@@ -651,6 +651,7 @@ import {
 } from '../qa/vitest-filter-preflight/src/index.js';
 import { integrationTestFiles, unitTestFiles } from './vitest-tiers.js';
 import { readTierMode } from '../../scripts/nightly-tiers.mjs';
+import { memoryBackedTmpEnv } from '../../scripts/vitest-memory-tmp.mjs';
 
 // The two tiers, DERIVED from what the files DO — never written down — over
 // the population `OS_TEST_TIERS` selects. `vitest-tiers.ts` holds the
@@ -989,6 +990,15 @@ export default defineConfig({
     // `server.deps.external` entry above; each repeats the console-intercept
     // disarm — and, since #17647, the registry log level — because a
     // root-level value is inert under projects.
+    //
+    // Each project's `env` also moves `os.tmpdir()` onto /dev/shm when it is
+    // writable (#27306; scripts/vitest-memory-tmp.mjs). Every SQLite database
+    // these tests migrate lives in an `mkdtemp` under it. Measured on the
+    // fifteen `organization-ownership-*` files beside four `dd … conv=fsync`
+    // writers on the same disk: 145 of 232 cases timed out and the run took
+    // 1,809 s, against 232 of 232 green in 68 s idle; with TMPDIR there, under
+    // the same writers, 232 of 232 green in 59 s. Assertions and budgets are
+    // untouched.
     projects: [
       {
         extends: true,
@@ -999,7 +1009,9 @@ export default defineConfig({
           // #13517: quiet the registry's per-item registration chatter — the
           // engine's own `OS_REGISTRY_LOG` seam, not a change to its shipped
           // default. Enforced by scripts/check-registry-log-declared.mjs.
-          env: { OS_REGISTRY_LOG: 'warn' },
+          // TMPDIR on a memory-backed directory (#27306): see the root `test`
+          // block's note above `projects`.
+          env: { OS_REGISTRY_LOG: 'warn', ...memoryBackedTmpEnv() },
           include: UNIT_FILES,
         },
       },
@@ -1012,7 +1024,9 @@ export default defineConfig({
           // #13517: quiet the registry's per-item registration chatter — the
           // engine's own `OS_REGISTRY_LOG` seam, not a change to its shipped
           // default. Enforced by scripts/check-registry-log-declared.mjs.
-          env: { OS_REGISTRY_LOG: 'warn' },
+          // TMPDIR on a memory-backed directory (#27306): see the root `test`
+          // block's note above `projects`.
+          env: { OS_REGISTRY_LOG: 'warn', ...memoryBackedTmpEnv() },
           include: INTEGRATION_FILES,
         },
       },
 
@@ -1,6 +1,7 @@
 import path from 'node:path';
 import { fileURLToPath } from 'node:url';
 import { configDefaults, defineConfig } from 'vitest/config';
+import { memoryBackedTmpEnv } from '../../scripts/vitest-memory-tmp.mjs';
 
 const HERE = path.dirname(fileURLToPath(import.meta.url));
 
@@ -38,5 +39,17 @@ export default defineConfig({
     // its `test` script.
     exclude: [...configDefaults.exclude, 'src/templates/**'],
     environment: 'node',
+    // Every scaffold these tests write lands under `os.tmpdir()`, children
+    // included (#27306; scripts/vitest-memory-tmp.mjs). The cost on a loaded
+    // disk is not fsync (four calls in the whole suite, by strace) but creating
+    // and removing those trees: ~4,000 unlinks and ~6,000 rmdirs. Measured
+    // beside four `dd … conv=fsync` writers on the same disk: all 301 cases
+    // passed, but the `afterAll` that removes the scaffolded tree in
+    // `created-summary.test.ts` and `scaffold-skills-single-copy.test.ts` ran
+    // past its 10 s hook budget, so two files failed with no failed case, and
+    // the run took 236 s against 21 s idle. With TMPDIR there, under the same
+    // writers, 19 of 19 files green in 20 s. Assertions and budgets are
+    // untouched.
+    env: memoryBackedTmpEnv(),
   },
 });
@@ -2,6 +2,7 @@
 
 import { defineConfig } from 'vitest/config';
 import path from 'path';
+import { memoryBackedTmpEnv } from '../../../scripts/vitest-memory-tmp.mjs';
 
