diff --git a/src/infra/gateway-watch-tmux.test.ts b/src/infra/gateway-watch-tmux.test.ts
index 3da4336e629..6089779acbb 100644
--- a/src/infra/gateway-watch-tmux.test.ts
+++ b/src/infra/gateway-watch-tmux.test.ts
@@ -26,13 +26,22 @@ function requireRecord(value: unknown, label: string): Record<string, unknown> {
 
 function spawnCall(mock: unknown, callIndex: number) {
   const calls = (mock as { mock?: { calls?: Array<Array<unknown>> } }).mock?.calls ?? [];
-  const call = calls.at(callIndex);
+  const call = calls[callIndex];
   if (!call) {
     throw new Error(`Expected spawn call ${callIndex + 1}`);
   }
   return call;
 }
 
+function spawnShellCommand(mock: unknown, callIndex: number): string {
+  const call = spawnCall(mock, callIndex);
+  const args = call[1];
+  if (!Array.isArray(args) || typeof args[6] !== "string") {
+    throw new Error(`Expected spawn call ${callIndex + 1} shell command`);
+  }
+  return args[6];
+}
+
 function expectSpawn(mock: unknown, callIndex: number, command: string, args: Array<unknown>) {
   const call = spawnCall(mock, callIndex);
   expect(call[0]).toBe(command);
@@ -110,7 +119,7 @@ describe("gateway-watch tmux wrapper", () => {
     });
 
     expect(code).toBe(0);
-    const command = spawnSync.mock.calls.at(1)?.[1]?.[6] as string;
+    const command = spawnShellCommand(spawnSync, 1);
     expect(command).toContain("'OPENCLAW_RUN_NODE_CPU_PROF_DIR=.artifacts/gateway-watch-profiles'");
     expect(command).toContain("'OPENCLAW_TRACE_SYNC_IO=0'");
     expect(command).not.toContain("--benchmark");
@@ -142,7 +151,7 @@ describe("gateway-watch tmux wrapper", () => {
     });
 
     expect(code).toBe(0);
-    const command = spawnSync.mock.calls.at(1)?.[1]?.[6] as string;
+    const command = spawnShellCommand(spawnSync, 1);
     expect(command).toContain("'OPENCLAW_TRACE_SYNC_IO=1'");
     expect(command).toContain(
       "'OPENCLAW_RUN_NODE_OUTPUT_LOG=.artifacts/gateway-watch-profiles/gateway-watch-output.log'",
@@ -174,7 +183,7 @@ describe("gateway-watch tmux wrapper", () => {
     });
 
     expect(code).toBe(0);
-    const command = spawnSync.mock.calls.at(1)?.[1]?.[6] as string;
+    const command = spawnShellCommand(spawnSync, 1);
     expect(command).toContain("'OPENCLAW_RUN_NODE_CPU_PROF_DIR=.artifacts/gateway-watch-profiles'");
     expect(command).not.toContain("--benchmark-no-force");
     expect(command).toContain("'gateway'");