diff --git a/AGENTS.md b/AGENTS.md
index e6d2c1a..bcdb0c5 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -28,5 +28,5 @@
 - 改动说明只在交流中记录。
 
 ## 当前重点
-- 主线已完成：高级优化器 → 并行查询 → M1 会话隔离 → M2 快照事务 → M3 WAL 与崩溃恢复 → M4 表结构进 WAL → M5 系统表统一命名 → M6 检查点按 LSN 部分截断。
+- 主线已完成：高级优化器 → 并行查询 → M1 会话隔离 → M2 快照事务 → M3 WAL 与崩溃恢复 → M4 表结构进 WAL → M5 系统表统一命名 → M6 检查点按 LSN 部分截断 → M7 事务写写冲突检测。
 - 进度与计数见 `agent_credientials/project-plan.txt`，限制与风险见 `agent_credientials/known-issues.txt`。
diff --git a/chusql-server/src/ChuSQL/Server/Session.hs b/chusql-server/src/ChuSQL/Server/Session.hs
index 8d13334..99ce9a3 100644
--- a/chusql-server/src/ChuSQL/Server/Session.hs
+++ b/chusql-server/src/ChuSQL/Server/Session.hs
@@ -264,13 +264,15 @@ commitTransaction session = do
             liveResult <- beSnapshot backend
             case liveResult of
                 Left err -> pure (Left (engineErrorCode err))
-                Right live -> do
-                    applied <- beApplyTransaction backend (transactionOps (txBase transaction) (txStaged transaction) live)
-                    case applied of
-                        Left err -> pure (Left (engineErrorCode err))
-                        Right () -> do
-                            writeIORef (ssTransaction session) Nothing
-                            pure (Right (emptyResult (Just current)))
+                Right live -> case transactionConflict (txBase transaction) (txStaged transaction) live of
+                    Just msg -> pure (Left (SessionError "serialization_failure" (T.pack msg)))
+                    Nothing -> do
+                        applied <- beApplyTransaction backend (transactionOps (txBase transaction) (txStaged transaction) live)
+                        case applied of
+                            Left err -> pure (Left (engineErrorCode err))
+                            Right () -> do
+                                writeIORef (ssTransaction session) Nothing
+                                pure (Right (emptyResult (Just current)))
 
 -- | 回滚：快照丢掉，存储层没被改过
 rollbackTransaction :: Session -> IO (Either SessionError QueryResult)
@@ -303,45 +305,79 @@ resultOfStaged db statement rows = resultOf (StatementResult cols rows)
         [] -> columnsFromStatement db statement
         (r : _) -> map fst r
 
+-- | 一张表的提交差异与冲突
+data TableDiff = TableDiff
+    { tdDeletes :: [Int]
+    , tdWrites :: [Row]
+    , tdReplace :: Maybe [Row]
+    , tdConflict :: Maybe String
+    }
+
 -- | 提交差分：相对快照列出要删、要写与要替换的行
 transactionOps :: Database -> Database -> Database -> [TxnOp]
-transactionOps base staged live =
-    concatMap opsFor names
+transactionOps base staged live = concatMap opsOf (transactionDiffs base staged live)
+  where
+    -- | 一张表的请求：先删后写
+    opsOf (name, diff) =
+        [TxnDelete name (tdDeletes diff) | not (null (tdDeletes diff))]
+            ++ [TxnUpsert name (tdWrites diff) | not (null (tdWrites diff))]
+            ++ [TxnReplace name rows | Just rows <- [tdReplace diff]]
+
+-- | 提交前校验：本事务动过的行是否已被别人改掉
+transactionConflict :: Database -> Database -> Database -> Maybe String
+transactionConflict base staged live =
+    case [msg | (_, diff) <- transactionDiffs base staged live, Just msg <- [tdConflict diff]] of
+        (msg : _) -> Just msg
+        [] -> Nothing
+
+-- | 逐表算提交差异，顺带找出冲突
+transactionDiffs :: Database -> Database -> Database -> [(String, TableDiff)]
+transactionDiffs base staged live = [(name, diffFor name) | name <- names]
   where
     -- | 事务里碰得着的表：数据语句不增删表
     names = nubKeys (map fst base ++ map fst staged)
 
