diff --git a/package.json b/package.json
index cb1ab5c..2dadeaa 100644
--- a/package.json
+++ b/package.json
@@ -139,6 +139,7 @@
     "check:client": "tsc -p tsconfig.check.client.json",
     "check:readmes": "node scripts/verify-readmes.mjs",
     "verify:self-contained": "node scripts/verify-self-contained.mjs",
+    "check:lockfile": "node scripts/check-lockfile-drift.mjs",
     "verify:artifacts": "node scripts/verify-artifacts.mjs",
     "test:conformance": "node test/protocol-conformance/run.mjs"
   },
diff --git a/scripts/check-lockfile-drift.mjs b/scripts/check-lockfile-drift.mjs
new file mode 100644
index 0000000..da60b99
--- /dev/null
+++ b/scripts/check-lockfile-drift.mjs
@@ -0,0 +1,31 @@
+#!/usr/bin/env node
+// scripts/check-lockfile-drift.mjs - fail fast when package.json and pnpm-lock.yaml disagree.
+//
+// Why this exists: the 0.1.6-alpha.2 batch shipped two releases whose `package.json`
+// had gained a peer while `pnpm-lock.yaml` had not been regenerated. Nothing in the
+// local acceptance chain noticed, so the first thing that failed was the release
+// workflow, with `ERR_PNPM_OUTDATED_LOCKFILE` (see the batch report, R22).
+//
+// `--frozen-lockfile` makes pnpm refuse to rewrite the lockfile and `--lockfile-only`
+// keeps it from touching `node_modules`, so this is a read-only probe: exit 0 means the
+// two files agree, exit non-zero means someone changed one without the other.
+//
+// Exit codes: 0 = in sync; 1 = drift (regenerate the lockfile and commit it).
+
+import { spawnSync } from 'node:child_process'
+import process from 'node:process'
+
+const result = spawnSync(
+  'pnpm',
+  ['install', '--frozen-lockfile', '--lockfile-only', '--ignore-scripts'],
+  { stdio: 'inherit', shell: process.platform === 'win32' },
+)
+
+if (result.status !== 0) {
+  console.error('')
+  console.error('lockfile drift: package.json and pnpm-lock.yaml disagree.')
+  console.error('Fix: `pnpm install --lockfile-only`, then commit the lockfile.')
+  process.exit(1)
+}
+
+console.log('lockfile ok: package.json and pnpm-lock.yaml agree')