eUI

An agent answers with an interface: a card with the numbers, a form for the missing fields, a button that does the next step. The model decides what to show; the app decides how it looks and what it may do. A2UI is the open protocol for this: the agent sends components from a catalog both sides agree on, a data model and the actions it wants back, all as data. eUI speaks A2UI v0.9 (the pinned v0.9.1 spec, vendored byte for byte).

Word	What it is
Surface	One rendered UI: a component tree, its data model and its theme.
Catalog	The components a surface may use and their props. The model reads it; the renderer implements it.
Theme	A JSON file of tokens and component recipes, loaded at runtime.
Extension	Extra components: an extension catalog plus a painter per platform.
Host plugin	What the embedding app lends a surface: transport, functions, data bindings, policy.

Messages

Four messages: createSurface, updateComponents (components added or replaced by id), updateDataModel (a value at a JSON pointer) and deleteSurface, one per line as JSONL, over SSE, a WebSocket or inside an MCP tool result. Components arrive as a flat list that names children by id, so a model can stream them in any order and patch one without resending the tree. A component the renderer does not know becomes a visible placeholder and a reported issue.

{"version":"v0.9","createSurface":{"surfaceId":"main","catalogId":"https://ui.exponential.at/catalogs/core/v1"}}
{"version":"v0.9","updateComponents":{"surfaceId":"main","components":[
  {"id":"root","component":"Card","title":"Deploy 0.18.84?","children":["note","go"]},
  {"id":"note","component":"Text","text":{"path":"/summary"}},
  {"id":"go","component":"Button","label":"Deploy","on":{"press":{"event":{"name":"deploy","context":{"version":"0.18.84"}}}}}
]}}
{"version":"v0.9","updateDataModel":{"surfaceId":"main","path":"/summary","value":"3 platforms, all checks green."}}

Data and actions

Every surface has one JSON data model. A bindable prop takes {"path": "/summary"} and follows the model; inputs bind both ways. A List repeats a template per item, with relative paths reading the current item. Interactive components fire events: an event action goes back to the agent with its context resolved against the data model, and a function call runs on the client (built-ins on every renderer, plus the host’s own, namespaced and behind a policy gate).

The catalog

The core catalog has 91 components with shadcn’s names, variants and sizes: 51 natives each renderer paints itself and 40 macros that expand into natives before painting. Every component and prop carries one sentence written for the model; catalogPrompt() turns them into a compact system prompt. Core lite is the subset without overlays, media and charts. A2UI’s basic catalog maps onto the core through one table (catalog/basic-map.json), so a surface written for any A2UI client renders here too.

Themes, extensions, hosts

A theme is one JSON file of tokens (colours per mode, radius, spacing, type, shadows, motion) and recipes (a style per component part, variant and state); extends overrides only what changes. Four are built in: neutral, exponential, playful and orange. An extension adds components: an extension catalog plus a painter per platform, or a macro that needs no painter. The host lends a surface its transport, functions, data bindings by source URI, and the policy for functions, URLs and media; deny wins, then allow, then ask, then the default.

How the renderers agree

The TypeScript reference in @exponential_app/eui and the Rust core eui-core replay the same fixtures byte for byte. React, Solid and the web component run on the TypeScript side; SwiftUI and Compose reach the Rust core through UniFFI, and gpui links it directly. The core lays out every native surface (taffy); the platform measures text and paints each node at its frame. The web follows the same layout in CSS, within a pixel. The conformance suite checks every renderer case by case.

A2UI messages ─▶ reducer ─▶ UiNode tree ─▶ layout (taffy) ─▶ frames + resolved visuals ─▶ painter

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