diff --git a/.changeset/guard-missing-resolution-graph-builder.md b/.changeset/guard-missing-resolution-graph-builder.md
new file mode 100644
index 0000000000..952db509e7
--- /dev/null
+++ b/.changeset/guard-missing-resolution-graph-builder.md
@@ -0,0 +1,6 @@
+---
+"@pnpm/deps.graph-builder": patch
+"pnpm": patch
+---
+
+Fix `TypeError: Cannot use 'in' operator to search for 'directory' in undefined` during `pnpm install --frozen-lockfile` when a peer-dep variant snapshot omits its `resolution` field (the variant inherits resolution from the base entry, so this shape is valid in the lockfile but the graph builder didn't guard the access).
diff --git a/deps/graph-builder/src/lockfileToDepGraph.ts b/deps/graph-builder/src/lockfileToDepGraph.ts
index 0d2ddf4e2a..2a1dec5b64 100644
--- a/deps/graph-builder/src/lockfileToDepGraph.ts
+++ b/deps/graph-builder/src/lockfileToDepGraph.ts
@@ -214,7 +214,11 @@ async function buildGraphFromPackages (
         return
       }
 
-      const isDirectoryDep = 'directory' in pkgSnapshot.resolution && pkgSnapshot.resolution.directory != null
+      // Peer-dep variant snapshots (e.g. `pkg@1.0.0(peer@2.0.0)`) inherit
+      // their resolution from the base entry and may omit `resolution`
+      // themselves. Guard so the `'directory' in …` check doesn't crash
+      // with `Cannot use 'in' operator to search for 'directory' in undefined`.
+      const isDirectoryDep = pkgSnapshot.resolution != null && 'directory' in pkgSnapshot.resolution && pkgSnapshot.resolution.directory != null
       if (isDirectoryDep && opts.ignoreLocalPackages) {
         logger.info({
           message: `Skipping local dependency ${pkgName}@${pkgVersion} (file: protocol)`,
diff --git a/installing/deps-restorer/test/fixtures/peer-variant-missing-resolution/package.json b/installing/deps-restorer/test/fixtures/peer-variant-missing-resolution/package.json
new file mode 100644
index 0000000000..83e000174f
--- /dev/null
+++ b/installing/deps-restorer/test/fixtures/peer-variant-missing-resolution/package.json
@@ -0,0 +1,8 @@
+{
+  "name": "peer-variant-missing-resolution",
+  "version": "1.0.0",
+  "private": true,
+  "dependencies": {
+    "pkg-a": "workspace:*"
+  }
+}
diff --git a/installing/deps-restorer/test/fixtures/peer-variant-missing-resolution/packages/peer/package.json b/installing/deps-restorer/test/fixtures/peer-variant-missing-resolution/packages/peer/package.json
new file mode 100644
index 0000000000..2b8b6606e6
--- /dev/null
+++ b/installing/deps-restorer/test/fixtures/peer-variant-missing-resolution/packages/peer/package.json
@@ -0,0 +1,5 @@
+{
+  "name": "peer",
+  "version": "1.0.0",
+  "private": true
+}
diff --git a/installing/deps-restorer/test/fixtures/peer-variant-missing-resolution/packages/pkg-a/package.json b/installing/deps-restorer/test/fixtures/peer-variant-missing-resolution/packages/pkg-a/package.json
new file mode 100644
index 0000000000..d0a16e5de2
--- /dev/null
+++ b/installing/deps-restorer/test/fixtures/peer-variant-missing-resolution/packages/pkg-a/package.json
@@ -0,0 +1,8 @@
+{
+  "name": "pkg-a",
+  "version": "1.0.0",
+  "private": true,
+  "peerDependencies": {
+    "peer": "1.0.0"
+  }
+}
diff --git a/installing/deps-restorer/test/fixtures/peer-variant-missing-resolution/pnpm-lock.yaml b/installing/deps-restorer/test/fixtures/peer-variant-missing-resolution/pnpm-lock.yaml
new file mode 100644
index 0000000000..367a2ecfb7
--- /dev/null
+++ b/installing/deps-restorer/test/fixtures/peer-variant-missing-resolution/pnpm-lock.yaml
@@ -0,0 +1,45 @@
+lockfileVersion: '9.0'
+
+settings:
+  autoInstallPeers: true
+  excludeLinksFromLockfile: false
+  injectWorkspacePackages: true
+
+importers:
+
+  .:
+    dependencies:
+      pkg-a:
+        specifier: workspace:*
+        version: 'file:packages/pkg-a(peer@1.0.0)'
+
+  packages/peer: {}
+
+  packages/pkg-a:
+    peerDependencies:
+      peer:
+        specifier: 1.0.0
+        version: 1.0.0
+
+packages:
+
+  # Base entry — has `resolution` describing the workspace directory.
+  'pkg-a@file:packages/pkg-a':
+    resolution: { directory: packages/pkg-a, type: directory }
+
+  # Peer-variant entry — inherits resolution from the base, intentionally
+  # omits its own `resolution`. This shape is what pnpm itself writes for a
+  # workspace dep with peer dependencies that resolve differently per
+  # consumer. Before the fix it crashed `lockfileToDepGraph` with
+  # `TypeError: Cannot use 'in' operator to search for 'directory' in undefined`.
+  'pkg-a@file:packages/pkg-a(peer@1.0.0)':
+    dependencies:
+      peer: 1.0.0
+
+snapshots:
+
+  'pkg-a@file:packages/pkg-a': {}
+
+  'pkg-a@file:packages/pkg-a(peer@1.0.0)':
+    dependencies:
+      peer: 1.0.0
diff --git a/installing/deps-restorer/test/fixtures/peer-variant-missing-resolution/pnpm-workspace.yaml b/installing/deps-restorer/test/fixtures/peer-variant-missing-resolution/pnpm-workspace.yaml
new file mode 100644
index 0000000000..9af4682d1d
--- /dev/null
+++ b/installing/deps-restorer/test/fixtures/peer-variant-missing-resolution/pnpm-workspace.yaml
@@ -0,0 +1,3 @@
+packages:
+  - "packages/*"
+injectWorkspacePackages: true
diff --git a/installing/deps-restorer/test/index.ts b/installing/deps-restorer/test/index.ts
index 518d40a1e1..94e05399a2 100644
--- a/installing/deps-restorer/test/index.ts
+++ b/installing/deps-restorer/test/index.ts
@@ -919,3 +919,25 @@ test('installing a package deeply installs all required dependencies', async ()
     expect(projectAssertion.requireModule('is-positive')).toBeTruthy()
   }
 })
+
+// Regression test: `lockfileToDepGraph` crashed with
+// `TypeError: Cannot use 'in' operator to search for 'directory' in undefined`
+// when a peer-dep variant snapshot in the lockfile omitted its `resolution`
+// field. The variant intentionally inherits resolution from the base entry,
+// so this shape is valid pnpm output but the graph builder accessed
+// `pkgSnapshot.resolution` without guarding for undefined.
+test('headlessInstall: peer-variant snapshot without `resolution` does not crash', async () => {
+  const workspaceFixture = f.prepare('peer-variant-missing-resolution')
+  const projects = [
+    workspaceFixture,
+    path.join(workspaceFixture, 'packages', 'pkg-a'),
+    path.join(workspaceFixture, 'packages', 'peer'),
+  ]
+
+  await expect(
+    headlessInstall(await testDefaults({
+      lockfileDir: workspaceFixture,
+      projects,
+    }))
+  ).resolves.not.toThrow()
+})