-    -- | 一张表的差分
-    opsFor name
-        | all (isJust . rowKey) (baseRows ++ stagedRows) = keyedOps
-        | stagedRows == baseRows = []
-        | otherwise = [TxnReplace name stagedRows]
+    -- | 一张表的差异
+    diffFor name
+        | all (isJust . rowKey) (baseRows ++ stagedRows) = keyedDiff
+        -- 没动过这张表：别人改成什么样都不关本事务
+        | stagedRows == baseRows = emptyDiff
+        -- 动过，库里还是基线：整表替换
+        | liveRows == baseRows = emptyDiff {tdReplace = Just stagedRows}
+        | otherwise = emptyDiff {tdConflict = Just ("write conflict on table " ++ name)}
       where
         baseRows = rowsOf name base
         stagedRows = rowsOf name staged
         liveRows = rowsOf name live
 
+        baseByKey = [(k, r) | r <- baseRows, Just k <- [rowKey r]]
         liveByKey = [(k, r) | r <- liveRows, Just k <- [rowKey r]]
         stagedByKey = [(k, r) | r <- stagedRows, Just k <- [rowKey r]]
-        baseKeys = [k | r <- baseRows, Just k <- [rowKey r]]
 
-        -- 开事务时就存在、现在没了：删；内容变了的行也先删再写
-        gone = [k | (k, _) <- liveByKey, k `elem` baseKeys, not (any ((== k) . fst) stagedByKey)]
-        rewritten = [k | (k, r) <- stagedByKey, Just liveRow <- [lookup k liveByKey], liveRow /= r]
-        deleted = gone ++ rewritten
+        -- 本事务动过的键：内容改过、新增，或者删掉
+        written = [k | (k, r) <- stagedByKey, lookup k baseByKey /= Just r]
+        removed = [k | (k, _) <- baseByKey, not (any ((== k) . fst) stagedByKey)]
+        touched = written ++ removed
 
-        -- 新增的行，以及跟库里不一样的行；并发插进来的行不动
-        written =
-            [ r
-            | (k, r) <- stagedByKey
-            , case lookup k liveByKey of
-                Nothing -> True
-                Just liveRow -> liveRow /= r
+        -- 两边都删了算一致；其余只要跟基线不一样就是冲突
+        victims =
+            [ k
+            | k <- touched
+            , lookup k liveByKey /= lookup k baseByKey
+            , not (k `elem` removed && lookup k liveByKey == Nothing)
             ]
 
-        keyedOps =
-            [TxnDelete name deleted | not (null deleted)]
-                ++ [TxnUpsert name written | not (null written)]
+        keyedDiff =
+            emptyDiff
+                { tdDeletes = [k | k <- touched, isJust (lookup k liveByKey)]
+                , tdWrites = [r | (k, r) <- stagedByKey, k `elem` written]
+                , tdConflict = case victims of
+                    (k : _) -> Just ("write conflict on table " ++ name ++ ", id " ++ show k)
+                    [] -> Nothing
+                }
+
+    -- | 没有差异的空结果
+    emptyDiff = TableDiff [] [] Nothing Nothing
 
     -- | 表名去重但保持顺序
     nubKeys = foldr (\name acc -> if name `elem` acc then acc else name : acc) []
diff --git a/chusql-server/test/Spec.hs b/chusql-server/test/Spec.hs
index ddc80e0..b3c21a6 100644
--- a/chusql-server/test/Spec.hs
+++ b/chusql-server/test/Spec.hs
@@ -652,6 +652,10 @@ sessionAge session wanted = do
 failureMessage :: Either SessionError QueryResult -> Text
 failureMessage outcome = either sessMessage (const "") outcome
 
