Feature flags for safe deployments decoupled from releases

Features must be fully tested before release, which forces long-lived feature branches that diverge from main and cause painful merge conflicts. Deploying and releasing are coupled, preventing incremental delivery.

Merge code to main behind a feature flag—deploy it disabled, then enable it gradually:

// Using LaunchDarkly / Unleash / custom flag store
const flags = await flagClient.getFlags(userId);

if (flags['new-checkout-flow']) {
  return renderNewCheckout();
} else {
  return renderLegacyCheckout();
}

In CI, test both code paths:

- name: Test with flag enabled
  env:
    FEATURE_NEW_CHECKOUT: 'true'
  run: npm test

- name: Test with flag disabled
  env:
    FEATURE_NEW_CHECKOUT: 'false'
  run: npm test

Flag rollout: 1% -> 10% -> 50% -> 100%, with automatic rollback if error rate exceeds threshold.

Decoupling deploy from release means code ships continuously without waiting for a release window. Rollback is a flag flip, not a code revert and redeploy.