diff --git a/cloud/tests/test_usage_quota.py b/cloud/tests/test_usage_quota.py
index 91eea31..1e4c8fa 100644
--- a/cloud/tests/test_usage_quota.py
+++ b/cloud/tests/test_usage_quota.py
@@ -288,6 +288,36 @@ class LedgerTestCase(unittest.TestCase):
             self.assertNotIn("token", cols)
             self.assertGreater(row_id, 0)
 
+    def test_concurrent_records_keep_chain_intact(self):
+        # round-12 修复轮（项 5，P11）：R12 真机同款并发复测——
+        # 4 线程 × 10 条，0 写错，但旧 record() 读-改-写无锁，
+        # verify_chain 返回 (False, first_bad=11)，链 G、链 H
+        # 两次复现同一断点。修后两轮都须全真、40 条一条不丢。
+        for _round in range(2):
+            with tempfile.TemporaryDirectory() as tmp:
+                db_path = _make_db(tmp)
+                ledger = UsageLedger(db_path)
+                errors = []
+
+                def _worker():
+                    try:
+                        for _ in range(10):
+                            ledger.record("u1", "message", 1)
+                    except Exception as e:  # noqa: BLE001 - 汇总后断言
+                        errors.append(e)
+
+                threads = [threading.Thread(target=_worker)
+                           for _ in range(4)]
+                for t in threads:
+                    t.start()
+                for t in threads:
+                    t.join()
+                self.assertEqual(errors, [])
+                self.assertEqual(ledger.sum_kind("u1", "message"), 40)
+                ok, bad = ledger.verify_chain("u1")
+                self.assertTrue(ok, bad)
+                self.assertIsNone(bad)
+
 
 class WarningTestCase(unittest.TestCase):
     def test_once_per_window_at_80_percent(self):
diff --git a/cloud/usage.py b/cloud/usage.py
index 882a07e..6bf566a 100644
--- a/cloud/usage.py
+++ b/cloud/usage.py
@@ -32,6 +32,7 @@ import hashlib
 import json
 import os
 import sqlite3
+import threading
 import time
 from datetime import datetime, timezone
 
@@ -227,6 +228,13 @@ class UsageLedger:
         self._conn = _db_connect(db_path)
         self._conn.execute(self._DDL)
         self._conn.commit()
+        # round-12 修复轮（项 5，P11）：record() 读-改-写的
+        # 串行锁。无锁时两线程读到同一 prev_hash、各写一条，
+        # 链分叉（R12 真机：4 线程 × 10 条，0 写错，
+        # verify_chain (False, first_bad=11)，链 G、链 H 两次
+        # 同断点）。锁只串行化同一实例；跨实例/进程的并发由
+        # record() 里的 BEGIN IMMEDIATE 持写锁兜底。
+        self._lock = threading.Lock()
 
     def _new_conn(self) -> sqlite3.Connection:
         """Fresh connection per write/read (mirrors CostStore).
@@ -256,26 +264,34 @@ class UsageLedger:
         created_at = time.time()
         conn = self._new_conn()
         try:
-            prev = conn.execute(
-                "SELECT entry_hash FROM usage_ledger"
-                " WHERE user_id = ? ORDER BY id DESC LIMIT 1",
-                (user_id,)).fetchone()
-            prev_hash = prev[0] if prev is not None else None
-            fields = {"user_id": user_id, "kind": kind,
-                      "amount": int(amount), "unit": unit,
-                      "task_id": task_id or "", "model": model or "",
-                      "source": source or "", "day": day,
-                      "created_at": created_at}
-            digest = _entry_hash(prev_hash, fields)
-            cursor = conn.execute(
-                """INSERT INTO usage_ledger
-                   (user_id, kind, amount, unit, task_id, model,
-                    source, day, created_at, prev_hash, entry_hash)
-                   VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)""",
-                (user_id, kind, int(amount), unit, task_id, model,
-                 source, day, created_at, prev_hash, digest))
-            conn.commit()
-            return int(cursor.lastrowid or 0)
+            # 读-改-写原子化：锁串行化同一实例；BEGIN IMMEDIATE
+            # 在 SELECT 前即持库级写锁（WAL + busy_timeout 下等
+            # 待而非立即失败），使另一实例/进程读到的链头必是已
+            # 提交的最新值——链不断、条不丢。锁与事务缺一不可：
+            # 光有事务、同一实例的两线程仍可能各持一个连接同时读
+            # 到同一链头。
+            with self._lock:
+                conn.execute("BEGIN IMMEDIATE")
+                prev = conn.execute(
+                    "SELECT entry_hash FROM usage_ledger"
+                    " WHERE user_id = ? ORDER BY id DESC LIMIT 1",
+                    (user_id,)).fetchone()
+                prev_hash = prev[0] if prev is not None else None
+                fields = {"user_id": user_id, "kind": kind,
+                          "amount": int(amount), "unit": unit,
+                          "task_id": task_id or "", "model": model or "",
+                          "source": source or "", "day": day,
+                          "created_at": created_at}
+                digest = _entry_hash(prev_hash, fields)
+                cursor = conn.execute(
+                    """INSERT INTO usage_ledger
+                       (user_id, kind, amount, unit, task_id, model,
+                        source, day, created_at, prev_hash, entry_hash)
+                       VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)""",
+                    (user_id, kind, int(amount), unit, task_id, model,
+                     source, day, created_at, prev_hash, digest))
+                conn.commit()
+                return int(cursor.lastrowid or 0)
         finally:
             if conn is not self._conn:
                 conn.close()