Island Mode Server — Development skill for Claude Code

island-mode-server

An [Agent Skill](https://agentskills.io) that teaches an agent to build an **on-premise fallback server** — a Node box at a physical site holding a live two-way replica of that site's slice of Firestore, which takes over serving LAN terminals when the internet drops and flushes offline work back on reconnect. Local server is Node/NestJS + RxDB; the cloud is Firestore with Next.js route handlers as the reference API, both stated as substitutable.

While the internet is up the box is invisible: it replicates in the background and heartbeats so the cloud knows it is alive. When the cloud stops answering, kiosk and staff PWAs on the LAN switch their API base URL to it and the site keeps taking bookings, moving stock and toggling hardware from the replica.

The insight that shapes the whole skill: **sync is two systems, not one.** Document-level RxDB replication keeps state current but cannot run the cloud's business logic; an HTTP flush of business events on reconnect runs that logic — atomic stock increments, audit ingestion — but keeps nothing current. Neither path alone is sufficient, and both are keyed on client-generated IDs so their overlap converges instead of double-counting.

Extracted from a production shooting-range management system: a NestJS + RxDB local server deployed at physical range locations, replicating 8 Firestore collections for a Next.js 16 cloud app, with kiosk/staff PWA terminals and HMAC-authenticated lock hardware. The templates are **hardened, not faithful**: ten defects found during the extraction audit are fixed in the code shown, and every deviation — including what was designed but never run in production — is recorded in [`references/provenance.md`](references/provenance.md).

Install

One command, via [skills.sh](https://www.skills.sh) — the open Agent Skills directory. Its [CLI](https://github.com/vercel-labs/skills) resolves a GitHub repository directly, so this works today, with no manual clone and no per-age