv the provider saw. Extracted from `android-lifecycle.test.ts`, which sits over the
- * test-file tripwire (`src/__tests__/test-file-size-ratchet.test.ts`).
+ * test-file tripwire (`scripts/__tests__/test-file-size-ratchet.test.ts`).
  */
 export function assertAndroidSettingsContract(world: AndroidSettingsWorld): void {
   const { adbCalls } = world;
diff --git a/vitest.config.ts b/vitest.config.ts
index 26ab5634e1..3e85d1f5dc 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -73,6 +73,13 @@ export default defineConfig({
             'scripts/__tests__/help-conformance-expectation-falsification.test.ts',
             'scripts/__tests__/help-conformance-sample-outputs.test.ts',
             'scripts/__tests__/help-conformance-topic-coverage.test.ts',
+            // Lives here rather than under src/ on purpose: it measures the repository's own
+            // files and git history, which Stryker's sandbox copy cannot answer (the copies are
+            // rewritten, and there is no origin/main). The mutation lane admits only root/package
+            // `src` tests, so this address makes it unreachable there by construction instead of
+            // by a classifier that has to recognise it — see KERNEL_TEST_FILE_RE in
+            // scripts/mutation/modules.ts.
+            'scripts/__tests__/test-file-size-ratchet.test.ts',
             'scripts/__tests__/agent-setup-startup-contract.test.ts',
             'scripts/__tests__/npm-skills-exclusion.test.ts',
             'scripts/__tests__/simulator-skills-contract.test.ts',