Real-time Features Skill

Degree of freedom: MIXED. Which transport and event [HIGH freedom]; existing-subscription probes and unmount cleanup [LOW freedom — run exactly].

How to reason

1. Observe — existing channels, sockets, hooks, cleanup
2. Interpret — new subscription vs reuse vs transport mismatch
3. Classify — postgres-changes / presence / broadcast / SSE / optimistic
4. Severity — leaked or duplicate subscription outranks a missing typing indicator

Worked example

Observe: chat mounts useRealtimeMessages twice; no untrack on leave; package.json already has @supabase/supabase-js. Interpret: duplicate INSERT listeners; leftover presence. Classify: one channel hook with unmount cleanup; reuse Supabase Realtime — do not add Socket.io. Verify: one postgres_changes subscription; removeChannel on unmount; no second socket lib.