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A2UI - Agent-to-User Interface Protocol

A2UI is an open-source protocol from Google that enables AI agents to generate rich, interactive user interfaces through declarative JSON rather than executable code. It solves the critical challenge of safely transmitting UI across trust boundaries in multi-agent systems - agents describe UI intent using a flat list of pre-approved components, and clients render them natively across web, mobile, and desktop. The format is optimized for LLM generation with streaming support, incremental updates, and framework-agnostic rendering.

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When to use this skill

Trigger this skill when the user:

• Wants to build an agent that generates A2UI JSON responses
• Needs to set up a client renderer (Lit, React, Angular, Flutter) for A2UI
• Is working with A2UI message types (surfaceUpdate, createSurface, etc.)
• Wants to create or customize an A2UI component catalog
• Needs to implement data binding between components and a data model
• Is integrating A2UI with A2A Protocol or AG UI transport
• Wants to handle client-to-server actions (button clicks, form submissions)
• Is building custom components or extending the basic catalog

Do NOT trigger this skill for:

• General UI framework questions unrelated to agent-generated interfaces
• A2A Protocol questions that don't involve UI rendering

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Setup & authentication

Environment variables

GEMINI_API_KEY=your-gemini-api-key

Agent-side installation (Python with Google ADK)

pip install google-adk
adk create my_agent

Client-side installation

# Lit (web components)
npm install @a2ui/web-lib lit @lit-labs/signals

# React
npm install @a2ui/react @a2ui/web-lib

# Angular
npm install @a2ui/angular @a2ui/web-lib

# Flutter
flutter pub add flutter_genui

Quickstart (full demo)

git clone https://github.com/google/a2ui.git
cd a2ui
export GEMINI_API_KEY="your_key"
cd samples/client/lit
npm install
npm run demo:all

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Core concepts

Adjacency list model: A2UI uses a flat list of components with ID references instead of nested trees. This is easier for LLMs to generate incrementally and enables progressive rendering. Each component has an id, type, and properties.

Surfaces: A surface is a UI container identified by surfaceId. Components and data models are scoped to surfaces. Multiple surfaces can exist independently. A surface is locked to a catalog for its lifetime.

Data binding: Components connect to application state via JSON Pointer paths (RFC 6901). The data model is separate from UI structure, enabling reactive updates. Input components (TextField, CheckBox) bind bidirectionally.

Catalogs: JSON Schema files defining which components, functions, and themes an agent can use. The Basic Catalog provides standard components. Production apps define custom catalogs matching their design system. Agents can only request pre-approved components from the negotiated catalog.

Two specification versions:

Version	Status	Key differences
v0.8	Stable	surfaceUpdate/dataModelUpdate/beginRendering, nested component syntax, literalString wrappers
v0.9	Draft	createSurface/updateComponents/updateDataModel, flat component syntax, direct strings, required catalogId

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Common tasks

Generate a v0.9 A2UI surface with components

Create a surface, add components, set data, in JSONL format (one JSON per line):

{"version": "v0.9", "createSurface": {"surfaceId": "main", "catalogId": "https://a2ui.org/specification/v0_9/basic_catalog.json"}}
{"version": "v0.9", "updateComponents": {"surfaceId": "main", "components": [
  {"id": "header", "component": "Text", "text": "Book Your Table", "variant": "h1"},
  {"id": "date-input", "component": "DateTimeInput", "label": "Select Date", "value": {"path": "/reservation/date"}, "enableDate": true},
  {"id": "submit-btn", "component": "Button", "child": "btn-text", "variant": "primary", "action": {"event": {"name": "confirm_booking"}}}
]}}
{"version": "v0.9", "updateDataModel": {"surfaceId": "main", "path": "/reservation", "value": {"date": "2025-12-15", "time": "19:00", "guests": 2}}}

Generate a v0.8 A2UI surface (legacy)

{"surfaceUpdate": {"surfaceId": "main", "components": [
  {"id": "header", "component": {"Text": {"text": {"literalString": "Book Your Table"}, "usageHint": "h1"}}},
  {"id": "date-picker", "component": {"DateTimeInput": {"label": {"literalString": "Select Date"}, "value": {"path": "/reservation/date"}, "enableDate": true}}},
  {"id": "submit-btn", "component": {"Button": {"child": "submit-text", "action": {"name": "confirm_booking"}}}}
]}}
{"dataModelUpdate": {"surfaceId": "main", "contents": [
  {"key": "reservation", "valueMap": [
    {"key": "date", "valueString": "2025-12-15"},
    {"key": "time", "valueString": "19:00"},
    {"key": "guests", "valueInt": 2}
  ]}
]}}
{"beginRendering": {"surfaceId": "main", "root": "header"}}

v0.8 requires beginRendering before the client renders. v0.9 renders on createSurface.

