fix(tui): stop file-watcher polling for a missing file with a throwing probe

src/ui/tui/hooks/__tests__/file-watcher.test.ts

@@ -1,3 +1,4 @@
+import * as fs from 'fs';
 import { mkdtempSync, rmSync, writeFileSync, renameSync, unlinkSync } from 'fs';
 import { tmpdir } from 'os';
 import path from 'path';
@@ -6,6 +7,18 @@ import {
   type FileWatcherHandle,
 } from '@ui/tui/hooks/file-watcher';
 
+// Wrap `fs` so we can assert the watcher never reaches for the throwing
+// `accessSync` existence probe. All other fs calls pass through untouched.
+vi.mock('fs', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('fs')>();
+  return {
+    ...actual,
+    accessSync: vi.fn((...args: Parameters<typeof actual.accessSync>) =>
+      actual.accessSync(...args),
+    ),
+  };
+});
+
 const wait = (ms: number) => new Promise((resolve) => setTimeout(resolve, ms));
 
 describe('startFileWatcher', () => {
@@ -88,6 +101,27 @@ describe('startFileWatcher', () => {
     expect(onUpdate.mock.calls.at(-1)?.[0]).toEqual({ ready: true });
   });
 
+  it('never probes for the missing file with a throwing accessSync', async () => {
+    // Regression: the attach-retry loop used to poll with `fs.accessSync`,
+    // which throws ENOENT every tick while the file is absent. With exception
+    // autocapture on, each throw was reported as a spurious `$exception`.
+    const accessSync = vi.mocked(fs.accessSync);
+    accessSync.mockClear();
+    const onUpdate = vi.fn();
+    const target = path.join(workdir, 'never.json');
+
+    handle = startFileWatcher(target, onUpdate, {
+      pollIntervalMs: 20,
+      attachRetryIntervalMs: 20,
+    });
+
+    // Several retry ticks elapse with the file still absent.
+    await wait(120);
+
+    expect(accessSync).not.toHaveBeenCalled();
+    expect(onUpdate).not.toHaveBeenCalled();
+  });
+
   it('swallows invalid JSON without throwing', async () => {
     const onUpdate = vi.fn();
     const target = path.join(workdir, 'data.json');

src/ui/tui/hooks/file-watcher.ts

@@ -66,15 +66,23 @@ export function startFileWatcher(
     // File doesn't exist yet — retry attaching the watch periodically until
     // it appears. The poll above already covers updates; this just upgrades
     // latency once the file shows up.
+    //
+    // Probe with a *non-throwing* `existsSync` rather than `accessSync`: the
+    // "file not there yet" state is expected and hit on every tick until the
+    // agent writes the file, and a throw here gets picked up by exception
+    // autocapture and reported as a spurious `$exception` once a second.
     const attachInterval = setInterval(() => {
+      if (!fs.existsSync(path)) return; // Still waiting for the file.
       try {
-        fs.accessSync(path);
+        // Attach the watch first, then stop retrying — so a file that is
+        // removed between the existence check and `fs.watch` just leaves the
+        // poll running for the next tick instead of dropping the watcher.
+        watchers.push(fs.watch(path, () => read(true)));
         clearInterval(attachInterval);
         const idx = intervals.indexOf(attachInterval);
         if (idx >= 0) intervals.splice(idx, 1);
-        watchers.push(fs.watch(path, () => read(true)));
       } catch {
-        // Still waiting.
+        // Raced with the file disappearing; keep polling and retry.
       }
     }, attachRetryIntervalMs);
     intervals.push(attachInterval);

src/ui/tui/primitives/LogViewer.tsx

@@ -96,13 +96,17 @@ export const LogViewer = ({ filePath, height }: LogViewerProps) => {
       watcher = fs.watch(filePath, () => scheduleRead());
     } catch {
       const interval = setInterval(() => {
+        // Probe with a *non-throwing* `existsSync` rather than `accessSync`:
+        // the "file not there yet" state is expected on every tick until the
+        // file appears, and a throw here gets picked up by exception
+        // autocapture and reported as a spurious `$exception`.
+        if (!fs.existsSync(filePath)) return; // Still waiting for the file.
         try {
-          fs.accessSync(filePath);
+          watcher = fs.watch(filePath, () => scheduleRead());
           readTail();
           clearInterval(interval);
-          watcher = fs.watch(filePath, () => scheduleRead());
         } catch {
-          // Still waiting for the file to appear
+          // Raced with the file disappearing; keep polling and retry.
         }
       }, 1000);