diff --git a/frontend/src/setupTests.ts b/frontend/src/setupTests.ts
index 1c857b4a..f7d022a8 100644
--- a/frontend/src/setupTests.ts
+++ b/frontend/src/setupTests.ts
@@ -1,6 +1,31 @@
 import "@testing-library/jest-dom";
 import { beforeEach, vi } from "vitest";
 
+// Default WebSocket stub: every `new WebSocket(...)` immediately fires
+// `onerror` + `onclose` on the next microtask. Without this, `rpcCall` from
+// `./api/rpc` (added by 770's HTTP→read-RPC migration) opens a real jsdom
+// WebSocket that hangs ~9s before firing its connection-failure error,
+// making any test that mounts a component calling `listAgents()` time out.
+// Tests that need real WS responses should override per-test with
+// `vi.stubGlobal("WebSocket", ...)`.
+class FailingWebSocket {
+	onopen: ((ev: Event) => void) | null = null;
+	onmessage: ((ev: MessageEvent) => void) | null = null;
+	onerror: ((ev: Event) => void) | null = null;
+	onclose: ((ev: CloseEvent) => void) | null = null;
+	readyState = 0;
+	constructor(_url: string) {
+		queueMicrotask(() => {
+			this.readyState = 3;
+			this.onerror?.(new Event("error"));
+			this.onclose?.(new CloseEvent("close"));
+		});
+	}
+	send(_data: string) {}
+	close() {}
+}
+vi.stubGlobal("WebSocket", FailingWebSocket);
+
 // Provide a default fetch mock so components that call API endpoints on mount
 // don't throw URL-parse errors in the jsdom test environment.  Tests that need
 // specific responses should mock the relevant `api.*` method as usual.