Handle client-to-server actions

Wire a button to dispatch an event with context from the data model:

{
  "id": "submit-btn",
  "component": "Button",
  "child": "btn-text",
  "action": {
    "event": {
      "name": "submit_reservation",
      "context": {
        "time": {"path": "/reservationTime"},
        "size": {"path": "/partySize"}
      }
    }
  }
}

Add validation checks that auto-disable the button:

{
  "checks": [
    {
      "condition": {"call": "required", "args": {"value": {"path": "/partySize"}}},
      "message": "Party size is required"
    }
  ]
}

Build an agent with Google ADK

from google.adk import Agent
import json
import jsonschema

# Load A2UI schema for validation
with open("a2ui_schema.json") as f:
    a2ui_schema = json.load(f)

AGENT_INSTRUCTION = """You are a restaurant booking agent.
When the user wants to book, generate A2UI JSON after the delimiter ---a2ui_JSON---
Output a JSON list of A2UI messages using the v0.9 format."""

agent = Agent(
    model="gemini-2.5-flash",
    name="booking_agent",
    instruction=AGENT_INSTRUCTION,
)

# Validate generated A2UI before sending
def validate_a2ui(json_string):
    parsed = json.loads(json_string)
    jsonschema.validate(instance=parsed, schema=a2ui_schema)
    return parsed

Use data binding with dynamic lists

Render a list of items from the data model using templates:

{"version": "v0.9", "updateComponents": {"surfaceId": "main", "components": [
  {"id": "product-list", "component": "List", "direction": "vertical",
   "template": {"dataBinding": "/products", "componentId": "product-card"}},
  {"id": "product-card", "component": "Card", "children": ["product-name", "product-price"]},
  {"id": "product-name", "component": "Text", "text": {"path": "/name"}},
  {"id": "product-price", "component": "Text", "text": {"path": "/price"}}
]}}
{"version": "v0.9", "updateDataModel": {"surfaceId": "main", "path": "/products", "value": [
  {"name": "Widget A", "price": "$9.99"},
  {"name": "Widget B", "price": "$14.99"}
]}}

Inside templates, paths are scoped to each array item (e.g., /name refers to the current item's name).

Set up a Lit web renderer

import { A2uiMessageProcessor } from '@a2ui/web_core/data/model-processor';
import { SurfaceModel } from '@a2ui/web_core/v0_9';
import type * as Types from '@a2ui/web_core/types/types';

// Process incoming JSONL stream
const processor = new A2uiMessageProcessor();
processor.onSurface((surfaceModel: SurfaceModel) => {
  // Render the surface using Lit components
  renderSurface(surfaceModel);
});

// Feed messages from transport
function handleMessage(jsonLine: string) {
  processor.processMessage(JSON.parse(jsonLine));
}

Create a custom catalog

{
  "catalogId": "https://myapp.com/catalog/v1",
  "components": {
    "StockTicker": {
      "type": "object",
      "properties": {
        "symbol": {"type": "string"},
        "refreshInterval": {"type": "number", "default": 5000}
      },
      "required": ["symbol"]
    }
  },
  "functions": [],
  "theme": {}
}

Build with the catalog tool:

uv run tools/build_catalog/assemble_catalog.py \
  my_components.json \
  --extend-basic-catalog \
  --output-name my-catalog \
  --catalog-id "https://myapp.com/catalog/v1"

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Error handling

Error	Cause	Resolution
VALIDATION_FAILED	Component properties don't match catalog schema	Check component type exists in catalog and all required properties are set
Unknown component type	Agent used a component not in the negotiated catalog	Verify agent prompt includes correct catalog; add component to custom catalog
Data binding resolution failure	JSON Pointer path doesn't exist in data model	Send updateDataModel before referencing the path; check for typos in path
Surface not found	Operating on a surfaceId that hasn't been created	Send createSurface (v0.9) or surfaceUpdate (v0.8) first
Catalog negotiation failure	No matching catalog between agent and client	Include supportedCatalogIds in client metadata; check agent's advertised catalogs

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References

For detailed content on specific sub-domains, read the relevant file from the references/ folder:

• references/components.md - full component reference with all types and properties
• references/messages.md - complete message format for v0.8 and v0.9
• references/catalogs-and-actions.md - catalog schema, negotiation, and client-to-server actions
• references/renderer-guide.md - implementation checklist for building custom renderers

Only load a references file if the current task requires it - they are long and will consume context.

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Related skills

When this skill is activated, check if the following companion skills are installed. For any that are missing, mention them to the user and offer to install before proceeding with the task. Example: "I notice you don't have [skill] installed yet - it pairs well with this skill. Want me to install it?"

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Install a companion: npx skills add AbsolutelySkilled/AbsolutelySkilled --skill <name>