+-- | 取失败里的错误码
+failureCode :: Either SessionError QueryResult -> Text
+failureCode outcome = either sessCode (const "") outcome
+
 -- | 取结果第一列的整数
 intValues :: [[Value]] -> [Int]
 intValues rows = [n | row <- rows, Just (VInt n) <- [nth 0 row]]
@@ -713,6 +717,77 @@ transactionSpec = describe "server transactions" $ do
         denied <- runStatementCoded session "use other"
         failureMessage denied `shouldBe` "cannot switch database inside a transaction"
 
+    it "refuses COMMIT when another session changed the same row" $ withTwoSessions $ \first second -> do
+        _ <- mustSql first "begin"
+        _ <- mustSql second "begin"
+        _ <- mustSql first "update users set age = 31 where id = 1"
+        _ <- mustSql second "update users set age = 42 where id = 1"
+        _ <- mustSql first "commit"
+        denied <- runStatementCoded second "commit"
+        failureCode denied `shouldBe` "serialization_failure"
+        failureMessage denied `shouldBe` "write conflict on table users, id 1"
+        -- 事务保留：回滚之后看到的还是先提交的那个值
+        _ <- mustSql second "rollback"
+        sessionAge second 1 `shouldReturn` Just 31
+
+    it "refuses COMMIT when the row was deleted meanwhile" $ withTwoSessions $ \first second -> do
+        _ <- mustSql first "begin"
+        _ <- mustSql second "begin"
+        _ <- mustSql first "delete from users where id = 2"
+        _ <- mustSql second "update users set age = 42 where id = 2"
+        _ <- mustSql first "commit"
+        denied <- runStatementCoded second "commit"
+        failureMessage denied `shouldBe` "write conflict on table users, id 2"
+        _ <- mustSql second "rollback"
+        ids <- sessionIds second
+        ids `shouldMatchList` [1, 3, 4, 5]
+
+    it "refuses COMMIT when deleting a row that changed meanwhile" $ withTwoSessions $ \first second -> do
+        _ <- mustSql first "begin"
+        _ <- mustSql second "begin"
+        _ <- mustSql first "update users set age = 31 where id = 3"
+        _ <- mustSql second "delete from users where id = 3"
+        _ <- mustSql first "commit"
+        denied <- runStatementCoded second "commit"
+        failureMessage denied `shouldBe` "write conflict on table users, id 3"
+        _ <- mustSql second "rollback"
+        sessionAge second 3 `shouldReturn` Just 31
+
+    it "refuses COMMIT when another session inserted the same id" $ withTwoSessions $ \first second -> do
+        _ <- mustSql first "begin"
+        _ <- mustSql second "begin"
+        _ <- mustSql first "insert into users (id, name, age) values (7, 'seven', 27)"
+        _ <- mustSql second "insert into users (id, name, age) values (7, 'other', 70)"
+        _ <- mustSql first "commit"
+        denied <- runStatementCoded second "commit"
+        failureMessage denied `shouldBe` "write conflict on table users, id 7"
+        _ <- mustSql second "rollback"
+        ids <- sessionIds second
+        ids `shouldMatchList` [1, 2, 3, 4, 5, 7]
+
+    it "keeps other sessions' rows untouched when the write sets do not overlap" $ withTwoSessions $ \first second -> do
+        _ <- mustSql first "begin"
+        _ <- mustSql second "begin"
+        _ <- mustSql first "insert into users (id, name, age) values (6, 'six', 26)"
+        _ <- mustSql first "update users set age = 31 where id = 1"
+        _ <- mustSql second "update users set age = 42 where id = 2"
+        _ <- mustSql first "commit"
+        _ <- mustSql second "commit"
+        sessionAge second 1 `shouldReturn` Just 31
+        sessionAge second 2 `shouldReturn` Just 42
+        ids <- sessionIds second
+        ids `shouldMatchList` [1, 2, 3, 4, 5, 6]
+
+    it "lets both sessions delete the same row" $ withTwoSessions $ \first second -> do
+        _ <- mustSql first "begin"
+        _ <- mustSql second "begin"
+        _ <- mustSql first "delete from users where id = 4"
+        _ <- mustSql second "delete from users where id = 4"
+        _ <- mustSql first "commit"
+        _ <- mustSql second "commit"
+        ids <- sessionIds second
+        ids `shouldMatchList` [1, 2, 3, 5]
+
     it "needs a current database to start" $ do
         db <- newMVar testDb
         settings <- tempSettingsPath "transaction-bare"
@@ -807,3 +882,28 @@ ipcConcurrencySpec = describe "server backend on the real storage" $ do
         rows <- rowsOf (beStatement alpha "SELECT id FROM t")
         intColumn "id" rows `shouldBe` [1]
 
