diff --git a/internal/tools/exec_backend_test.go b/internal/tools/exec_backend_test.go
index 9974ce7..1e088e0 100644
--- a/internal/tools/exec_backend_test.go
+++ b/internal/tools/exec_backend_test.go
@@ -137,13 +137,13 @@ func TestSandboxHomeHidden(t *testing.T) {
 		t.Skipf("cannot create home fixture: %v", err)
 	}
 	defer os.Remove(secret)
-	out, state := sandboxRun(t, "cat \"$HOME/.agent-ext16-marker\" 2>err.txt; cat err.txt; exit 7", 30*time.Second)
+	out, state := sandboxRun(t, "cat \"$HOME/.agent-ext16-marker\" 2>err.txt; cat err.txt; echo SANDBOX_SCRIPT_RAN; exit 7", 30*time.Second)
+	if state != "exit=7" || !strings.Contains(out, "SANDBOX_SCRIPT_RAN") {
+		t.Fatalf("sandbox script did not run: state=%s out=%q", state, out)
+	}
 	if strings.Contains(out, "topsecret") {
 		t.Fatalf("sandbox read real home secret (state=%s out=%q)", state, out)
 	}
-	if state == "exit=0" || state == "exit=7" && strings.Contains(out, "topsecret") {
-		t.Fatalf("unexpected success: state=%s out=%q", state, out)
-	}
 }
 
 func TestSandboxSiblingSessionUnreadable(t *testing.T) {
@@ -164,7 +164,7 @@ func TestSandboxSiblingSessionUnreadable(t *testing.T) {
 	defer cancel()
 	var buf bytes.Buffer
 	spec := ExecSpec{
-		Bin: "/bin/bash", Args: []string{"-c", "cat '" + marker + "'"}, Dir: a,
+		Bin: "/bin/bash", Args: []string{"-c", "cat '" + marker + "' 2>&1; echo SANDBOX_SCRIPT_RAN; exit 7"}, Dir: a,
 		Env: []string{"PATH=/usr/bin:/bin", "HOME=/home/agent"},
 	}
 	proc, err := sandboxBackend{}.Start(ctx, spec, &buf)
@@ -172,7 +172,11 @@ func TestSandboxSiblingSessionUnreadable(t *testing.T) {
 		t.Fatal(err)
 	}
 	werr := proc.Wait()
-	if werr == nil || buf.String() == "others-data" {
+	exit, ok := werr.(*exec.ExitError)
+	if !ok || exit.ExitCode() != 7 || !strings.Contains(buf.String(), "SANDBOX_SCRIPT_RAN") {
+		t.Fatalf("sandbox script did not run: wait=%v out=%q", werr, buf.String())
+	}
+	if strings.Contains(buf.String(), "others-data") {
 		t.Fatalf("sibling session readable inside sandbox: wait=%v out=%q", werr, buf.String())
 	}
 }