diff --git a/src/apps/shell/src/runtime/pty-manager.ts b/src/apps/shell/src/runtime/pty-manager.ts
index a681dfe..45d5309 100644
--- a/src/apps/shell/src/runtime/pty-manager.ts
+++ b/src/apps/shell/src/runtime/pty-manager.ts
@@ -1,5 +1,9 @@
 import pty, { type IPty } from 'node-pty';
 import fs from 'fs';
+import path from 'path';
+import { createRequire } from 'module';
+import { execFileSync } from 'child_process';
+import { lacksExecuteBit } from './shell-config.js';
 import type { PtyEntry, PaneInfo, ShellEmitter } from '../shell-types.js';
 import {
   globalPtys,
@@ -9,6 +13,55 @@ import {
 } from './shell-state.js';
 import { noteCursorReportRequests } from './cursor-report.js';
 
+let spawnHelperChecked = false;
+
+/**
+ * Ensure node-pty's `spawn-helper` is runnable on macOS.
+ *
+ * On macOS, node-pty's native code launches the shell via a small `spawn-helper`
+ * binary using posix_spawn(), which requires that file to exist AND be executable.
+ * The error surfaces (misleadingly) as "posix_spawnp failed." pnpm's content-addressable
+ * store materialization — and downloaded prebuilds — can drop the execute bit or leave a
+ * macOS quarantine xattr, so a freshly installed tree throws on every pane/agent launch.
+ * node-pty's own post-install does not chmod the helper, so we repair it here.
+ *
+ * Idempotent, darwin-only, best-effort — never throws (spawning must still proceed).
+ */
+export function ensureSpawnHelperExecutable(): void {
+  if (spawnHelperChecked) return;
+  spawnHelperChecked = true;
+  if (process.platform !== 'darwin') return;
+
+  try {
+    const require_ = createRequire(import.meta.url);
+    const pkgJson = require_.resolve('node-pty/package.json');
+    const helper = path.join(path.dirname(pkgJson), 'build', 'Release', 'spawn-helper');
+
+    if (!fs.existsSync(helper)) {
+      console.error(
+        `[shell] node-pty spawn-helper is missing at ${helper}. ` +
+        `This usually means node-pty was not built for this platform/arch — ` +
+        `run "pnpm rebuild node-pty" (or reinstall) on this machine.`,
+      );
+      return;
+    }
+
+    if (lacksExecuteBit(fs.statSync(helper).mode)) {
+      fs.chmodSync(helper, 0o755);
+      console.error(`[shell] restored execute permission on node-pty spawn-helper: ${helper}`);
+    }
+
+    // Strip the macOS quarantine attribute if present; harmless when absent.
+    try {
+      execFileSync('xattr', ['-d', 'com.apple.quarantine', helper], { stdio: 'ignore' });
+    } catch {
+      // not quarantined, or xattr unavailable — nothing to do
+    }
+  } catch (err) {
+    console.error('[shell] spawn-helper check failed:', (err as Error).message);
+  }
+}
+
 /** Send SIGHUP, then SIGKILL after 2 s if still alive. */
 export function killPty(p: IPty): void {
   try {
@@ -56,6 +109,10 @@ export function spawnGlobalPty(
   cols = Number.isInteger(cols) && cols >= 1 ? Math.min(cols, 500) : 80;
   rows = Number.isInteger(rows) && rows >= 1 ? Math.min(rows, 500) : 24;
 
+  // macOS: make sure node-pty's spawn-helper is executable before the first spawn,
+  // otherwise pty.spawn throws "posix_spawnp failed." (runs once, no-op elsewhere).
+  ensureSpawnHelperExecutable();
+
   const spawnOpts = {
     name: 'xterm-256color',
     cols,
diff --git a/src/apps/shell/src/runtime/shell-config.test.ts b/src/apps/shell/src/runtime/shell-config.test.ts
index 68005f2..1fdb936 100644
--- a/src/apps/shell/src/runtime/shell-config.test.ts
+++ b/src/apps/shell/src/runtime/shell-config.test.ts
@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest';
-import { pickUnixShell, withDefaultPath } from './shell-config.js';
+import { lacksExecuteBit, pickUnixShell, withDefaultPath } from './shell-config.js';
 
 describe('pickUnixShell', () => {
   it('returns $SHELL when it exists', () => {
@@ -44,3 +44,18 @@ describe('withDefaultPath', () => {
     expect(env.PATH).toBe('/custom/bin');
   });
 });
+
+describe('lacksExecuteBit', () => {
+  it('is true when no execute bit is set (e.g. 0o644)', () => {
+    // Regression: a non-executable node-pty spawn-helper makes posix_spawn fail on macOS.
+    expect(lacksExecuteBit(0o644)).toBe(true);
+    expect(lacksExecuteBit(0o600)).toBe(true);
+  });
+
+  it('is false when any execute bit is set', () => {
+    expect(lacksExecuteBit(0o755)).toBe(false); // user+group+other
+    expect(lacksExecuteBit(0o744)).toBe(false); // user only
+    expect(lacksExecuteBit(0o711)).toBe(false);
+    expect(lacksExecuteBit(0o100)).toBe(false); // owner-exec only
+  });
+});
diff --git a/src/apps/shell/src/runtime/shell-config.ts b/src/apps/shell/src/runtime/shell-config.ts
index 7f68083..2e82997 100644
--- a/src/apps/shell/src/runtime/shell-config.ts
+++ b/src/apps/shell/src/runtime/shell-config.ts
@@ -34,6 +34,11 @@ export function pickUnixShell(exists: (p: string) => boolean, shellEnv: string |
   return '/bin/sh';
 }
 
+/** True when a file mode has no execute bit set for any of user/group/other. */
+export function lacksExecuteBit(mode: number): boolean {
+  return (mode & 0o111) === 0;
+}
+
 export function safeEnv(extra: Record<string, string> = {}): Record<string, string> {
   if (process.platform === 'win32') {
     const env: Record<string, string> = {};