What is Snug?

Snug lets a user of any AI-assisted product say “build me a habit tracker” — and get a real, running app: sandboxed, persistent, versioned, and alive, because at runtime it keeps thinking through the same agent that built it.

The body/mind split

A Snug app does not embed a model, an engine, or an API key. It is a body: UI, local state, and a set of actions it can ask its mind — the host agent — to perform. A Snug chess game sends the board to the host LLM over a standard postMessage envelope and animates the JSON reply. The relationship is runtime, not codegen: the same app gets smarter when the user upgrades their model, and cheaper when they point it at a small local one.

Two protocol features make that practical:

• Envelopes — thirteen versioned JSON frame types over postMessage, published as JSON Schemas. An app works in any conformant host.
• Runtime contracts — each app carries a compact, version-pinned description of what it is and what a good answer looks like. Its turns are assembled from that contract, never from the conversation that built it, so a turn costs very little and runs well on any brain.

One file, owned by the user

Everything a user builds lives in one portable SQLite file — the .snug file: every app’s code and version history, each app’s own isolated database, the chats, the settings. It runs from the browser (OPFS) or a desktop disk, syncs to an origin the user picks, exports and imports whole, and can be sealed with a passphrase only the user holds.

There is no vendor database of your apps. The file is the account.

Secure by construction

The security model is enforced by architecture, not by policy:

• Apps run in an iframe sandbox with allow-scripts only and no network of their own.
• The app’s only path to the network is a host-mediated request pair, governed by a human-approved, host-frozen ceiling of allowed hosts.
• Credentials live in the user’s own file and are injected by the host executor — they never enter the app iframe, never reach the LLM, and never reach an app publisher.

The threat model and security argument are written down, including what is accepted and not mitigated.

Where Snug runs today

• The Playground — the hosted demo hub, zero install.
• The desktop app — Snug at full strength: fully private, local file, local models, LAN devices (Philips Hue), linked-device connections (personal WhatsApp).
• Your product — any SaaS can embed the runner + SDK and let its users build micro apps powered by its existing assistant. Start at the implementor quickstart.

Normative source: the specification. This page is the tour, not the contract.