diff --git a/codex-rs/core/tests/common/test_codex.rs b/codex-rs/core/tests/common/test_codex.rs
index de3a31b673..7c63fc90af 100644
--- a/codex-rs/core/tests/common/test_codex.rs
+++ b/codex-rs/core/tests/common/test_codex.rs
@@ -493,13 +493,22 @@ impl TestCodexBuilder {
     pub async fn build_with_auto_env(
         &mut self,
         server: &wiremock::MockServer,
+    ) -> anyhow::Result<TestCodex> {
+        let test_env = test_env().await?;
+        self.build_with_environment(server, test_env).await
+    }
+
+    /// Builds a test runtime using an explicitly selected execution environment.
+    pub async fn build_with_environment(
+        &mut self,
+        server: &wiremock::MockServer,
+        test_env: TestEnv,
     ) -> anyhow::Result<TestCodex> {
         let home = match self.home.clone() {
             Some(home) => home,
             None => Arc::new(TempDir::new()?),
         };
         let base_url = format!("{}/v1", server.uri());
-        let test_env = test_env().await?;
         Box::pin(self.build_with_home_and_base_url(
             base_url, home, /*resume_from*/ None, test_env,
             /*include_local_environment*/ false,
diff --git a/codex-rs/exec-server/testing/exec_server.rs b/codex-rs/exec-server/testing/exec_server.rs
index 36c7685ceb..9c6773995b 100644
--- a/codex-rs/exec-server/testing/exec_server.rs
+++ b/codex-rs/exec-server/testing/exec_server.rs
@@ -7,31 +7,32 @@
 use codex_exec_server::ExecServerRuntimePaths;
 use codex_http_client::HttpClientFactory;
 use codex_http_client::OutboundProxyPolicy;
-#[cfg(unix)]
 use std::ffi::OsStr;
 
-#[tokio::main]
-async fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+const CODEX_LINUX_SANDBOX_EXE_ENV_VAR: &str = "CODEX_TEST_LINUX_SANDBOX_EXE";
+
+fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+    let mut args = std::env::args_os();
+    let _ = args.next();
+    let argv1 = args.next();
     #[cfg(unix)]
-    {
-        let mut args = std::env::args_os();
-        let _ = args.next();
-        if args.next().as_deref()
-            == Some(OsStr::new(
-                codex_exec_server::CODEX_ARG0_EXEC_HELPER_ARG1,
-            ))
-        {
-            codex_exec_server::run_arg0_exec_helper_main();
-        }
+    if argv1.as_deref() == Some(OsStr::new(codex_exec_server::CODEX_ARG0_EXEC_HELPER_ARG1)) {
+        codex_exec_server::run_arg0_exec_helper_main();
+    }
+    if argv1.as_deref() == Some(OsStr::new(codex_exec_server::CODEX_FS_HELPER_ARG1)) {
+        codex_exec_server::run_fs_helper_main();
     }
 
     let current_exe = std::env::current_exe()?;
-    let runtime_paths =
-        ExecServerRuntimePaths::new(current_exe, /*codex_linux_sandbox_exe*/ None)?;
-    codex_exec_server::run_main(
-        "ws://127.0.0.1:0",
-        runtime_paths,
-        HttpClientFactory::new(OutboundProxyPolicy::ReqwestDefault),
-    )
-    .await
+    let codex_linux_sandbox_exe =
+        std::env::var_os(CODEX_LINUX_SANDBOX_EXE_ENV_VAR).map(std::path::PathBuf::from);
+    let runtime_paths = ExecServerRuntimePaths::new(current_exe, codex_linux_sandbox_exe)?;
+    tokio::runtime::Builder::new_multi_thread()
+        .enable_all()
+        .build()?
+        .block_on(codex_exec_server::run_main(
+            "ws://127.0.0.1:0",
+            runtime_paths,
+            HttpClientFactory::new(OutboundProxyPolicy::ReqwestDefault),
+        ))
 }