 export default defineConfig({
   test: {
@@ -14,6 +15,15 @@ export default defineConfig({
     disableConsoleIntercept: true,
     globals: true,
     environment: 'node',
+    // TMPDIR on /dev/shm when it is writable (#27306;
+    // scripts/vitest-memory-tmp.mjs): the SQLite file tests open their
+    // databases in an `mkdtemp` under it. Measured on the 23 test files that
+    // do, beside four `dd … conv=fsync` writers on the same disk: 3 of 192
+    // cases timed out (posture conformance, reclaim-space, declared-index
+    // retired keys) and the run took 98 s, against all green in 28 s idle; with
+    // TMPDIR there, under the same writers, all green in 28 s. Assertions and
+    // budgets are untouched.
+    env: memoryBackedTmpEnv(),
     // #9350: the live-dialect files each own a schema (Postgres) / database
     // (MySQL), named in the connection so knex's `client.database()` and the
     // session cannot disagree. That naming has an ordering constraint — MySQL
 
@@ -1,7 +1,7 @@
 // Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
 
-import { accessSync, constants } from 'node:fs';
 import { defineConfig } from 'vitest/config';
+import { memoryBackedTmpEnv } from '../../../scripts/vitest-memory-tmp.mjs';
 
 // This package re-exported the shared preset until #27076. It now carries one
 // key of its own, `test.env.TMPDIR`, so the preset's one setting is restated
@@ -27,20 +27,9 @@ import { defineConfig } from 'vitest/config';
 // tmpfs the same code still runs, `sync()` included: what each case asserts
 // (bytes on disk, atomic rename, the flush chain, the failure report) is
 // unchanged; only the wait for unrelated I/O is gone. Budgets, fixtures and
-// cases are untouched.
-//
-// A host without a writable /dev/shm (macOS, Windows) keeps its own temp
-// directory: the setting is absent there, not an error.
-const MEMORY_BACKED_TMP = '/dev/shm';
-
-function writableDir(dir: string): boolean {
-  try {
-    accessSync(dir, constants.W_OK);
-    return true;
-  } catch {
-    return false;
-  }
-}
+// cases are untouched. The setting itself, and the host without a writable
+// /dev/shm, live in scripts/vitest-memory-tmp.mjs, shared since #27306 with the
+// other packages whose tests write SQLite files under `os.tmpdir()`.
 
 export default defineConfig({
   test: {
@@ -51,6 +40,6 @@ export default defineConfig({
     // Mechanism + measured costs: examples/app-showcase/vitest.config.ts.
     // Enforced repo-wide by scripts/check-console-intercept-disarm.mjs.
     disableConsoleIntercept: true,
-    env: writableDir(MEMORY_BACKED_TMP) ? { TMPDIR: MEMORY_BACKED_TMP } : {},
+    env: memoryBackedTmpEnv(),
   },
 });
@@ -1,4 +1,31 @@
 // Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
 
-// This package needs nothing beyond the shared preset; see its header.
-export { default } from '../../../vitest.preset.mts';
+import { defineConfig } from 'vitest/config';
+import { memoryBackedTmpEnv } from '../../../scripts/vitest-memory-tmp.mjs';
+
+// This package re-exported the shared preset until #27306. It now carries one
+// key of its own, `test.env`, so the preset's one setting is restated below (a
+// merged or wrapped preset is not what scripts/check-console-intercept-disarm.mjs
+// judges in a config's place). Every other key stays on vitest's defaults, as
+// under the bare re-export.
+//
+// `test.env` moves `os.tmpdir()` onto /dev/shm when it is writable
+// (scripts/vitest-memory-tmp.mjs): the local and remote-face tests open `file:`
+// databases in an `mkdtemp` under it. Measured on the 14 test files that do,
+// beside four `dd … conv=fsync` writers on the same disk: 21 of 502 cases
+// timed out across 4 files (inherited-members, external-object-parity,
+// drift-detection, migration-entries) and the run took 211 s, against all
+// green in 17 s idle; with TMPDIR there, under the same writers, all green in
+// 17 s. Assertions and budgets are untouched.
+export default defineConfig({
+  test: {
+    // A late console.* must not redden a green suite (#10374): vitest's worker
+    // forwards console output over RPC and discards the promise, and a write
+    // landing after teardown's rpcDone() snapshot is rejected into an unhandled
+    // error — a fully green run that exits 1. Disarming removes the mechanism.
+    // Mechanism + measured costs: examples/app-showcase/vitest.config.ts.
+    // Enforced repo-wide by scripts/check-console-intercept-disarm.mjs.
+    disableConsoleIntercept: true,
+    env: memoryBackedTmpEnv(),
+  },
+});
@@ -34,6 +34,7 @@
 // author can see it.
 
