Add a lightweight feature flag system to my app. I don't need a fancy vendor; I need to be able to ship risky code turned off, flip it on for me first, roll out to a percentage of users, and kill it fast if it misbehaves. Implement this: 1. A flag store: environment variables for simple boolean flags is fine for a small app; use a database table (or a hosted flag service like LaunchDarkly/PostHog if I already use one) once flags need per-user or percentage rollouts. Ask which fits before choosing. 2. A single check function: is_flag_enabled(flag_name, user) that returns a boolean and handles the 'flag missing' case by defaulting to off (fail closed for anything risky). 3. Support three rollout modes: off, on for specific users (me and a test list), and on for a percentage of users (hashed on user id so a given user's answer is stable). 4. Use flags for the right things: new features that are risky, migrations between old and new code paths, and kill switches for third-party integrations. Don't wrap every trivial change; flags are debt if left in the code forever. 5. Add a small admin UI or documented process for flipping flags without a deploy. That's the whole point; if changing a flag requires a redeploy, it's not solving the problem. 6. Log every flag evaluation with the flag name, user, and result (sampled if volume is high) so I can debug 'why did this user see the new thing'. Don't ship a huge flag framework; keep it small. The deliverable is the flag check function, the store (env or table), the rollout modes working, one real feature migrated behind a flag as a proof of concept, and a note in the docs about cleaning up flags after full rollout.