diff --git a/workspaces/arborist/lib/arborist/reify.js b/workspaces/arborist/lib/arborist/reify.js
index 960510a86531e..4b10e78ff1c3f 100644
--- a/workspaces/arborist/lib/arborist/reify.js
+++ b/workspaces/arborist/lib/arborist/reify.js
@@ -26,7 +26,8 @@ const optionalSet = require('../optional-set.js')
 const relpath = require('../relpath.js')
 const retirePath = require('../retire-path.js')
 const treeCheck = require('../tree-check.js')
-const { defaultLockfileVersion } = require('../shrinkwrap.js')
+const Shrinkwrap = require('../shrinkwrap.js')
+const { defaultLockfileVersion } = Shrinkwrap
 const { saveTypeMap, hasSubKey } = require('../add-rm-pkg-deps.js')
 const { IsolatedNode, IsolatedLink } = require('../isolated-classes.js')
 
@@ -115,12 +116,15 @@ module.exports = cls => class Reifier extends cls {
     await this[_loadTrees](options)
 
     const oldTree = this.idealTree
+    // Kept to serialize the hidden lockfile from the on-disk .store/symlink layout.
+    let isolatedTree = null
     if (linked) {
       // swap out the tree with the isolated tree
       // this is currently technical debt which will be resolved in a refactor
       // of Node/Link trees
       log.warn('reify', 'The "linked" install strategy is EXPERIMENTAL and may contain bugs.')
       this.idealTree = await this.createIsolatedTree()
+      isolatedTree = this.idealTree
       if (this.actualTree) {
         this.#linkedActualForDiff = this.#buildLinkedActualForDiff(
           this.idealTree, this.actualTree
@@ -241,17 +245,24 @@ module.exports = cls => class Reifier extends cls {
       calcDepFlags(this.idealTree)
     }
 
-    // save the ideal's meta as a hidden lockfile after we actualize it
-    this.idealTree.meta.filename =
-      this.idealTree.realpath + '/node_modules/.package-lock.json'
-    this.idealTree.meta.hiddenLockfile = true
-    this.idealTree.meta.lockfileVersion = defaultLockfileVersion
+    // save the ideal's meta as a hidden lockfile after we actualize it.
+    // Under linked the logical tree is the hoisted layout, so the hidden lockfile is serialized from the isolated tree instead.
+    if (!linked) {
+      this.idealTree.meta.filename =
+        this.idealTree.realpath + '/node_modules/.package-lock.json'
+      this.idealTree.meta.hiddenLockfile = true
+      this.idealTree.meta.lockfileVersion = defaultLockfileVersion
+    }
 
     this.actualTree = this.idealTree
     this.idealTree = null
 
     if (!this.options.global && !this.options.dryRun) {
-      await this.actualTree.meta.save()
+      if (linked) {
+        await this.#saveLinkedHiddenLockfile(isolatedTree)
+      } else {
+        await this.actualTree.meta.save()
+      }
       const ignoreScripts = !!this.options.ignoreScripts
       // if we aren't doing a dry run or ignoring scripts and we actually made changes to the dep
       // tree, then run the dependencies scripts
@@ -834,6 +845,47 @@ module.exports = cls => class Reifier extends cls {
     return join(filePath)
   }
 
+  // Serialize the hidden lockfile from the isolated tree, which mirrors the on-disk .store/symlink layout.
+  // Its children are every materialized node_modules entry: store package dirs and all symlinks.
+  async #saveLinkedHiddenLockfile (isolatedTree) {
+    const path = isolatedTree.realpath
+    const meta = new Shrinkwrap({
+      path,
+      hiddenLockfile: true,
+      lockfileVersion: defaultLockfileVersion,
+      resolveOptions: this.options,
+    })
+    meta.reset()
+    meta.filename = resolve(path, 'node_modules/.package-lock.json')
+    const storeRe = /^(.*\/\.store\/.+?)\/node_modules\//
+    const containers = new Set()
+    const nodes = new Set()
+    for (const node of isolatedTree.children.values()) {
+      // Tree-only undeclared workspace self-links aren't on disk.
+      if (node.isUndeclaredWorkspaceLink) {
+        continue
+      }
+      nodes.add(node)
+      // Record the enclosing .store/<key> dir so loadVirtual can resolve a store package's sibling deps.
+      // node.location uses the platform separator; lockfile keys are posix.
+      const m = node.location.replace(/\\/g, '/').match(storeRe)
+      if (m) {
+        containers.add(m[1])
+      }
+    }
+    // Workspace dirs hold their own dep symlinks; record them so the cache can validate those subtrees.
+    for (const ws of isolatedTree.fsChildren) {
+      nodes.add(ws)
+    }
+    for (const node of nodes) {
+      meta.add(node)
+    }
+    for (const loc of containers) {
+      meta.data.packages[loc] = {}
+    }
+    await meta.save()
+  }
+
   // Build a flat actual tree wrapper for linked installs so the diff can correctly match store entries that already exist on disk.
   // The proxy tree from createIsolatedTree() is flat (all children on root), but loadActual() produces a nested tree where store entries are deep link targets.
   // This wrapper surfaces them at the root level for comparison.
diff --git a/workspaces/arborist/lib/shrinkwrap.js b/workspaces/arborist/lib/shrinkwrap.js
index cf27bcf4ac726..03cc86edbd4e9 100644
--- a/workspaces/arborist/lib/shrinkwrap.js
+++ b/workspaces/arborist/lib/shrinkwrap.js
@@ -172,6 +172,35 @@ const assertNoNewer = async (path, data, lockTime, dir, seen) => {
     return
   }
 
