A2ui Rendering

A2UI Rendering

A2UI is a declarative JSON protocol for delivering rich UI from agents to client surfaces. UI is composed as a flat list of components where layout containers reference children by ID. The protocol enforces typed value wrappers and a fixed catalog of 18 primitives.

When to Compose A2UI

Compose A2UI only in response to a user message that needs rich visual output. Do not generate A2UI on session creation or proactively. Plain text remains the default for narrative responses.

Workflow

A2UI Render Progress:
- [ ] Decide what information to present
- [ ] Choose primitives (Text, Card, Column/Row, etc.)
- [ ] Compose the surfaceUpdate JSON with unique component IDs
- [ ] Call validate_and_emit_a2ui with the surfaceUpdate JSON
- [ ] If validation fails, fix the violations and re-validate
- [ ] Compose the beginRendering JSON pointing to the root component
- [ ] Call validate_and_emit_a2ui with the beginRendering JSON
- [ ] Wrap both validated payloads in ```a2ui markdown fences

validate_and_emit_a2ui MUST be called twice per surface: once for surfaceUpdate and once for beginRendering. Without beginRendering, the frontend will not render the surface.

Message Structure

surfaceUpdate (defines components)

{
  "surfaceUpdate": {
    "surfaceId": "my-surface",
    "components": [
      {"id": "unique-id", "component": {"TypeName": { ...props }}}
    ]
  }
}

• surfaceId: unique identifier for this UI surface.
• components: flat array of component definitions (NOT nested trees).
• Each component has id (unique string) and component (object with exactly one key = the type name).

beginRendering (tells the frontend which component is root)

{
  "beginRendering": {
    "surfaceId": "my-surface",
    "root": "root-component-id"
  }
}

• surfaceId: must match the surfaceUpdate's surfaceId.
• root: the id of the outermost component (usually a Card).

Typed Value Wrappers

CRITICAL: Never use raw JSON primitives. All values MUST be wrapped.

Raw "value", raw 42, or raw true as property values will be rejected by validate_and_emit_a2ui.

Container Children

Layout containers reference children by ID:

• Multiple children: "children": {"explicitList": ["child-id-1", "child-id-2"]} (NEVER raw arrays)
• Single child: "child": "child-id" (used by Card, Button)

Component ID Rules

• IDs MUST be unique within a surfaceUpdate.
• Use descriptive names: "title-heading", "main-card", "details-column".
• All ID references (child, children.explicitList, entryPointChild, contentChild, tabItems[].child) MUST resolve to IDs in the same components array.

Component Catalog

For the full catalog of 18 primitives, with required and optional props plus per-type notes, see components.md.

Examples

For three complete surfaceUpdate + beginRendering payloads (simple info card, data list card, dashboard with action button), see examples.md.