diff --git a/codex-rs/linux-sandbox/src/linux_run_main.rs b/codex-rs/linux-sandbox/src/linux_run_main.rs
index 27cdb5032..346b1f14c 100644
--- a/codex-rs/linux-sandbox/src/linux_run_main.rs
+++ b/codex-rs/linux-sandbox/src/linux_run_main.rs
@@ -1242,7 +1242,10 @@ fn synthetic_mount_marker_dir(path: &Path) -> PathBuf {
 }
 
 fn synthetic_mount_registry_root() -> PathBuf {
-    std::env::temp_dir().join("codex-bwrap-synthetic-mount-targets")
+    let effective_uid = unsafe { libc::geteuid() };
+    std::env::temp_dir().join(format!(
+        "codex-bwrap-synthetic-mount-targets-{effective_uid}"
+    ))
 }
 
 fn hash_path(path: &Path) -> u64 {
diff --git a/codex-rs/linux-sandbox/src/linux_run_main_tests.rs b/codex-rs/linux-sandbox/src/linux_run_main_tests.rs
index 95db9d9de..4441af780 100644
--- a/codex-rs/linux-sandbox/src/linux_run_main_tests.rs
+++ b/codex-rs/linux-sandbox/src/linux_run_main_tests.rs
@@ -302,6 +302,17 @@ fn cleanup_synthetic_mount_targets_removes_only_empty_mount_targets() {
     assert!(!missing_file.exists());
 }
 
+#[test]
+fn synthetic_mount_registry_root_is_unique_to_effective_user() {
+    let effective_uid = unsafe { libc::geteuid() };
+    assert_eq!(
+        synthetic_mount_registry_root(),
+        std::env::temp_dir().join(format!(
+            "codex-bwrap-synthetic-mount-targets-{effective_uid}"
+        ))
+    );
+}
+
 #[test]
 fn cleanup_synthetic_mount_targets_waits_for_other_active_registrations() {
     let temp_dir = tempfile::TempDir::new().expect("tempdir");