+  // The walk above can't reach two linked-strategy layouts: a store package's sibling deps under .store/<key>/node_modules (.store is a skipped dot-dir), and an undeclared workspace not symlinked into root node_modules. Walk those dirs, derived from the lockfile entries.
+  // A dir reachable through a link entry is skipped: it was already reached (or, if the symlink is stale, correctly stays unseen so the lockfile is rejected). This keeps the hoisted strategy's stale-symlink detection intact, since there every workspace is a link target.
+  const linkTargets = new Set()
+  for (const loc in data.packages) {
+    const { link, resolved } = data.packages[loc]
+    if (link && resolved) {
+      linkTargets.add(resolved.replace(/\\/g, '/'))
+    }
+  }
+  const extraDirs = new Set()
+  for (const loc in data.packages) {
+    const store = loc.match(/^(.*\/\.store\/.+?)\/node_modules\//)
+    if (store) {
+      // .store/<key> is never walked but has no entry, so mark it seen.
+      seen.add(store[1])
+      extraDirs.add(`${store[1]}/node_modules`)
+      continue
+    }
+    // A workspace/fsChild dir outside node_modules, e.g. packages/a.
+    const i = loc.indexOf('/node_modules/')
+    const root = i === -1 ? loc : loc.slice(0, i)
+    if (root && !/(^|\/)node_modules(\/|$)/.test(root) && !linkTargets.has(root)) {
+      extraDirs.add(root)
+    }
+  }
+  for (const rel of extraDirs) {
+    await assertNoNewer(path, data, lockTime, resolve(path, rel), seen)
+  }
+
   // assert that all the entries in the lockfile were seen
   for (const loc in data.packages) {
     if (!seen.has(loc)) {
diff --git a/workspaces/arborist/test/arborist/reify.js b/workspaces/arborist/test/arborist/reify.js
index becde0b6fa4c2..e16563a7722bf 100644
--- a/workspaces/arborist/test/arborist/reify.js
+++ b/workspaces/arborist/test/arborist/reify.js
@@ -4333,6 +4333,119 @@ t.test('install strategy linked', async (t) => {
     t.ok(abbrev.isSymbolicLink(), 'abbrev got installed')
   })
 
