a2ui

name: "a2ui" description: "Use when you need to render generative UI surfaces — forms, cards, layouts, richer controls — back to the user inside the chat. A2UI is enabled by default in the harness unless a session explicitly disables it with enableA2ui: false. Emit A2UI v0.9 envelopes through the a2ui tool."

A2UI Generative UI Skill

Render agent-authored UI surfaces inside the chat using the A2UI v0.9 streaming protocol.

When to use

• The user asks for a richer response than plain text (a form, a card, a list of options, a layout with headings).
• You want to summarize structured data (tables, KPIs) in a way that scans better than markdown.
• The requested information is inherently visual (cards with images, etc.).

Avoid A2UI for pure prose answers. Use it when the shape of the output warrants a dedicated UI.

Protocol cheat sheet

Every envelope MUST carry "version": "v0.9" and exactly one of:

• createSurface — create a named surface with a component tree + data model.
• updateComponents — upsert components (by id), replace a subtree, or delete components.
• updateDataModel — patch the surface's JSON data model at a JSON-pointer path.
• deleteSurface — remove a surface.

Send envelopes via the a2ui tool:

{
  "envelopes": [
    {
      "version": "v0.9",
      "createSurface": {
        "surfaceId": "order-confirmation",
        "catalogId": "https://a2ui.org/specification/v0_9/basic_catalog.json",
        "theme": { "primaryColor": "#0f766e" },
        "root": {
          "id": "root",
          "type": "Column",
          "children": [
            { "id": "title",  "type": "Heading", "props": { "text": "Order placed", "level": 2 } },
            { "id": "total",  "type": "Text",    "props": { "text": { "formatString": "Total: $${/amountUsd}" } } },
            { "id": "thanks", "type": "Paragraph", "props": { "text": "We'll email a receipt shortly." } }
          ]
        },
        "dataModel": { "amountUsd": 42.37 }
      }
    }
  ]
}

Supported components (basic catalog v0.9)

The desktop renderer supports these component types out of the box:

Unknown component types are shown as a diagnostic fallback. If you're streaming updates for a new component type, include a short Paragraph fallback nearby so the user still sees meaningful content.

The mobile renderer supports the same catalog as a read-only preview for now.

Dynamic bindings

Props may reference the data model:

• { "path": "/user/name" } — read a value by JSON-pointer.
• { "$ref": "/items/0/title" } — alias for path.
• { "literal": 42 } — force the value through literally.
• { "formatString": "Hi ${/name}, you have ${/count} items." } — interpolate pointer expressions. Unknown tokens render as empty string.

The renderer does not evaluate arbitrary expressions. Stick to plain JSON-pointer paths inside ${...}.

Functions (v0.9)

In addition to plain bindings, prop values may carry a single-key "function call" object. Supported helpers:

Example using if inside a Text.text:

{
  "id": "status",
  "type": "Text",
  "props": {
    "text": {
      "if": {
        "cond": { "path": "/online" },
        "then": { "concat": ["Online since ", { "path": "/since" }] },
        "else": "Offline"
      }
    }
  }
}

Security rules

• All text renders as plain text — HTML tags are NOT parsed. Don't try to inject <script> or <img onerror=...>; they render as literal strings.
• Image URLs are restricted to http:, https:, and data: schemes.
• The desktop renderer caps component depth and string length to avoid runaway surfaces.

Interaction model

Interactive controls (Button, TextField, Checkbox) now round-trip back to you when the user interacts with them:

• Button — a click fires eventType: "click".
• TextField — pressing Enter fires eventType: "submit" with payload: { value }; losing focus after editing fires eventType: "change" with the current value.
• Checkbox — toggling fires eventType: "change" with payload: { value: boolean }.

The harness delivers each action as a structured user/steer message starting with [a2ui.action]. When a turn is already running, the action is folded in as a steer; otherwise it starts a new turn.

Typical response: emit another a2ui tool call to update the surface (e.g. updateDataModel to reflect new state), or reply in plain text.

Use the ask tool when you need a modal, blocking question. Use a2ui surfaces when you want a richer or stateful UI.

Typical workflow

1. Emit a single createSurface envelope with the full tree.
2. When data changes, emit updateDataModel to patch a specific path.
3. When structure changes, emit updateComponents with the affected components (keyed by id).
4. When the surface is no longer needed, emit deleteSurface.

Always reuse a stable surfaceId across updates so the client replaces the surface in place rather than creating duplicates.

Failure handling

The a2ui tool returns { applied, failed, results: [...] }. On failed entries, the error field explains what was rejected (version mismatch, unknown surface, resolved-state too large, etc). Read the error and send a corrective envelope.

Example: incremental update

// 1) Initial surface
{ "version": "v0.9", "createSurface": { "surfaceId": "counter", "catalogId": "https://a2ui.org/specification/v0_9/basic_catalog.json",
  "root": { "id": "root", "type": "Column", "children": [
    { "id": "label", "type": "Heading", "props": { "text": { "formatString": "Count: ${/count}" }, "level": 2 } }
  ]},
  "dataModel": { "count": 0 }
}}

// 2) After doing some work, bump the counter
{ "version": "v0.9", "updateDataModel": { "surfaceId": "counter", "path": "/count", "value": 3 } }

// 3) Done — tear down.
{ "version": "v0.9", "deleteSurface": { "surfaceId": "counter" } }