diff --git a/src/apps/shell/src/runtime/shell-config.test.ts b/src/apps/shell/src/runtime/shell-config.test.ts
new file mode 100644
index 0000000..68005f2
--- /dev/null
+++ b/src/apps/shell/src/runtime/shell-config.test.ts
@@ -0,0 +1,46 @@
+import { describe, expect, it } from 'vitest';
+import { pickUnixShell, withDefaultPath } from './shell-config.js';
+
+describe('pickUnixShell', () => {
+  it('returns $SHELL when it exists', () => {
+    const exists = (p: string) => p === '/opt/homebrew/bin/fish';
+    expect(pickUnixShell(exists, '/opt/homebrew/bin/fish')).toBe('/opt/homebrew/bin/fish');
+  });
+
+  it('ignores $SHELL when the path does not exist', () => {
+    const exists = (p: string) => p === '/bin/zsh';
+    expect(pickUnixShell(exists, '/nonexistent/shell')).toBe('/bin/zsh');
+  });
+
+  it('falls back to an ABSOLUTE path (never a bare name) when $SHELL is unset', () => {
+    // Regression: bare "bash" makes posix_spawnp fail when the child env has no PATH (macOS daemon/GUI launch).
+    const exists = (p: string) => p === '/bin/zsh';
+    const result = pickUnixShell(exists, undefined);
+    expect(result.startsWith('/')).toBe(true);
+    expect(result).toBe('/bin/zsh');
+  });
+
+  it('prefers zsh, then bash, then sh', () => {
+    expect(pickUnixShell((p) => ['/bin/zsh', '/bin/bash', '/bin/sh'].includes(p), undefined)).toBe('/bin/zsh');
+    expect(pickUnixShell((p) => ['/bin/bash', '/bin/sh'].includes(p), undefined)).toBe('/bin/bash');
+    expect(pickUnixShell((p) => p === '/bin/sh', undefined)).toBe('/bin/sh');
+  });
+
+  it('never returns a bare command even when nothing is found', () => {
+    const result = pickUnixShell(() => false, undefined);
+    expect(result.startsWith('/')).toBe(true);
+  });
+});
+
+describe('withDefaultPath', () => {
+  it('injects a sane PATH when the env has none', () => {
+    const env = withDefaultPath({ TERM: 'xterm-256color' });
+    expect(env.PATH).toBeTruthy();
+    expect(env.PATH).toContain('/bin');
+  });
+
+  it('preserves an existing PATH', () => {
+    const env = withDefaultPath({ PATH: '/custom/bin' });
+    expect(env.PATH).toBe('/custom/bin');
+  });
+});
diff --git a/src/apps/shell/src/runtime/shell-config.ts b/src/apps/shell/src/runtime/shell-config.ts
index b9464cd..7f68083 100644
--- a/src/apps/shell/src/runtime/shell-config.ts
+++ b/src/apps/shell/src/runtime/shell-config.ts
@@ -5,6 +5,35 @@ export type { ShellConfig };
 
 const ENV_ALLOWLIST_UNIX = ['HOME', 'PATH', 'USER', 'SHELL', 'LANG', 'LC_ALL', 'SSH_AUTH_SOCK'];
 
+/** Absolute shells tried, in order, when $SHELL is unset/missing. zsh is the macOS default since Catalina. */
+const UNIX_SHELL_CANDIDATES = ['/bin/zsh', '/bin/bash', '/bin/sh', '/usr/bin/zsh', '/usr/bin/bash', '/usr/bin/sh'];
+
+/** Conservative default PATH for spawn envs launched without one (daemon/GUI launch). */
+const DEFAULT_UNIX_PATH = '/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin';
+
+/**
+ * Ensure a spawn env has a PATH. A macOS daemon or GUI-launched process can have a sparse
+ * environment with no PATH; without it node-pty's posix_spawnp cannot locate a bare command
+ * and fails. Pure/testable — does not read process.env.
+ */
+export function withDefaultPath(env: Record<string, string>): Record<string, string> {
+  if (env.PATH) return env;
+  return { ...env, PATH: DEFAULT_UNIX_PATH };
+}
+
+/**
+ * Resolve a macOS/Linux shell to an ABSOLUTE path. Returning a bare name ("bash") makes
+ * posix_spawnp depend on PATH being present in the child env, which fails under daemon/GUI
+ * launch. Pure/testable — caller injects the existence check and $SHELL value.
+ */
+export function pickUnixShell(exists: (p: string) => boolean, shellEnv: string | undefined): string {
+  if (shellEnv && exists(shellEnv)) return shellEnv;
+  for (const sh of UNIX_SHELL_CANDIDATES) {
+    if (exists(sh)) return sh;
+  }
+  return '/bin/sh';
+}
+
 export function safeEnv(extra: Record<string, string> = {}): Record<string, string> {
   if (process.platform === 'win32') {
     const env: Record<string, string> = {};
@@ -18,18 +47,18 @@ export function safeEnv(extra: Record<string, string> = {}): Record<string, stri
   for (const key of ENV_ALLOWLIST_UNIX) {
     if (process.env[key] !== undefined) env[key] = process.env[key]!;
   }
-  return { ...env, ...extra };
+  return withDefaultPath({ ...env, ...extra });
 }
 
 function resolveDefaultShell(): string {
-  const userShell = process.env.SHELL;
-  if (userShell && fs.existsSync(userShell)) return userShell;
   if (process.platform === 'win32') {
+    const userShell = process.env.SHELL;
+    if (userShell && fs.existsSync(userShell)) return userShell;
     const gitBash = 'C:\\Program Files\\Git\\bin\\bash.exe';
     if (fs.existsSync(gitBash)) return gitBash;
     return 'cmd.exe';
   }
-  return 'bash';
+  return pickUnixShell(fs.existsSync, process.env.SHELL);
 }
 
 export const SHELL_CONFIGS: Record<string, ShellConfig> = {