+  t.test('hidden lockfile records the linked .store layout and round-trips', async t => {
+    // Regression for #9612: the hidden lockfile must record the on-disk .store/symlink layout so it round-trips as a valid cache.
+    const Shrinkwrap = require('../../lib/shrinkwrap.js')
+    const path = t.testdir({
+      'package.json': JSON.stringify({
+        name: 'root',
+        version: '1.0.0',
+        // once depends on wrappy, so the store has a transitive symlink to validate
+        dependencies: { once: '1.4.0' },
+      }),
+    })
+
+    createRegistry(t, true)
+    await reify(path, { installStrategy: 'linked' })
+
+    const hidden = require(resolve(path, 'node_modules/.package-lock.json'))
+    const locs = Object.keys(hidden.packages)
+    // the layout is recorded at .store paths, not hoisted node_modules/<name>
+    t.ok(locs.some(l => /^node_modules\/\.store\/once@/.test(l)),
+      'once is recorded under .store')
+    t.ok(locs.some(l => /^node_modules\/\.store\/.*\/node_modules\/wrappy$/.test(l)),
+      'the transitive wrappy symlink is recorded inside the store')
+    t.notOk(locs.includes('node_modules/wrappy'),
+      'wrappy is not recorded at a hoisted path')
+
+    // the cache is accepted on reload: assertNoNewer matches it against the real disk layout
+    const meta = await Shrinkwrap.load({ path, hiddenLockfile: true })
+    t.equal(meta.loadedFromDisk, true, 'hidden lockfile is a valid cache of the disk layout')
+
+    // loadActual must reconstruct the tree from the cache with once->wrappy resolved through the store.
+    const actual = await newArb({ path, installStrategy: 'linked' }).loadActual()
+    const onceNode = [...actual.inventory.values()].find(n => n.name === 'once' && !n.isLink)
+    t.ok(onceNode, 'once is in the cached actual tree')
+    const wrappyEdge = onceNode.edgesOut.get('wrappy')
+    t.ok(wrappyEdge && !wrappyEdge.missing, 'once resolves its wrappy dep through the cached store layout')
+  })
+
+  t.test('hidden lockfile round-trips with an undeclared workspace', async t => {
+    // Regression for #9612: an undeclared workspace materializes deps in its own node_modules but isn't linked into root, and the cache must still validate that subtree.
+    const Shrinkwrap = require('../../lib/shrinkwrap.js')
+    const path = t.testdir({
+      'package.json': JSON.stringify({
+        name: 'host',
+        version: '1.0.0',
+        workspaces: ['packages/a'],
+        // root does not depend on the workspace, so it stays undeclared
+      }),
+      packages: {
+        a: {
+          'package.json': JSON.stringify({
+            name: 'a',
+            version: '1.0.0',
+            dependencies: { once: '1.4.0' },
+          }),
+        },
+      },
+    })
+
+    createRegistry(t, true)
+    await reify(path, { installStrategy: 'linked' })
+
+    // the workspace's dep is materialized under its own node_modules, not the root's
+    t.ok(fs.lstatSync(resolve(path, 'packages/a/node_modules/once')).isSymbolicLink(),
+      'once is symlinked into the workspace node_modules')
+    t.notOk(fs.existsSync(resolve(path, 'node_modules/a')),
+      'the undeclared workspace is not symlinked into the root node_modules')
+
+    const meta = await Shrinkwrap.load({ path, hiddenLockfile: true })
+    t.equal(meta.loadedFromDisk, true, 'hidden lockfile validates the undeclared workspace subtree')
+  })
+
+  t.test('hidden lockfile round-trips with an undeclared workspace and no store entries', async t => {
+    // Regression for #9612: with only local deps there is no .store, so the cache must still walk the undeclared workspace subtree to validate it.
+    const Shrinkwrap = require('../../lib/shrinkwrap.js')
+    const path = t.testdir({
+      'package.json': JSON.stringify({
+        name: 'host',
+        version: '1.0.0',
+        workspaces: ['packages/w', 'packages/a', 'packages/b'],
+        // only w is declared; a and b stay undeclared, and a depends on b locally
+        dependencies: { w: '1.0.0' },
+      }),
+      packages: {
+        w: { 'package.json': JSON.stringify({ name: 'w', version: '1.0.0' }) },
+        a: {
+          'package.json': JSON.stringify({
+            name: 'a',
+            version: '1.0.0',
+            dependencies: { b: '1.0.0' },
+          }),
+        },
+        b: { 'package.json': JSON.stringify({ name: 'b', version: '1.0.0' }) },
+      },
+    })
+
+    createRegistry(t, false)
+    await reify(path, { installStrategy: 'linked' })
+
+    t.notOk(fs.existsSync(resolve(path, 'node_modules/.store')),
+      'no store is created for an all-local graph')
+    t.ok(fs.lstatSync(resolve(path, 'packages/a/node_modules/b')).isSymbolicLink(),
+      'the undeclared workspace links its local dep')
+
+    const meta = await Shrinkwrap.load({ path, hiddenLockfile: true })
+    t.equal(meta.loadedFromDisk, true, 'hidden lockfile validates the subtree without any store entry')
+
+    // loadActual must reconstruct the undeclared workspace from the cache with its local dep resolved.
+    const actual = await newArb({ path, installStrategy: 'linked' }).loadActual()
+    const aNode = [...actual.inventory.values()].find(n => n.name === 'a' && !n.isLink)
+    const bEdge = aNode && aNode.edgesOut.get('b')
+    t.ok(bEdge && !bEdge.missing, 'the undeclared workspace resolves its local dep through the cache')
+  })
+
   t.test('does not re-create a workspace dir removed from manifest', async t => {
     // Regression test for https://github.com/npm/cli/issues/9331
     const path = t.testdir({