diff --git a/packages/qa/dogfood/test/per-file-cwd.global-setup.ts b/packages/qa/dogfood/test/per-file-cwd.global-setup.ts
index eaa656093d..284a6f6c9e 100644
--- a/packages/qa/dogfood/test/per-file-cwd.global-setup.ts
+++ b/packages/qa/dogfood/test/per-file-cwd.global-setup.ts
@@ -10,22 +10,34 @@
 //      left by a developer's earlier run on a tree without this isolation, or by
 //      a crashed run. The guard judges only what THIS run leaves, so an old
 //      leftover never reds a run that wrote nothing.
-//   2. It creates ONE temporary root for the run and hands it to every worker
-//      through `provide` / `inject`. Each test file makes its own working
-//      directory under that root.
+//   2. It reserves a TAG for the run, `os-dogfood-run-XXXXXX`, as a directory
+//      `mkdtempSync` creates under the system temp directory, and hands the tag
+//      (a name, never a path) to every worker through `provide` / `inject`.
+//      Each test file makes its own working directory directly under the system
+//      temp directory, named `<tag>-file-XXXXXX`.
 //
-// At the END of the run it removes that root, and with it every per-file
-// directory. The removal is run-level, not per-file: on the `shared-showcase`
-// project (`isolate: false`) one memoized boot serves every file on a worker,
-// and its SQLite handles stay open in the directory of the file that booted it.
+// At the END of the run it removes every directory whose name starts with this
+// run's `<tag>-file-`, then the reservation itself. Another run's directories
+// carry another tag, so a concurrent run on the same machine is never touched.
+// The removal is run-level, not per-file: on the `shared-showcase` project
+// (`isolate: false`) one memoized boot serves every file on a worker, and its
+// SQLite handles stay open in the directory of the file that booted it.
+//
+// Why a tag and not a shared parent path (#21924): every `mkdtempSync` base in
+// this tree must be one the tree's scratch-directory scan can read, so that an
+// in-tree fixture root can never hide behind an expression
+// (`scripts/pm/dispatch-gates.mjs`, "no mkdtempSync site in this tree takes a
+// base the scan cannot read"). A path handed over through `inject()` is such an
+// expression. `join(tmpdir(), ...)` is not: it is outside the tree by
+// construction, whatever name follows it.
 //
 // ⛔ This teardown never JUDGES anything. On vitest 4.1.11 an error thrown from
 // a globalSetup teardown is printed as `error during close` and the run still
 // exits 0 (measured), so a guard placed here would be a false green. The guard
 // is a throwing `afterAll` in the per-file module, which fails a test file.
-import { mkdtempSync, rmSync } from 'node:fs';
+import { mkdtempSync, readdirSync, rmSync } from 'node:fs';
 import { tmpdir } from 'node:os';
-import { join } from 'node:path';
+import { basename, join } from 'node:path';
 import { fileURLToPath } from 'node:url';
 import type { TestProject } from 'vitest/node';
 
@@ -34,19 +46,29 @@ const PACKAGE_ROOT = fileURLToPath(new URL('..', import.meta.url));
 
 declare module 'vitest' {
   export interface ProvidedContext {
-    /** The run's temporary root; each test file makes its working directory under it. */
-    dogfoodCwdRoot: string;
+    /** The run's tag; each test file makes its working directory as `join(tmpdir(), '<tag>-file-')`. */
+    dogfoodRunTag: string;
   }
 }
 
-let runRoot: string | undefined;
+/** The prefix of every per-file directory a run tagged `tag` creates under the system temp directory. */
+export function perFileDirPrefix(tag: string): string {
+  return `${tag}-file-`;
+}
+
+let reservation: string | undefined;
 
 export function setup(project: TestProject): void {
   rmSync(join(PACKAGE_ROOT, '.objectstack'), { recursive: true, force: true });
-  runRoot = mkdtempSync(join(tmpdir(), 'os-dogfood-run-'));
-  project.provide('dogfoodCwdRoot', runRoot);
+  reservation = mkdtempSync(join(tmpdir(), 'os-dogfood-run-'));
+  project.provide('dogfoodRunTag', basename(reservation));
 }
 
