diff --git a/packages/electron/main.mjs b/packages/electron/main.mjs
index d2b85e0e..a308b5ce 100644
--- a/packages/electron/main.mjs
+++ b/packages/electron/main.mjs
@@ -1,4 +1,4 @@
-import { app, BrowserWindow, dialog, ipcMain, Menu, nativeTheme, Notification, session, shell } from 'electron';
+import { app, BrowserWindow, dialog, ipcMain, Menu, nativeTheme, Notification, powerMonitor, session, shell } from 'electron';
 import contextMenu from 'electron-context-menu';
 import log from 'electron-log/main.js';
 import dgram from 'node:dgram';
@@ -2366,6 +2366,12 @@ app.whenReady().then(async () => {
   const { initialUrl, localOrigin, bootOutcome } = await resolveInitialUrl();
   await activateMainWindow(initialUrl, localOrigin, bootOutcome);
   startQuitRiskPoller();
+
+  // Notify renderer on OS wake-from-sleep so the SSE event pipeline can
+  // reconnect immediately instead of waiting for the heartbeat watchdog.
+  powerMonitor.on('resume', () => {
+    emitToAllWindows('openchamber:system-resume', { timestamp: Date.now() });
+  });
 }).catch((error) => {
   log.error('[electron] startup failed:', error);
   app.exit(1);
diff --git a/packages/ui/src/sync/__tests__/event-pipeline-resume.test.js b/packages/ui/src/sync/__tests__/event-pipeline-resume.test.js
new file mode 100644
index 00000000..3bb6ec29
--- /dev/null
+++ b/packages/ui/src/sync/__tests__/event-pipeline-resume.test.js
@@ -0,0 +1,112 @@
+import { afterEach, describe, expect, it } from 'bun:test';
+import { createEventPipeline } from '../event-pipeline';
+
+const savedDocument = globalThis.document;
+const savedWindow = globalThis.window;
+
+afterEach(() => {
+  globalThis.document = savedDocument;
+  globalThis.window = savedWindow;
+});
+
+describe('createEventPipeline — system resume reconnect', () => {
+  it('reconnects immediately on openchamber:system-resume event', async () => {
+    const winListeners = {};
+    globalThis.document = {
+      visibilityState: 'visible',
+      addEventListener() {},
+      removeEventListener() {},
+    };
+    globalThis.window = {
+      location: {
+        href: 'http://127.0.0.1:3000/',
+        origin: 'http://127.0.0.1:3000',
+      },
+      addEventListener(event, handler) { winListeners[event] = handler; },
+      removeEventListener(event) { delete winListeners[event]; },
+    };
+
+    const disconnectReasons = [];
+    let reconnectCount = 0;
+    const eventCalls = [];
+
+    let sdkCallIndex = 0;
+    let releaseFirstStream;
+    const firstHold = new Promise((resolve) => { releaseFirstStream = resolve; });
+
+    const sdk = {
+      global: {
+        // Accept options with signal so the mock generator can abort.
+        event: async (options) => {
+          const callIndex = sdkCallIndex++;
+          eventCalls.push(callIndex);
+          const signal = options?.signal;
+          if (callIndex === 0) {
+            return {
+              stream: (async function* () {
+                yield {
+                  payload: { type: 'session.status', properties: { sessionID: 's1', status: { type: 'idle' } } },
+                };
+                // Wait for either the hold promise or abort signal.
+                await Promise.race([
+                  firstHold,
+                  new Promise((_, reject) => {
+                    if (signal?.aborted) { reject(signal.reason || new DOMException('Aborted', 'AbortError')); return; }
+                    signal?.addEventListener('abort', () => {
+                      reject(signal.reason || new DOMException('Aborted', 'AbortError'));
+                    });
+                  }),
+                ]);
+              })(),
+            };
+          }
+          return {
+            stream: (async function* () {
+              yield {
+                payload: { type: 'session.status', properties: { sessionID: 's1', status: { type: 'idle' } } },
+              };
+              await new Promise(() => {});
+            })(),
+          };
+        },
+      },
+    };
+
+    const recovered = new Promise((resolve) => {
+      const { cleanup } = createEventPipeline({
+        sdk,
+        transport: 'sse',
+        heartbeatTimeoutMs: 60_000,
+        reconnectDelayMs: 60_000,
+        onEvent: () => {},
+        onDisconnect: (reason) => {
+          disconnectReasons.push(reason);
+        },
+        onReconnect: () => {
+          reconnectCount += 1;
+          // onReconnect fires on the initial connect too (count=1),
+          // so wait for the second reconnect (count=2) triggered by resume.
+          if (reconnectCount === 2) {
+            cleanup();
+            resolve();
+          }
+        },
+      });
+
+      // Wait for first SSE attempt to start and deliver the event, then
+      // simulate OS resume by invoking the registered handler directly.
+      setTimeout(() => {
+        const handler = winListeners['openchamber:system-resume'];
+        if (handler) handler();
+      }, 80);
+    });
+
+    await recovered;
+    releaseFirstStream();
+
+    // Should have made two SDK calls: initial connect + reconnect after resume.
+    expect(eventCalls.length).toBe(2);
+    // Disconnect reason should include system_resume.
+    expect(disconnectReasons.some((r) => r.includes('system_resume'))).toBe(true);
+  });
+});
diff --git a/packages/ui/src/sync/event-pipeline.ts b/packages/ui/src/sync/event-pipeline.ts
index 43e5e488..342e189c 100644
--- a/packages/ui/src/sync/event-pipeline.ts
+++ b/packages/ui/src/sync/event-pipeline.ts
@@ -152,6 +152,8 @@ type AttemptAbortReason =
   | "pipeline_stopped"
   | "ws_heartbeat_timeout"
   | "sse_heartbeat_timeout"
+  | "ws_system_resume"
+  | "sse_system_resume"
   | null
 
 export function createEventPipeline(input: EventPipelineInput) {
@@ -616,16 +618,33 @@ export function createEventPipeline(input: EventPipelineInput) {
     attempt?.abort()
   }
 
+  // OS wake-from-sleep (Electron powerMonitor.resume). The SSE connection
+  // is almost certainly dead after sleep — abort immediately so the
+  // reconnect loop fires on the next tick with retryDelayMs = 0.
+  const onSystemResume = () => {
+    attemptAbortReason = `${activeTransport}_system_resume`
+    attempt?.abort()
+  }
+
   if (typeof document !== "undefined") {
     document.addEventListener("visibilitychange", onVisibility)
     window.addEventListener("pageshow", onPageShow)
   }
 
+  // Use globalThis (not window) for the system-resume listener so that
+  // test environments can replace globalThis.window with a stub.
+  if (typeof globalThis.window !== "undefined") {
+    globalThis.window.addEventListener("openchamber:system-resume", onSystemResume)
+  }
+
   const cleanup = () => {
     if (typeof document !== "undefined") {
       document.removeEventListener("visibilitychange", onVisibility)
       window.removeEventListener("pageshow", onPageShow)
     }
+    if (typeof globalThis.window !== "undefined") {
+      globalThis.window.removeEventListener("openchamber:system-resume", onSystemResume)
+    }
     abort.abort()
     flushAll()
   }