Exact – a new cross platform UI framework

Exact Write an app once. It runs natively on the web, macOS, iOS, and Linux, and an AI agent can build it, run it, see it, and test it on every one of them. Tip Try it with a coding agent. On a Mac with Xcode, Rust, Bun, and Chrome installed, give Claude Code this prompt: Clone https://github.com/expo/exact and follow its README to make a new Exact app with `exact new`. Run `bun scripts/exact.mjs setup` once first. Make a todo list where I can add items, check them off, delete them, and see how many are left. Put the view in Contract and keep the list in `app.ts`. Write an `app.test.contract`, pass it on web, macOS, and the iOS Simulator with `scripts/agent.mjs`, then open the app for me on all three. A fresh agent given an earlier version of this prompt on a clean clone (Hermes already in the machine cache, Cargo's cache warm) finished in about 57 minutes, and its tests passed on all three platforms. About 25 of those minutes were the first macOS and iOS builds, roughly 13 minutes each; later builds take a minute or two. On a machine that has lacked Hermes, its source build came first and added time. Current setup downloads and verifies the pinned host/iOS bundles once instead; ordinary builds stay offline. Web iOS macOS The browser's own DOM and CSS UIKit views, native controls and the system's tab bar AppKit views and native controls One app.contract , the same screen on three hosts, captured by the agent driver. Softwear is a demonstration store you can generate from exact-templates : it shows Alternative Apparel's catalog, is not affiliated with that brand, and takes no real orders or payments. Exact is a cross-platform application runtime. You describe your interface in Contract , a small declarative language, and you write your data logic in TypeScript or Rust . The Contract compiler turns the interface into a compact plan . On the web, that plan becomes one ES module over a ~20 KB runtime that drives the real DOM. On macOS, iOS, and Linux, a Rust runner executes the same plan, a Rust layout kernel computes every box with CSS rules, and each platform draws with its own tools: AppKit, UIKit, or a GPU painter. There is no webview and no JavaScript bridge in the UI, and no app JavaScript runs before the first pixel. Note Exact is pre-1.0 and changes daily. There is no API stability, no backwards compatibility, and no migration guide, by design. Upgrade note: Snapback mutation replies with ok: false now throw refused(why) and expose failure(slot) ; the snapback4 package's README has the before/after and inherited-app commands. Contents Three principles How it works Quick start Contract Example apps What works today, and what doesn't yet Repository map Working on Exact Three principles 1. Agent native Exact assumes much of the code will be written by AI agents. An agent can make an app, run it, look at it, operate it, and prove it works without a person in the loop. Ten operations, the same on every host. tree · screenshot · tap · type · state · layout · logs · clock · prefer · perf drive the web, macOS, iOS (Simulator or a real iPhone), and Linux through one script, scripts/agent.mjs . There are ten on purpose: an agent API grows one reasonable addition at a time until no one can hold it in mind. The clock belongs to the agent. Between two operations nothing moves. Instead of sleeping until an animation finishes, the agent says clock +60000 or clock settle and reads the result. No timing flakes, no waits. Tests are scripts of those operations. An app.test.contract file runs unchanged on any host, with no second evaluator. Every tool speaks to machines. contract build --json gives diagnostics with stable ids and source ranges. contract symbols gives navigation. A development source map leads from any node on screen back to the line that declared it. The driver refuses to drive a build older than its sources and names the rebuild. Contract is small on purpose. A whole app fits in a context window. Every write is declared, every list is keyed, every value is typed. Mistakes are refusals that say what to do, not quiet misbehavior. bun scripts/agent.mjs web tree " tap change-station " " type station-search Palo " \ " clock +60000 " state " screenshot out.png " 2. Speed is king Speed of the app and speed of the loop. Each budget in rules/RULES.md is tracked on every commit, and a regression is a P0 with a name on it. Budget Target Cold start to interactive first frame 100 ms p50 Dev restart, edit to present 100 ms p50 App JavaScript executed before first pixel none Touch one line, rebuild that crate 30 s The whole blocking check suite 65 s How the design keeps those numbers: The boot path executes and compiles nothing. TypeScript is compiled to Hermes bytecode at build time and loads after the first pixel. The first frame comes from data baked into the plan. Pay only for what you use. GPU rendering, the Markdown editor, text flow, and browser SQLite are separate artifacts, loaded on demand. Nothing is a feature flag on a core crate. The web gets its own small runtime. On the RealWorld ("Conduit") benchmark, Exact's runtime was up about 300 ms before the React SSR build had hydrated. It used 26 KB of code against React's 81 KB (the RealWorld benchmark in apps/realworld ). Edits land in about a tenth of a second. On a fresh app made by exact new , a saved Contract or TypeScript edit was rebuilt and in the page in 99 ms. 3. Write the web, ship the platform What you write is CSS: the vocabulary, the layout model, the development loop. What you leave unsaid is the platform's: controls, colour, type, motion, gestures, the keyboard and accessibility come out as an app built by hand with UIKit or AppKit would have them, and on the web as a carefully built web app would. What you write wins. The order is author, then platform, then CSS's default, and one test settles a dispute: would someone who knows the platform notice? Layout is CSS's, because a person can't see a layout default, only its result. A bare node is display: block , box-sizing: content-box , flex-direction: row , flex-shrink: 1 , the same as a bare