diff --git a/packages/fs/fs-sandbox/src/containment.ts b/packages/fs/fs-sandbox/src/containment.ts
index 782ceb151d..41b9bdd08a 100644
--- a/packages/fs/fs-sandbox/src/containment.ts
+++ b/packages/fs/fs-sandbox/src/containment.ts
@@ -9,9 +9,11 @@ import type { BigIntStats } from 'node:fs'
 import { stat } from 'node:fs/promises'
 import { dirname, sep } from 'node:path'
 
+const MISSING_CODES: ReadonlySet<NodeJS.ErrnoException['code']> = new Set(['ENOENT', 'ENOTDIR'])
+
 function isMissing(error: unknown): boolean {
   const code = (error as NodeJS.ErrnoException).code
-  return code === 'ENOENT' || code === 'ENOTDIR'
+  return MISSING_CODES.has(code)
 }
 
 function comparablePath(path: string, caseSensitive: boolean): string {
diff --git a/packages/fs/fs-sandbox/tests/containment.spec.ts b/packages/fs/fs-sandbox/tests/containment.spec.ts
index 30821cc826..35dc52029b 100644
--- a/packages/fs/fs-sandbox/tests/containment.spec.ts
+++ b/packages/fs/fs-sandbox/tests/containment.spec.ts
@@ -27,6 +27,7 @@ describe('filesystem sandbox containment', () => {
 
   it('uses case-insensitive lexical comparison for Windows-style containment', async () => {
     expect(await isPathUnder(join(base.toUpperCase(), 'child'), base.toLowerCase(), false)).toBe(true)
+    expect(await isPathUnder(join(base, 'case-sensitive-child'), base, true)).toBe(true)
   })
 
   it('recognizes an alias-equivalent root by filesystem identity for a missing target', async () => {