In CI environments (such as GitHub Actions), the local main branch (or the branch specified in GITHUB_BASE_REF) might not exist locally because of shallow clones or because only the PR branch is checked out. However, the remote tracking branch (e.g., origin/main) is typically available. Using only baseBranch directly can cause git diff to fail with fatal: ambiguous argument in CI.

We can make this more robust by checking if the local ref exists, and falling back to the remote tracking branch (origin/${base}) if it does not.

// Define the base branch to compare against, falling back to the remote tracking branch if the local ref is missing (common in CI shallow clones)
const baseBranch = (() => {
  const base = process.env.GITHUB_BASE_REF || 'main';
  try {
    execFileSync('git', ['rev-parse', '--verify', base], {stdio: 'ignore'});
    return base;
  } catch {
    return 'origin/' + base;
  }
})();

On Windows, executing npx directly via execFileSync will fail with an ENOENT error because npx is a shell script (npx.cmd) rather than a binary executable. To ensure cross-platform compatibility for developers running the linter locally on Windows, we should resolve the command name dynamically based on the platform.

    const npxCmd = process.platform === 'win32' ? 'npx.cmd' : 'npx';
    execFileSync(npxCmd, ['prettier', '--check', ...filesToCheck], {
      stdio: 'inherit',
    });