Runtime contracts
The obvious way to let a built app keep talking to the agent is to keep the build conversation around and append to it. It’s also the wrong way: turns get more expensive forever, the app’s behavior drifts with the chat, and nothing smaller than a frontier model can host it.
Snug specifies the alternative: the runtime contract.
What it is
Section titled “What it is”When an app version is created, the host assigns it a compact, version-pinned contract — a structured description of what this app is and what a good answer looks like for its actions. Every runtime turn for that app is assembled from the contract, never from the conversation that built it.
The measured effect in the reference implementation is on the order of a kilobyte saved per turn — which is precisely what makes a small local model a viable mind for an app.
The properties that matter
Section titled “The properties that matter”- Pinned to the version. Edit the app and the contract moves with the edit; revert the app and the contract of the target version is restored. The app you run is always the app the contract describes.
- Trust has provenance. An imported file’s contracts are dropped unless the hub already holds byte-identical ones — a foreign file cannot smuggle instructions into your agent’s system assembly.
- Model-portable. Because a turn carries the contract and the action payload rather than a history, the same app runs well on a frontier model, a mid-tier one, or the small local brain — the user’s choice, per app if they like.
The app chat surface
Section titled “The app chat surface”The contract also powers the app-attached chat. A message beside an installed app is intent-classified first, and the intent picks both the context assembled and the tools offered:
- Data questions run agent-authored SQL on an isolated copy of the app’s own database — “what did I spend on food last month?” works on a budget app that never shipped that screen.
- Data changes are proposed with the verbatim statements and row counts, execute only on the user’s approval, and re-validate before applying.
Normative source: Part IV — runtime contracts and the app chat surface.