 export function teardown(): void {
-  if (runRoot) rmSync(runRoot, { recursive: true, force: true });
+  if (!reservation) return;
+  const prefix = perFileDirPrefix(basename(reservation));
+  for (const name of readdirSync(tmpdir())) {
+    if (name.startsWith(prefix)) rmSync(join(tmpdir(), name), { recursive: true, force: true });
+  }
+  rmSync(reservation, { recursive: true, force: true });
 }
diff --git a/packages/qa/dogfood/test/per-file-cwd.setup.ts b/packages/qa/dogfood/test/per-file-cwd.setup.ts
index 0991eb9c55..dc1d3de3d4 100644
--- a/packages/qa/dogfood/test/per-file-cwd.setup.ts
+++ b/packages/qa/dogfood/test/per-file-cwd.setup.ts
@@ -19,10 +19,18 @@
 // ## What it does
 //
 // At module top level, which runs before the test file's own imports, it makes
-// a directory under the run's temporary root and `chdir`s into it. `afterAll`
-// restores the previous working directory. The directories are removed at the
-// end of the run by the globalSetup, not here: the memoized `shared-showcase`
-// boot keeps its SQLite handles open in the first file's directory.
+// a directory directly under the system temp directory, named with the run's
+// tag (`<tag>-file-XXXXXX`), and `chdir`s into it. `afterAll` restores the
+// previous working directory. The directories are removed at the end of the
+// run by the globalSetup, which sweeps its own tag, not here: the memoized
+// `shared-showcase` boot keeps its SQLite handles open in the first file's
+// directory.
+//
+// The base is spelled `join(tmpdir(), ...)` on purpose (#21924): the tree's
+// scratch-directory scan must be able to read every `mkdtempSync` base, and a
+// path received through `inject()` is one it cannot read. Only the run's TAG
+// comes through `inject()`, as a name component, and it is refused below if
+// it could carry a separator.
 //
 // The invariant for every dogfood author: a file runs in its own temporary
 // cwd, so anything cwd-relative it writes is its own and disappears with the
@@ -38,26 +46,31 @@
 // what this run leaves.
 import { afterAll, inject } from 'vitest';
 import { existsSync, mkdtempSync, readdirSync } from 'node:fs';
+import { tmpdir } from 'node:os';
 import { join } from 'node:path';
 import { fileURLToPath } from 'node:url';
+import { perFileDirPrefix } from './per-file-cwd.global-setup.js';
 
 /** `packages/qa/dogfood`, resolved from this module's own location. */
 const PACKAGE_ROOT = fileURLToPath(new URL('..', import.meta.url));
 /** What a file must never leave in the package directory. */
 const LEFTOVER = join(PACKAGE_ROOT, '.objectstack', 'data');
 
-const runRoot = inject('dogfoodCwdRoot');
-if (!runRoot) {
+const runTag = inject('dogfoodRunTag');
+if (!runTag) {
   throw new Error(
-    'per-file-cwd.setup.ts: no run root was provided. The globalSetup ' +
+    'per-file-cwd.setup.ts: no run tag was provided. The globalSetup ' +
       '`test/per-file-cwd.global-setup.ts` must be wired in packages/qa/dogfood/vitest.config.ts; ' +
       'without it this file would run in the package directory.',
   );
 }
+if (/[\\/]|\.\./.test(runTag)) {
+  throw new Error(`per-file-cwd.setup.ts: the run tag ${JSON.stringify(runTag)} is not a plain directory name.`);
+}
 
 const previousCwd = process.cwd();
 const presentAtStart = existsSync(LEFTOVER);
-process.chdir(mkdtempSync(join(runRoot, 'file-')));
+process.chdir(mkdtempSync(join(tmpdir(), perFileDirPrefix(runTag))));
 
 afterAll(() => {
   process.chdir(previousCwd);
diff --git a/packages/qa/dogfood/vitest.config.ts b/packages/qa/dogfood/vitest.config.ts
index bbef0be21e..841365bfeb 100644
--- a/packages/qa/dogfood/vitest.config.ts
+++ b/packages/qa/dogfood/vitest.config.ts
@@ -143,7 +143,7 @@ runProjectCliOverridePreflight({
 //     `.objectstack/data` exists in the package directory: that throw is the guard.
 //   - The `globalSetup` below is ROOT-level: one run, one call, covering both
 //     projects and each `OS_TEST_SHARD` slice (measured). It clears a stale
-//     `.objectstack` at the start and removes the run's temporary root at the end.
+//     `.objectstack` at the start and removes the run's per-file directories at the end.
 //     Its teardown judges nothing, because a throw there exits 0 on vitest 4.1.11.
 // Both modules' headers carry the rest, including what a dogfood author owes.
 const PER_FILE_CWD = './test/per-file-cwd.setup.ts';