diff --git a/cloud/sandbox.py b/cloud/sandbox.py
index a2fab57..9456251 100644
--- a/cloud/sandbox.py
+++ b/cloud/sandbox.py
@@ -896,7 +896,8 @@ class DockerExecutor(Executor):
                  user="1000:1000", worktrees_root=None, env=None,
                  env_key="XCM_API_KEY", timeout=None, extra_mounts=None,
                  model=None, security_opts=None, pids_limit=None,
-                 runtime=None, extra_env=None, ignore_host_budget=False):
+                 runtime=None, extra_env=None, ignore_host_budget=False,
+                 readonly_root=False):
         if host_py_path is None:
             here = os.path.dirname(os.path.abspath(__file__))
             host_py_path = os.path.join(os.path.dirname(here), "host.py")
@@ -935,6 +936,11 @@ class DockerExecutor(Executor):
         if not self.runtime:
             self.runtime = None
         self.extra_env = dict(extra_env or {})
+        # 严格模式：容器根文件系统只读（/work 挂载仍可写、/tmp 与
+        # /var/tmp 为 tmpfs，见 build_run_command）。默认关：codex
+        # 可能往 HOME 等容器内路径写，默认开会改现有任务行为；
+        # 开启是显式的 worker 旗标 --docker-readonly-root。
+        self.readonly_root = bool(readonly_root)
 
     @staticmethod
     def _parse_extra_mounts(extra_mounts):
@@ -966,6 +972,9 @@ class DockerExecutor(Executor):
                 raise ValueError("extra mount 拒绝危险宿主路径: %s" % spec)
             if cont_p in ("/", "/app"):
                 raise ValueError("extra mount 拒绝覆盖容器根或 /app 本身: %s" % spec)
+            if cont_p in ("/tmp", "/var/tmp"):
+                raise ValueError(
+                    "extra mount 拒绝覆盖 /tmp（容器 /tmp 固定为 tmpfs 隔离）: %s" % spec)
             if not os.path.exists(host_p):
                 raise ValueError("extra mount 宿主路径不存在，拒绝挂载: %s" % spec)
             out.append((host_p, cont_p, mode))
@@ -1052,6 +1061,8 @@ class DockerExecutor(Executor):
                "--memory", self.memory,
                "--cap-drop=ALL",
                "--security-opt=no-new-privileges"]
+        if self.readonly_root:
+            cmd += ["--read-only"]
         for opt in self.security_opts:
             if opt != "no-new-privileges":
                 cmd += ["--security-opt", opt]
@@ -1071,6 +1082,13 @@ class DockerExecutor(Executor):
             else self._parse_extra_mounts(extra_mounts)
         for host_p, cont_p, mode in mounts:
             cmd += ["-v", "%s:%s:%s" % (host_p, cont_p, mode)]
+        # /tmp 固定为 tmpfs：容器内 /tmp 绝不落在宿主 /tmp，也不受
+        # 额外挂载影响（/tmp 目标的额外挂载已在解析时拒绝）。
+        # 诚实边界：这是 Docker 外层能做的；内层 codex 的 bwrap
+        # workspace-write 仍由 codex 二进制控制，本仓库改不了它
+        # 的 --proc 挂载，宿主级 bwrap 的 /tmp 不隔离问题也不在
+        # 本仓库的修复范围内——不要把本改动说成“内层沙箱已修”。
+        cmd += ["--tmpfs", "/tmp", "--tmpfs", "/var/tmp"]
         if self.env_key:
             cmd += ["-e", self.env_key]
         merged_env = dict(self.extra_env)
diff --git a/cloud/tests/test_docker_sandbox.py b/cloud/tests/test_docker_sandbox.py
index 97a3ae7..278633e 100644
--- a/cloud/tests/test_docker_sandbox.py
+++ b/cloud/tests/test_docker_sandbox.py
@@ -362,6 +362,28 @@ class TestDockerExtraMounts(DockerCommandTestCase):
             self.assertIn(f"{work}:/app/work:rw", mounts)
             self.assertLess(mounts.index("/fake:/app:ro"), mounts.index(f"{work}:/app/work:rw"))
 
+    def test_tmpfs_tmp_and_readonly_root_flag(self):
+        with mock.patch("cloud.sandbox.shutil.which",
+                        return_value="/usr/bin/docker"):
+            ex = self._executor([])
+            cmd = ex.build_run_command(
+                {"id": "t1", "worktree": "/tmp/wt"}, ["echo", "hi"])
+        self.assertIn("--tmpfs", cmd)
+        self.assertIn("/tmp", cmd)
+        self.assertIn("/var/tmp", cmd)
+        self.assertNotIn("--read-only", cmd)
+        with mock.patch("cloud.sandbox.shutil.which",
+                        return_value="/usr/bin/docker"):
+            ex = DockerExecutor("/fake/host.py", runner=_ok_runner_factory([]),
+                                env={"XCM_API_KEY": "placeholder-not-a-real-key"},
+                                readonly_root=True)
+            cmd = ex.build_run_command(
+                {"id": "t1", "worktree": "/tmp/wt"}, ["echo", "hi"])
+        self.assertIn("--read-only", cmd)
+        with self.assertRaises(ValueError):
+            DockerExecutor("/fake/host.py",
+                           extra_mounts=["/tmp/host-tmp:/tmp"])
+
     def test_extra_mounts_reject_dangerous_paths(self):
         with self.assertRaises(ValueError):
             DockerExecutor("/fake/host.py", extra_mounts=["/: /x"])
diff --git a/cloud/worker.py b/cloud/worker.py
index 562e4e4..016f086 100644
--- a/cloud/worker.py
+++ b/cloud/worker.py
@@ -5843,7 +5843,8 @@ def _build_cli_executor(args, host_py):
         extra_mounts=args.docker_extra_mounts,
         security_opts=args.docker_security_opt,
         pids_limit=args.docker_pids_limit,
-        runtime=getattr(args, "docker_runtime", None))
+        runtime=getattr(args, "docker_runtime", None),
+        readonly_root=getattr(args, "docker_readonly_root", False))
 
 
 def main(argv=None):
@@ -5951,6 +5952,11 @@ def main(argv=None):
                         help="docker --runtime（不传 = docker 默认 runc；"
                              "传 runsc = 任务容器走 gVisor 沙箱，需 daemon"
                              "已注册该 runtime，否则起容器失败）")
+    parser.add_argument("--docker-readonly-root", action="store_true",
+                        help="docker --read-only：容器根只读（/work 挂载"
+                             "仍可写、/tmp 与 /var/tmp 为 tmpfs）。默认关："
+                             "codex 可能往容器内 HOME 等路径写，开之前"
+                             "需先在生产任务上实测，勿直接全量开")
     parser.add_argument("--egress-proxy-port", type=int, default=0,
                         help="egress 过滤代理端口（0 = 不启用；生产配"
                              " 8888）：启用后 Docker 任务容器的 HTTP(S)"