diff --git a/extensions/acpx/src/runtime.test.ts b/extensions/acpx/src/runtime.test.ts
index 35444ba1ac9..5d410573ded 100644
--- a/extensions/acpx/src/runtime.test.ts
+++ b/extensions/acpx/src/runtime.test.ts
@@ -111,7 +111,11 @@ function makeLeaseStore() {
 function readFirstEnsureSessionInput(ensure: {
   mock: { calls: Array<Array<unknown>> };
 }): Parameters<AcpRuntime["ensureSession"]>[0] {
-  const input = ensure.mock.calls[0]?.[0];
+  const [call] = ensure.mock.calls;
+  if (!call) {
+    throw new Error("Expected ensureSession to be called");
+  }
+  const [input] = call;
   if (typeof input !== "object" || input === null) {
     throw new Error("Expected ensureSession to be called with an input object");
   }
diff --git a/extensions/acpx/src/service.test.ts b/extensions/acpx/src/service.test.ts
index 022c7d7deab..37b5e61dd22 100644
--- a/extensions/acpx/src/service.test.ts
+++ b/extensions/acpx/src/service.test.ts
@@ -157,7 +157,11 @@ function createMockRuntime(overrides: Record<string, unknown> = {}) {
 }
 
 function readFirstRuntimeFactoryInput(runtimeFactory: { mock: { calls: Array<Array<unknown>> } }) {
-  const input = runtimeFactory.mock.calls[0]?.[0];
+  const [call] = runtimeFactory.mock.calls;
+  if (!call) {
+    throw new Error("Expected runtimeFactory to be called");
+  }
+  const [input] = call;
   if (typeof input !== "object" || input === null) {
     throw new Error("Expected runtimeFactory to be called with an options object");
   }