+    it "refuses the second COMMIT on the real storage" $ withIpcStorage $ do
+        base <- ipcBackend
+        mustRun (beStatement base "CREATE DATABASE alpha")
+        let alpha = beWithDatabase base "alpha"
+        mustRun (beStatement alpha "CREATE TABLE users (id int, name text, age int)")
+        mustRun (beStatement alpha "INSERT INTO users (id, name, age) VALUES (1, 'a', 21)")
+        settings <- tempSettingsPath "transaction-ipc-conflict"
+        removeIfExists settings
+        first <- newSession alpha (T.pack testDatabaseName) settings
+        second <- newSession alpha (T.pack testDatabaseName) settings
+        _ <- mustSql first "use alpha"
+        _ <- mustSql second "use alpha"
+        _ <- mustSql first "begin"
+        _ <- mustSql second "begin"
+        _ <- mustSql first "update users set age = 31 where id = 1"
+        _ <- mustSql second "update users set age = 42 where id = 1"
+        _ <- mustSql first "commit"
+        denied <- runStatementCoded second "commit"
+        failureCode denied `shouldBe` "serialization_failure"
+        failureMessage denied `shouldBe` "write conflict on table users, id 1"
+        _ <- mustSql second "rollback"
+        rows <- rowsOf (beStatement alpha "SELECT id, age FROM users")
+        intColumn "id" rows `shouldBe` [1]
+        intColumn "age" rows `shouldBe` [31]
+
diff --git a/docs/architecture.md b/docs/architecture.md
index e96ef7e..4bbdec2 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -49,7 +49,7 @@ chusql-server
 
 语句里的库前缀和表限定符由引擎在语义层解析：存储层只接受精确的表名键，因此 `FROM sales.orders` 与选定 `sales` 后的 `FROM orders` 指向同一张表。
 
-显式事务在会话层实现：`BEGIN` 把当前数据库整库读成内存快照，事务内的语句都在快照上执行；`COMMIT` 计算快照与库的差分，用一条 `apply_transaction` 请求交给存储层应用；`ROLLBACK` 直接丢弃快照。因此隔离级别是快照读加原子提交：事务里看不到别的事务未提交或后来提交的改动，提交对外一次性可见。
+显式事务在会话层实现：`BEGIN` 把当前数据库整库读成内存快照，事务内的语句都在快照上执行；`COMMIT` 先把快照、基线与库里现值按行 `id` 逐键比对，检测到写写冲突就报 `serialization_failure` 并保留事务；没有冲突才把差分用一条 `apply_transaction` 请求交给存储层应用；`ROLLBACK` 直接丢弃快照。因此隔离级别是快照读加原子提交：事务里看不到别的事务未提交或后来提交的改动，提交对外一次性可见，冲突由调用方回滚或重试。
 
 WAL 负责崩溃恢复：每条记录带自增 LSN，一组写操作与一条提交标记一次落盘；每组提交在数据和字典都刷盘之后把 `wal.checkpoint` 推进到这条提交的 LSN，含义是「到此为止的改动都已落盘」，再按这个边界丢掉日志前缀——所以日志里通常只有当组在飞的记录。进程重启时只重放检查点之后的记录，按标记分组取已提交的组、丢弃没有标记的整组，重放前逐条做幂等检查作为第二道保险，结束后再把检查点推到最后一条提交。LSN 从检查点与文件尾里较大的那个续号，跨重启单调，`SystemStatus` 应答里的 `last_lsn` 报告该检查点 LSN。建表、删表、建索引、删索引与数据改动走同一条 WAL，恢复时一并重放。