realtime-subscriptions

A guide for adding Supabase Realtime to a Next.js app, so the browser can receive database changes, temporary messages, or online-status updates. It covers Postgres Changes, Broadcast, and Presence channels.

In plain words
What is it for?
Use it for live database updates, typing indicators, shared cursors, presence lists, and collaborative features. It helps set up client-side subscriptions, cleanup, row-level security, and UI updates.
Why use it?
It helps avoid duplicate event handlers, leaked connections, and updates reaching users who are not allowed to read the data. It also explains how to reconnect and keep the live data aligned with the app's cache.

Skill for Claude CodeCodex

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add skills/m-binimran/dev-pack/realtime-subscriptions
Any agent
npx skills add m-binimran/dev-pack --skill realtime-subscriptions
Clone the repo
git clone --depth 1 https://github.com/m-binimran/dev-pack

Made for: Claude Code, Codex.

Per session 47 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 377 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00047 $0.00377
Opus 5 $0.00023 $0.00188
Sonnet 5 $0.00009 $0.00075
Haiku 4.5 $0.00005 $0.00038

Measured yesterday against content hash 114c9edb1b5d, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

realtime-subscriptions scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured yesterday.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

skills/realtime-subscriptions/SKILL.md · 34 lines

What it actually says

realtime-subscriptions

Live data is a client concern (it needs a persistent connection). Wire it carefully and always clean up.

Pick the mechanism

Need Use
React to DB inserts/updates/deletes Postgres Changes
Ephemeral messages (cursors, typing) Broadcast
Who's online / shared state Presence

Process

  1. Subscribe in a client component, inside useEffect. Realtime can't live in a server component.
  2. Always unsubscribe in the effect cleanup (supabase.removeChannel(channel)) — leaked channels pile up and cause duplicate handlers.
  3. RLS applies to Realtime. A client only receives change events for rows it's allowed to read. Enable Realtime on the table and confirm the policies.
  4. Reconcile with your cache: on an event, update the query cache (state-architect) rather than keeping a parallel copy. Handle reconnects (refetch on SUBSCRIBED after a drop).
  5. Don't over-subscribe: scope channels narrowly (filter by id); a firehose channel is a performance and cost problem.

Output

  • The channel setup, the cleanup, the RLS/Realtime enablement note, and how events update the UI/cache.

Guardrails

  • Every subscription has a matching cleanup — no exceptions (memory + duplicate-event bugs).
  • Realtime is not your security layer; RLS is. Don't broadcast data the user can't already read.
Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. yesterday First seen · 34 lines · 47 tokens per session scan A 114c9edb1b5d

Subscribe to this mod's changes

realtime-subscriptions is a skill published in the GitHub repository m-binimran/dev-pack (2 stars, last pushed 2mo ago), licensed MIT. It adds 47 tokens to every session and 377 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

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