diff --git a/docker_agent_sandbox/sandbox.py b/docker_agent_sandbox/sandbox.py
index 009a699..1c8e34a 100644
--- a/docker_agent_sandbox/sandbox.py
+++ b/docker_agent_sandbox/sandbox.py
@@ -6,11 +6,14 @@ import io
 import select as _select
 import socket as _socket
 import tarfile
+from contextlib import ExitStack
 from select import select as _original_select
 from pathlib import Path
 from typing import TYPE_CHECKING
 from unittest.mock import patch
 
+_original_poll = getattr(_select, "poll", None)
+
 import docker
 import docker.errors
 from docker.utils.socket import consume_socket_output, demux_adaptor, frames_iter
@@ -46,6 +49,7 @@ class DockerSandbox:
         security_opt: list[str] | None = None,
         cpu_limit: float = 8,
         memory_limit: str = "16g",
+        command: str | None = None,
     ) -> None:
         self.container_name = container_name
         self._image = image
@@ -60,6 +64,7 @@ class DockerSandbox:
         self._security_opt = security_opt
         self._nano_cpus = int(cpu_limit * 1e9)
         self._memory_limit = memory_limit
+        self._command = command
         self._client: docker.DockerClient = docker.from_env()
         self._container: docker.models.containers.Container | None = None
 
@@ -132,7 +137,7 @@ class DockerSandbox:
         run_kwargs["mem_limit"] = self._memory_limit
 
         try:
-            self._container = self._client.containers.run(self._image, **run_kwargs)
+            self._container = self._client.containers.run(self._image, self._command, **run_kwargs)
         except docker.errors.ImageNotFound:
             raise RuntimeError(
                 f"Image {self._image!r} not found locally. "
@@ -258,9 +263,12 @@ class DockerSandbox:
 
         # TODO(fragile): timeout enforcement relies on private docker-py internals
         # (frames_iter, demux_adaptor, consume_socket_output from docker.utils.socket)
-        # and monkey-patches select.select for the duration of the read — not thread-safe
-        # if multiple exec() calls run concurrently. Replace when docker-py adds native
-        # per-call timeout support. See https://github.com/docker/docker-py/issues/2651
+        # and monkey-patches select.select / select.poll for the duration of the read
+        # — not thread-safe if multiple exec() calls run concurrently. Replace when
+        # docker-py adds native per-call timeout support.
+        # See https://github.com/docker/docker-py/issues/2651
+        #
+        # On Linux docker-py uses select.poll (not select.select), so both are patched.
         try:
             exec_id = self._client.api.exec_create(
                 self._container.id,
@@ -271,17 +279,39 @@ class DockerSandbox:
             )
             sock = self._client.api.exec_start(exec_id["Id"], socket=True)
             sock._sock.settimeout(timeout)
-            with patch.object(
-                _select,
-                "select",
-                new=lambda rlist, wlist, xlist: _original_select(
-                    rlist, wlist, xlist, timeout
-                ),
-            ):
+
+            timeout_ms = timeout * 1000
+
+            class _PollWithTimeout:
+                def __init__(self):
+                    self._inner = _original_poll()
+
+                def register(self, fd, eventmask):
+                    return self._inner.register(fd, eventmask)
+
+                def poll(self, *args):
+                    result = self._inner.poll(timeout_ms)
+                    if not result:
+                        raise _socket.timeout(f"timed out after {timeout}s")
+                    return result
+
+            with ExitStack() as stack:
+                stack.enter_context(patch.object(
+                    _select, "select",
+                    new=lambda rlist, wlist, xlist: _original_select(
+                        rlist, wlist, xlist, timeout
+                    ),
+                ))
+                if _original_poll is not None:
+                    stack.enter_context(
+                        patch.object(_select, "poll", new=_PollWithTimeout)
+                    )
                 gen = (demux_adaptor(*frame) for frame in frames_iter(sock, tty=False))
                 stdout, stderr = consume_socket_output(gen, demux=True)
 
-            exit_code = self._client.api.exec_inspect(exec_id["Id"])["ExitCode"] or 0
+            exit_code = self._client.api.exec_inspect(exec_id["Id"])["ExitCode"]
+            if exit_code is None:
+                exit_code = 0
             output = (stdout or b"") + (stderr or b"")
             return exit_code, output.decode("utf-8", errors="replace")
         except _socket.timeout: