simstudioai--sim/.cursor/commands/add-feature-flag.md

Add Feature Flag Skill

You add a runtime, gated feature flag to Sim — one that can be turned on for specific orgs, users, or admins and changed on prod with no redeploy (AWS AppConfig). When AppConfig isn't the source of truth, the flag falls back to a single secret (on/off only).

When to use this vs env-flags.ts

• Feature flag (@/lib/core/config/feature-flags.ts): per-request, gated by userId/orgId/admin, changeable at runtime. This skill.
• Env flag (@/lib/core/config/env-flags.ts): deploy-time capability/environment detection (isProd, isHosted, isBillingEnabled). A module-load boolean. Do not add gated flags here.

If the user wants a fixed per-deployment toggle, send them to env-flags.ts instead.

The flag model

A flag's gating rule lives only in the hosted AppConfig document. It is ON for a context when any clause matches:

interface FeatureFlagRule {
  enabled?: boolean   // global default for everyone
  orgIds?: string[]   // allowlisted organization ids
  userIds?: string[]  // allowlisted user ids
  admins?: boolean    // platform admins (user.role === 'admin')
}

Critically, none of this is expressible in code — gating (especially admins) can only be set through AppConfig, so no environment can grant access from a code literal. Off-AppConfig (self-hosted/OSS/local), a flag is simply on or off, derived from its fallback secret.

Steps

1. Define the flag. Add one entry to the FEATURE_FLAGS registry in apps/sim/lib/core/config/feature-flags.ts. Each entry is the flag's whole definition — name (kebab-case key), description, and the fallback secret consulted when AppConfig isn't the source of truth (truthy ⇒ on globally):

   const FEATURE_FLAGS = {
     '<flag-name>': {
       description: '<what this gates>',
       fallback: '<FLAG_SECRET>',
     },
   }
   

   fallback is the env/secret key (typed as keyof typeof env), so add <FLAG_SECRET> to apps/sim/lib/core/config/env.ts first (and the deployment's secret store) — it won't typecheck otherwise. Do not add org/user/admin defaults here — that gating exists only in AppConfig. Adding the entry makes <flag-name> a valid FeatureFlagName.

2. Gate the call site. Call isFeatureEnabled with whatever ids you have — admin status is resolved internally, so callers never pass it:

   import { isFeatureEnabled } from '@/lib/core/config/feature-flags'
   
   if (await isFeatureEnabled('<flag-name>', { userId, orgId })) {
     // gated behavior
   }
   

   • Missing ids are fine — a clause with no matching id is skipped; with no userId, the admin clause resolves to false without a DB read.
   • Admin routes that already know the caller is an admin may pass { userId, isAdmin: true } to skip the role lookup.
   • Client/UI flags: resolve server-side (in a server component, route, or loader) and pass the boolean down as a prop. There is no client AppConfig.

3. (Prod) configure in AppConfig. The infra feature-flags profile schema is permissive, so a new flag needs no infra change. Operators add the flag under flags in the hosted feature-flags document — including any orgIds/userIds/admins gating — and start a sim-<env>-fast deployment (see the AppConfig runbook in the infra README — same flow as access-control). The fallback secret only applies when AppConfig is disabled.

4. Test. Add a case to apps/sim/lib/core/config/feature-flags.test.ts: use withAppConfig({ flags: { ... } }) to cover the gating rule (mock isPlatformAdmin for the admins clause), and toggle the fallback secret to cover the off-AppConfig path.

5. Clean up after rollout. When the feature ships to everyone, delete the flag's entry from FEATURE_FLAGS, the <FLAG_SECRET> env entry, the AppConfig document, the call sites, and the test. Leaving dead flags around is the main failure mode of flag systems.

Notes

• Flag keys are kebab-case.
• Never read flags via raw fetch or a new AppConfig client — always go through isFeatureEnabled / getFeatureFlags.
• Never bake gating into code. The fallback is a single boolean secret; org/user/admin scoping is AppConfig-only.
• The admin check reads the DB replica (dbReplica) and is resolved lazily, so an admin-gated flag adds at most one cheap replica read, and only when admins is the deciding clause.