diff --git a/codex-rs/windows-sandbox-rs/src/unified_exec/tests.rs b/codex-rs/windows-sandbox-rs/src/unified_exec/tests.rs
index a2bc7bebf..b0530a4fb 100644
--- a/codex-rs/windows-sandbox-rs/src/unified_exec/tests.rs
+++ b/codex-rs/windows-sandbox-rs/src/unified_exec/tests.rs
@@ -14,6 +14,8 @@ use std::io::Seek;
 use std::io::SeekFrom;
 use std::path::Path;
 use std::path::PathBuf;
+use std::sync::Mutex;
+use std::sync::MutexGuard;
 use std::sync::atomic::AtomicU64;
 use std::sync::atomic::Ordering;
 use std::time::Duration;
@@ -26,6 +28,13 @@ use tokio::sync::oneshot;
 use tokio::time::timeout;
 
 static TEST_HOME_COUNTER: AtomicU64 = AtomicU64::new(0);
+static LEGACY_PROCESS_TEST_LOCK: Mutex<()> = Mutex::new(());
+
+fn legacy_process_test_guard() -> MutexGuard<'static, ()> {
+    LEGACY_PROCESS_TEST_LOCK
+        .lock()
+        .expect("legacy Windows sandbox process test lock poisoned")
+}
 
 fn current_thread_runtime() -> tokio::runtime::Runtime {
     Builder::new_current_thread()
@@ -129,6 +138,7 @@ async fn collect_stdout_and_exit(
 
 #[test]
 fn legacy_non_tty_cmd_emits_output() {
+    let _guard = legacy_process_test_guard();
     let runtime = current_thread_runtime();
     runtime.block_on(async move {
         let cwd = sandbox_cwd();
@@ -167,6 +177,7 @@ fn legacy_non_tty_powershell_emits_output() {
     let Some(pwsh) = pwsh_path() else {
         return;
     };
+    let _guard = legacy_process_test_guard();
     let runtime = current_thread_runtime();
     runtime.block_on(async move {
         let cwd = sandbox_cwd();
@@ -351,6 +362,7 @@ fn legacy_capture_powershell_emits_output() {
     let Some(pwsh) = pwsh_path() else {
         return;
     };
+    let _guard = legacy_process_test_guard();
     let cwd = sandbox_cwd();
     let codex_home = sandbox_home("legacy-capture-pwsh");
     println!("capture pwsh codex_home={}", codex_home.path().display());
@@ -387,6 +399,7 @@ fn legacy_tty_powershell_emits_output_and_accepts_input() {
     let Some(pwsh) = pwsh_path() else {
         return;
     };
+    let _guard = legacy_process_test_guard();
     let runtime = current_thread_runtime();
     runtime.block_on(async move {
         let cwd = sandbox_cwd();