 import { configDefaults, defineConfig } from 'vitest/config';
+import { memoryBackedTmpEnv } from '../../scripts/vitest-memory-tmp.mjs';
 import { readFileSync } from 'node:fs';
 import path from 'node:path';
 import { parseCLI } from 'vitest/node';
@@ -231,6 +232,15 @@ export default defineConfig({
     // Each project re-declares the root block's test options: a ROOT-level
     // value is inert for a project run (measured on vitest 4.1.10, see
     // check-registry-log-declared / check-console-intercept-disarm).
+    //
+    // Each project's `env` also moves `os.tmpdir()` onto /dev/shm when it is
+    // writable (#27306; scripts/vitest-memory-tmp.mjs): the package-lifecycle
+    // integration tests keep their SQLite databases in an `mkdtemp` under it.
+    // Measured on the 51 test files that do, beside four `dd … conv=fsync`
+    // writers on the same disk: 25 of 685 cases timed out across 7 files
+    // (package revert, uninstall, duplicate-adopt, list-commits) and the run
+    // took 501 s, against all green in 102 s idle; with TMPDIR there, under the
+    // same writers, all green in 110 s. Assertions and budgets are untouched.
     projects: [
       {
         extends: true,
@@ -242,7 +252,9 @@ export default defineConfig({
           // #13517: quiet the registry's per-item registration chatter — the
           // engine's own `OS_REGISTRY_LOG` seam, not a change to its shipped
           // default. Enforced by scripts/check-registry-log-declared.mjs.
-          env: { OS_REGISTRY_LOG: 'warn' },
+          // TMPDIR on a memory-backed directory (#27306): see the note above
+          // `projects`.
+          env: { OS_REGISTRY_LOG: 'warn', ...memoryBackedTmpEnv() },
           // A late console.* must not redden a green suite (#10374); see the root
           // block. A ROOT-level value is inert for a project run, so it is
           // declared here as well (scripts/check-console-intercept-disarm.mjs).
@@ -258,7 +270,9 @@ export default defineConfig({
           // #13517: quiet the registry's per-item registration chatter — the
           // engine's own `OS_REGISTRY_LOG` seam, not a change to its shipped
           // default. Enforced by scripts/check-registry-log-declared.mjs.
-          env: { OS_REGISTRY_LOG: 'warn' },
+          // TMPDIR on a memory-backed directory (#27306): see the note above
+          // `projects`.
+          env: { OS_REGISTRY_LOG: 'warn', ...memoryBackedTmpEnv() },
           // A late console.* must not redden a green suite (#10374); see the root
           // block. A ROOT-level value is inert for a project run, so it is
           // declared here as well (scripts/check-console-intercept-disarm.mjs).
 
@@ -1,5 +1,6 @@
 import { defineConfig } from 'vitest/config';
 import path from 'node:path';
+import { memoryBackedTmpEnv } from '../../../scripts/vitest-memory-tmp.mjs';
 
 /**
  * [#12157] `plugin.ts` registers the ADR-0126 §4 activation ledger's object,
@@ -38,7 +39,13 @@ export default defineConfig({
     // ride a bare `console.warn` this level never gates — measured green with
     // the declaration in place. Enforced by
     // scripts/check-registry-log-declared.mjs.
-    env: { OS_REGISTRY_LOG: 'warn' },
+    // TMPDIR on /dev/shm when it is writable (#27306;
+    // scripts/vitest-memory-tmp.mjs): the three tests that persist runs to a
+    // SQLite file do it in an `mkdtemp` under it. Measured beside four
+    // `dd … conv=fsync` writers on the same disk: green in that run, but the
+    // slowest case went from 0.31 s idle to 4.35 s, 87% of its 5 s budget, for
+    // no change in the code; with TMPDIR there, under the same writers, 0.34 s.
+    env: { OS_REGISTRY_LOG: 'warn', ...memoryBackedTmpEnv() },
   },
   resolve: {
     alias: [
 
