fix(pty): validate command exists before spawning agent

Add validateCommand() that checks whether a binary exists (via `which`
for PATH names, `fs.accessSync` for absolute paths) before pty.spawn().
This gives users a clear "not found in PATH" error instead of the
cryptic "Process exited (?)" when a CLI agent is not installed.

Closes #6

Author: johannesjo

electron/ipc/pty.test.ts

@@ -0,0 +1,39 @@
+import { describe, it, expect } from 'vitest';
+import { validateCommand } from './pty.js';
+
+describe('validateCommand', () => {
+  it('does not throw for a command found in PATH', () => {
+    // /bin/sh always exists on macOS/Linux
+    expect(() => validateCommand('/bin/sh')).not.toThrow();
+  });
+
+  it('throws a descriptive error for a missing command', () => {
+    expect(() => validateCommand('nonexistent-binary-xyz')).toThrow(
+      /not found in PATH/,
+    );
+  });
+
+  it('throws a descriptive error naming the command', () => {
+    expect(() => validateCommand('nonexistent-binary-xyz')).toThrow(
+      /nonexistent-binary-xyz/,
+    );
+  });
+
+  it('throws for a nonexistent absolute path', () => {
+    expect(() => validateCommand('/nonexistent/path/binary')).toThrow(
+      /not found or not executable/,
+    );
+  });
+
+  it('does not throw for a bare command found in PATH', () => {
+    expect(() => validateCommand('sh')).not.toThrow();
+  });
+
+  it('throws for an empty command string', () => {
+    expect(() => validateCommand('')).toThrow(/must not be empty/);
+  });
+
+  it('throws for a whitespace-only command string', () => {
+    expect(() => validateCommand('   ')).toThrow(/must not be empty/);
+  });
+});

electron/ipc/pty.ts

@@ -1,4 +1,6 @@
 import * as pty from 'node-pty';
+import { execFileSync } from 'child_process';
+import fs from 'fs';
 import type { BrowserWindow } from 'electron';
 import { RingBuffer } from '../remote/ring-buffer.js';
 
@@ -43,6 +45,32 @@ const BATCH_INTERVAL = 8; // ms
 const TAIL_CAP = 8 * 1024;
 const MAX_LINES = 50;
 
+/** Verify that a command exists in PATH. Throws a descriptive error if not found. */
+export function validateCommand(command: string): void {
+  if (!command || !command.trim()) {
+    throw new Error('Command must not be empty.');
+  }
+  // Absolute paths: check directly via filesystem
+  if (command.startsWith('/')) {
+    try {
+      fs.accessSync(command, fs.constants.X_OK);
+      return;
+    } catch {
+      throw new Error(
+        `Command '${command}' not found or not executable. Check that it is installed.`,
+      );
+    }
+  }
+  // Bare names: resolve via `which` (execFileSync — no shell interpolation)
+  try {
+    execFileSync('which', [command], { encoding: 'utf8', timeout: 3000 });
+  } catch {
+    throw new Error(
+      `Command '${command}' not found in PATH. Make sure it is installed and available in your terminal.`,
+    );
+  }
+}
+
 export function spawnAgent(
   win: BrowserWindow,
   args: {
@@ -68,6 +96,8 @@ export function spawnAgent(
     throw new Error(`Command contains disallowed characters: ${command}`);
   }
 
+  validateCommand(command);
+
   const filteredEnv: Record<string, string> = {};
   for (const [k, v] of Object.entries(process.env)) {
     if (v !== undefined) filteredEnv[k] = v;

vitest.config.ts

@@ -2,6 +2,6 @@ import { defineConfig } from 'vitest/config';
 
 export default defineConfig({
   test: {
-    include: ['src/**/*.test.ts'],
+    include: ['src/**/*.test.ts', 'electron/**/*.test.ts'],
   },
 });