feat(cli): add runtime Node.js version check with clear error message

src/index.ts

@@ -1,4 +1,15 @@
 #!/usr/bin/env node
+
+// Guard: exit early with a clear message on unsupported Node.js versions.
+// This runs before any ESM/modern-syntax imports that would crash with cryptic errors.
+const major = Number(process.versions.node.split('.')[0]);
+if (major < 20) {
+  process.stderr.write(
+    `Error: testsprite requires Node.js >= 20 (found ${process.versions.node}).\nInstall the latest LTS from https://nodejs.org\n`,
+  );
+  process.exit(1);
+}
+
 import { Command, CommanderError } from 'commander';
 import { createAgentCommand } from './commands/agent.js';
 import { createAuthCommand } from './commands/auth.js';

src/version-guard.test.ts

@@ -0,0 +1,52 @@
+import { describe, expect, it } from 'vitest';
+
+/**
+ * The version guard logic extracted for testability.
+ * The real guard lives at the top of src/index.ts as imperative code.
+ * These tests verify the parsing and threshold logic.
+ */
+
+function parseMajorVersion(nodeVersion: string): number {
+  return Number(nodeVersion.split('.')[0]);
+}
+
+function shouldRejectVersion(nodeVersion: string): boolean {
+  return parseMajorVersion(nodeVersion) < 20;
+}
+
+describe('Node.js version guard logic', () => {
+  it('rejects Node 18.x', () => {
+    expect(shouldRejectVersion('18.19.1')).toBe(true);
+  });
+
+  it('rejects Node 16.x', () => {
+    expect(shouldRejectVersion('16.20.2')).toBe(true);
+  });
+
+  it('rejects Node 14.x', () => {
+    expect(shouldRejectVersion('14.21.3')).toBe(true);
+  });
+
+  it('accepts Node 20.x', () => {
+    expect(shouldRejectVersion('20.11.0')).toBe(false);
+  });
+
+  it('accepts Node 22.x', () => {
+    expect(shouldRejectVersion('22.1.0')).toBe(false);
+  });
+
+  it('accepts Node 21.x', () => {
+    expect(shouldRejectVersion('21.0.0')).toBe(false);
+  });
+
+  it('parses major version correctly from semver string', () => {
+    expect(parseMajorVersion('20.11.1')).toBe(20);
+    expect(parseMajorVersion('18.0.0')).toBe(18);
+    expect(parseMajorVersion('22.3.0')).toBe(22);
+  });
+
+  it('the running Node satisfies the guard (meta-test)', () => {
+    // This test is running on Node >= 20, so it must pass the guard.
+    expect(shouldRejectVersion(process.versions.node)).toBe(false);
+  });
+});