@@ -1,11 +1,12 @@
 // Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
 
-// This config exists for three settings; everything else stays on vitest's
+// This config exists for four settings; everything else stays on vitest's
 // defaults, deliberately — a key added here re-specifies behaviour for every
 // test file in the package (packages/cli/vitest.config.ts's header records the
 // incident that taught that).
 import { defineConfig } from 'vitest/config';
 import path from 'node:path';
+import { memoryBackedTmpEnv } from '../../../scripts/vitest-memory-tmp.mjs';
 
 export default defineConfig({
   test: {
@@ -19,7 +20,13 @@ export default defineConfig({
     // #13517: quiet the registry's per-item registration chatter — the
     // engine's own `OS_REGISTRY_LOG` seam, not a change to its shipped
     // default. Enforced by scripts/check-registry-log-declared.mjs.
-    env: { OS_REGISTRY_LOG: 'warn' },
+    // TMPDIR on /dev/shm when it is writable (#27306;
+    // scripts/vitest-memory-tmp.mjs): the two tests that open a SQLite file do
+    // it in an `mkdtemp` under it. Measured beside four `dd … conv=fsync`
+    // writers on the same disk: green in that run, but the slowest case went
+    // from 0.17 s idle to 3.04 s, 61% of its 5 s budget, for no change in the
+    // code; with TMPDIR there, under the same writers, 0.15 s.
+    env: { OS_REGISTRY_LOG: 'warn', ...memoryBackedTmpEnv() },
   },
   resolve: {
     // `hydration-convert-forward.test.ts` boots the REAL engine, protocol and
 
@@ -0,0 +1,11 @@
+// Types for `vitest-memory-tmp.mjs`, published for the package vitest configs
+// that import it. Several of those configs sit inside a package's
+// `tsconfig.test.json` program, where an untyped `.mjs` import is TS7016. See
+// `invoked-as.d.mts` for why this pairing exists; this file follows the same
+// convention, and `check:declaration-mirrors` holds it in step with the module.
+
+/**
+ * The `test.env` that moves `os.tmpdir()` onto /dev/shm:
+ * `{ TMPDIR: '/dev/shm' }` when that directory is writable, `{}` otherwise.
+ */
+export function memoryBackedTmpEnv(): Record<string, string>;
@@ -0,0 +1,51 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+import { accessSync, constants } from 'node:fs';
+
+// The `test.env` that moves a suite's temp directory onto a memory-backed
+// filesystem. A package whose tests write real files under `os.tmpdir()`
+// (SQLite databases, scaffolded projects) spreads it into its own config:
+//
+//   test: { disableConsoleIntercept: true, env: memoryBackedTmpEnv() }
+//
+// Not a key of `vitest.preset.mts`: that preset carries one setting for every
+// re-exporter, and this one belongs only to the suites whose wall time was
+// measured against the disk. A `.mjs` with a hand-written `.d.mts` beside it,
+// like `tsup-drop-sources-content.mjs`: a package's `tsconfig.test.json`
+// type-checks its `vitest.config.ts`, where a `.mts` import path is TS5097
+// and an untyped `.mjs` one is TS7016 (`check:declaration-mirrors` holds the
+// pair in step).
+//
+// ## Why
+//
+// On a Test Core shard the runner's disk is shared by every suite running
+// beside this one. A case that fsyncs a file, or a hook that unlinks a tree of
+// them, waits for that disk's queue, so its wall time measures the
+// neighbours' I/O, not the code. Measured with four `dd … conv=fsync` writers
+// on the same disk (#27076, #27306): cases that take tens of milliseconds idle
+// took seconds and timed out, while with TMPDIR on /dev/shm under the same
+// writers they stayed at their idle times. The code under test runs unchanged
+// on a tmpfs, `fsync` included; only the wait for unrelated I/O is gone.
+//
+// `os.tmpdir()` reads TMPDIR on every call, and a child process the suite
+// spawns inherits it, so the one setting covers both. A path built from the
+// package or the repository does not follow TMPDIR and is not moved by this.
+//
+// A host without a writable /dev/shm (macOS, Windows) keeps its own temp
+// directory: the setting is absent there, not an error.
+const MEMORY_BACKED_TMP = '/dev/shm';
+
+/** @param {string} dir */
+function writableDir(dir) {
+  try {
+    accessSync(dir, constants.W_OK);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+/** @returns {Record<string, string>} */
+export function memoryBackedTmpEnv() {
+  return writableDir(MEMORY_BACKED_TMP) ? { TMPDIR: MEMORY_BACKED_TMP } : {};
+}