diff --git a/packages/desktop/src/main/debug/index.ts b/packages/desktop/src/main/debug/index.ts
index a8962369..61455b59 100644
--- a/packages/desktop/src/main/debug/index.ts
+++ b/packages/desktop/src/main/debug/index.ts
@@ -34,6 +34,7 @@ let debugLogger: DebugLogger = {
   log: (input) => record(input.level, input),
   getRecentEvents: () => [],
   currentFilePath: () => '',
+  flush: () => Promise.resolve(),
 };
 
 export function initDebugLogger(debugDir: string): DebugLogger {
diff --git a/packages/desktop/src/main/debug/logger.ts b/packages/desktop/src/main/debug/logger.ts
index e87c31de..474dbf6f 100644
--- a/packages/desktop/src/main/debug/logger.ts
+++ b/packages/desktop/src/main/debug/logger.ts
@@ -1,4 +1,4 @@
-import { appendFileSync, existsSync, mkdirSync } from 'node:fs';
+import { createWriteStream, existsSync, mkdirSync, openSync, type WriteStream } from 'node:fs';
 import { join } from 'node:path';
 import type { DebugDomain, DebugEvent, DebugLevel } from '@clawwork/shared';
 import { sanitizeForLog } from '@clawwork/shared';
@@ -29,6 +29,7 @@ export interface DebugLogger {
   log: (input: LogEventInput & { level: DebugLevel }) => DebugEvent;
   getRecentEvents: (filter?: DebugLogFilter) => DebugEvent[];
   currentFilePath: () => string;
+  flush: () => Promise<void>;
 }
 
 export interface LogEventInput {
@@ -55,8 +56,38 @@ export function createDebugLogger(options: CreateDebugLoggerOptions): DebugLogge
   const maxEvents = options.maxEvents ?? 1000;
   const writeConsole = options.console ?? true;
   const recentEvents: DebugEvent[] = [];
+  const debugDir = options.debugDir;
 
-  ensureDir(options.debugDir);
+  // Write stream state — cached fd avoids open/write/close per event
+  let writeStream: WriteStream | null = null;
+  let currentLogDate = '';
+
+  ensureDir(debugDir);
+
+  function currentFilePath(): string {
+    const day = new Date().toISOString().slice(0, 10);
+    return join(debugDir, `debug-${day}.ndjson`);
+  }
+
+  function ensureStream(): void {
+    const today = new Date().toISOString().slice(0, 10);
+    if (writeStream && currentLogDate === today) return;
+
+    // Close previous day's stream
+    if (writeStream) {
+      writeStream.end();
+      writeStream = null;
+    }
+
+    ensureDir(debugDir);
+    currentLogDate = today;
+    const filePath = currentFilePath();
+    // Open fd synchronously so the file exists immediately and is append-only
+    const fd = openSync(filePath, 'a');
+    writeStream = createWriteStream(filePath, { fd, autoClose: true }).on('error', (err) => {
+      console.error('[debug] logger stream error:', err);
+    });
+  }
 
   function log(input: LogEventInput & { level: DebugLevel }): DebugEvent {
     const event: DebugEvent = sanitizeForLog({
@@ -69,7 +100,9 @@ export function createDebugLogger(options: CreateDebugLoggerOptions): DebugLogge
       recentEvents.splice(0, recentEvents.length - maxEvents);
     }
 
-    appendFileSync(currentFilePath(), `${JSON.stringify(event)}\n`, 'utf8');
+    // Async write via persistent stream — no more blocking the event loop
+    ensureStream();
+    writeStream!.write(`${JSON.stringify(event)}\n`, 'utf8');
 
     if (writeConsole) {
       const line = `[${event.level}] [${event.domain}] ${event.event}`;
@@ -82,6 +115,19 @@ export function createDebugLogger(options: CreateDebugLoggerOptions): DebugLogge
     return event;
   }
 
+  async function flush(): Promise<void> {
+    if (!writeStream || writeStream.closed || writeStream.destroyed) return;
+    // Write an empty chunk to serve as a flush marker.
+    // Writable maintains ordering — this callback fires only after
+    // all previously enqueued data has been written to the OS.
+    return new Promise<void>((resolve, reject) => {
+      writeStream!.write(Buffer.alloc(0), (err) => {
+        if (err) reject(err);
+        else resolve();
+      });
+    });
+  }
+
   return {
     debug: (input) => log({ ...input, level: 'debug' }),
     info: (input) => log({ ...input, level: 'info' }),
@@ -90,12 +136,8 @@ export function createDebugLogger(options: CreateDebugLoggerOptions): DebugLogge
     log,
     getRecentEvents: (filter) => filterEvents(recentEvents, filter),
     currentFilePath,
+    flush,
   };
-
-  function currentFilePath(): string {
-    const day = new Date().toISOString().slice(0, 10);
-    return join(options.debugDir, `debug-${day}.ndjson`);
-  }
 }
 
 function filterEvents(events: DebugEvent[], filter?: DebugLogFilter): DebugEvent[] {
diff --git a/packages/desktop/src/main/index.ts b/packages/desktop/src/main/index.ts
index 74617e64..3a7ee8be 100644
--- a/packages/desktop/src/main/index.ts
+++ b/packages/desktop/src/main/index.ts
@@ -277,13 +277,25 @@ app.on('window-all-closed', () => {
   }
 });
 
-app.on('before-quit', () => {
+app.on('before-quit', (event) => {
+  if (isQuitting) return;
+  event.preventDefault();
   isQuitting = true;
-  getDebugLogger().info({ domain: 'app', event: 'app.before-quit', data: { installingUpdate: isInstallingUpdate() } });
-  globalShortcut.unregisterAll();
-  unwatchAll();
-  destroyAllGateways();
-  destroyTray();
-  destroyQuickLaunch();
-  closeDatabase();
+
+  const logger = getDebugLogger();
+  logger.info({ domain: 'app', event: 'app.before-quit', data: { installingUpdate: isInstallingUpdate() } });
+
+  // Flush pending debug log writes before exit, then complete shutdown
+  logger
+    .flush()
+    .catch(() => {})
+    .finally(() => {
+      globalShortcut.unregisterAll();
+      unwatchAll();
+      destroyAllGateways();
+      destroyTray();
+      destroyQuickLaunch();
+      closeDatabase();
+      app.quit();
+    });
 });
diff --git a/packages/desktop/test/debug-observability.test.ts b/packages/desktop/test/debug-observability.test.ts
index e524bcdb..6f5b3f3a 100644
--- a/packages/desktop/test/debug-observability.test.ts
+++ b/packages/desktop/test/debug-observability.test.ts
@@ -42,7 +42,7 @@ describe('debug observability foundation', () => {
       rmSync(dir, { recursive: true, force: true });
     });
 
-    it('stores recent events and writes ndjson to disk', () => {
+    it('stores recent events and writes ndjson to disk', async () => {
       const seen: string[] = [];
       const logger = createDebugLogger({
         debugDir: dir,
@@ -56,6 +56,8 @@ describe('debug observability foundation', () => {
       logger.info({ domain: 'gateway', event: 'gateway.connect.start', gatewayId: 'gw-1' });
       logger.error({ domain: 'ipc', event: 'ipc.ws.send-message.failed', error: { message: 'not connected' } });
 
+      await logger.flush();
+
       const events = logger.getRecentEvents();
       expect(events).toHaveLength(2);
       expect(events[0]?.event).toBe('gateway.connect.start');