A2UI Agent implementation

The python/a2ui_agent/src/a2ui directory contains the Python implementation of the A2UI agent SDK.

Core Components

The following directories contain the base protocol logic, parsing, and schema operations directly under src/a2ui/:

Schema Management (src/a2ui/schema)

• manager.py: The A2uiSchemaManager handles loading specification schemas, managing catalogs, and generating system prompts for LLMs.
• catalog.py: Defines A2uiCatalog and CatalogConfig for handling component libraries. A2uiCatalog.validate_components checks components against the catalog schema with the a2ui-core PayloadValidator.

Parser (src/a2ui/parser)

• parser.py: Implementation of parse_response for synchronous parsing.
• streaming.py: Incremental streaming parsers with automatic JSON healing and validation.
• payload_fixer.py: Utilities to automatically correct common LLM output issues in A2UI payloads.

Basic Catalog (src/a2ui/basic_catalog)

• provider.py: Implementation of BasicCatalog for handling the basic A2UI components.

A2A (src/a2ui/a2a)

• extension.py: Utilities for managing the A2UI extension URI and activation logic.
• parts.py: Utilities for creating A2A Parts with A2UI data and helpers for response parsing.

ADK Extensions (src/a2ui/adk)

Support for the Agent Development Kit (ADK) and A2A protocol.

• send_a2ui_to_client_toolset.py: Implementation of SendA2uiToClientToolset to enable agents to send UI to clients via tool calls.

Running tests

1. Navigate to the package directory:

   cd python/a2ui_agent
   

2. Run the tests

   uv run pytest
   

Building the SDK

Building from source regenerates the Express parser from specification/inference_formats/express/Express.g4. That step runs ANTLR, which requires a Java runtime (JRE 11 or newer) on the PATH.

To build the SDK, run the following command from the python/a2ui_agent directory:

uv build

Formatting code

To format the code, run the following command from the python directory:

uv run pyink .

Releasing the SDK

See internal guidance at go/a2ui-release-pipy.

Disclaimer

Important: The sample code provided is for demonstration purposes and illustrates the mechanics of A2UI and the Agent-to-Agent (A2A) protocol. When building production applications, it is critical to treat any agent operating outside of your direct control as a potentially untrusted entity.

All operational data received from an external agent—including its AgentCard, messages, artifacts, and task statuses—should be handled as untrusted input. For example, a malicious agent could provide crafted data in its fields (e.g., name, skills.description) that, if used without sanitization to construct prompts for a Large Language Model (LLM), could expose your application to prompt injection attacks.

Similarly, any UI definition or data stream received must be treated as untrusted. Malicious agents could attempt to spoof legitimate interfaces to deceive users (phishing), inject malicious scripts via property values (XSS), or generate excessive layout complexity to degrade client performance (DoS). If your application supports optional embedded content (such as iframes or web views), additional care must be taken to prevent exposure to malicious external sites.

Developer Responsibility: Failure to properly validate data and strictly sandbox rendered content can introduce severe vulnerabilities. Developers are responsible for implementing appropriate security measures—such as input sanitization, Content Security Policies (CSP), strict isolation for optional embedded content, and secure credential handling—to protect their systems and users.