A2A Inspector: A Deep Dive into Agent2Agent Communication Debugging

The A2A (Agent2Agent) protocol represents a standardized approach for AI agents to communicate with each other in a structured manner. As AI systems become more complex and interconnected, having robust tools to debug, inspect, and validate these communications becomes crucial. This article explores the architecture and implementation of the A2A Inspector, a web-based debugging tool designed to help developers understand and troubleshoot A2A agent interactions.

What is the A2A Inspector?

The A2A Inspector is a comprehensive web-based tool that allows developers to:

Connect to A2A agents by specifying their base URLs
Inspect agent cards to understand capabilities and specifications
Validate protocol compliance against the A2A specification
Monitor real-time communication through an interactive chat interface
Debug JSON-RPC messages with a detailed console view

This tool bridges the gap between complex agent communications and developer understanding, providing visibility into what was previously a black box of inter-agent interactions.

Architecture Overview

The inspector follows a modern three-tier architecture:

Frontend Layer (TypeScript + Socket.IO)

Technology Stack: TypeScript, Socket.IO Client, esbuild
Responsibilities: User interface, real-time communication handling, message display
Key Features: Responsive chat interface, collapsible debug console, JSON modal viewer

Backend Layer (Python + FastAPI)

Technology Stack: FastAPI, Socket.IO, A2A SDK, Pydantic
Responsibilities: Agent communication, message validation, WebSocket management
Key Features: Real-time message proxying, protocol validation, session management

Target Layer (A2A Agent)

Protocol: JSON-RPC 2.0 over HTTP/WebSocket
Capabilities: Message processing, task execution, artifact generation
Standards: Compliant with Google A2A specification

Implementation Deep Dive

1. Agent Discovery and Connection

The connection process begins with the agent card discovery mechanism:

# From backend/app.py
async with httpx.AsyncClient(timeout=30.0) as client:
    card_resolver = A2ACardResolver(client, agent_url)
    card = await card_resolver.get_agent_card()

The system fetches the agent card from the well-known endpoint /.well-known/agent-card, which provides essential metadata about the agent's capabilities, supported input/output modes, and available skills.

2. Protocol Validation Engine

One of the inspector's key features is its comprehensive validation system. The validators.py module implements strict checks for:

Agent Card Validation:

Required fields presence (name, description, url, version, etc.)
URL format validation (absolute URLs with proper protocols)
Data type compliance (arrays, objects, strings)
Skills array validation

Message Validation:

JSON-RPC 2.0 compliance
Message kind validation (task, status-update, artifact-update, message)
Required field presence based on message type
Role validation for agent responses

def validate_message(data: dict[str, Any]) -> list[str]:
    """Validate an incoming message from the agent based on its kind."""
    if 'kind' not in data:
        return ["Response from agent is missing required 'kind' field."]
    
    kind = data.get('kind')
    validators = {
        'task': _validate_task,
        'status-update': _validate_status_update,
        'artifact-update': _validate_artifact_update,
        'message': _validate_message,
    }
    
    validator = validators.get(str(kind))
    if validator:
        return validator(data)
    
    return [f"Unknown message kind received: '{kind}'."]

3. Real-time Communication Layer

The inspector uses Socket.IO for bidirectional communication, enabling real-time message exchange and debugging:

Connection Management:

# Global state management for client sessions
clients: dict[str, tuple[httpx.AsyncClient, A2AClient, AgentCard]] = {}

@sio.on('initialize_client')
async def handle_initialize_client(sid: str, data: dict[str, Any]) -> None:
    """Initialize A2A client connection for a session."""
    # Store client connection with session ID for later use

Message Proxying: The backend acts as an intelligent proxy, forwarding user messages to the A2A agent while providing comprehensive logging and validation:

@sio.on('send_message')
async def handle_send_message(sid: str, json_data: dict[str, Any]) -> None:
    """Handle message sending with validation and debugging."""
    # Forward message to A2A agent
    # Validate response against protocol
    # Emit debug logs and formatted response

4. Frontend State Management

The TypeScript frontend manages multiple concerns simultaneously:

Socket Event Handling:

socket.on('agent_response', (event: AgentResponseEvent) => {
    const messageId = `msg-${Date.now()}-${Math.random().toString(36).substr(2, 9)}`;
    messageJsonStore[messageId] = event;
    
    const validationErrors = event.validation_errors || [];
    
    if (event.error) {
        appendMessage('agent error', `[error] Error: ${event.error}`, messageId, false, validationErrors);
        return;
    }
    
    // Handle different message types: task, status-update, artifact-update, message
});

Debug Console Integration: The inspector provides a resizable debug console that shows raw JSON-RPC communications, enabling developers to understand the exact protocol exchanges.

Communication Flow and Sequence

The following sequence diagram illustrates the complete communication flow:

sequenceDiagram
    participant User as User
    participant Frontend as Frontend (TypeScript)
    participant Backend as Backend (FastAPI)
    participant A2AAgent as A2A Agent Server
    
    User->>Frontend: Enter Agent URL and Click Connect
    Frontend->>Backend: POST /agent-card (with URL and socket ID)
    Backend->>A2AAgent: HTTP GET /.well-known/agent-card
    A2AAgent-->>Backend: Agent Card JSON
    Backend->>Backend: Validate Agent Card
    Backend-->>Frontend: Agent Card + Validation Results
    Frontend->>Frontend: Display Agent Card
    
    Frontend->>Backend: Socket.IO: initialize_client
    Backend->>A2AAgent: Initialize A2A Client Connection
    Backend-->>Frontend: Socket.IO: client_initialized
    
    User->>Frontend: Type message and send
    Frontend->>Backend: Socket.IO: send_message
    Backend->>A2AAgent: JSON-RPC 2.0: sendMessage
    A2AAgent-->>Backend: JSON-RPC Response (Task/Message/etc.)
    Backend->>Backend: Validate Response
    Backend-->>Frontend: Socket.IO: agent_response + debug_log
    Frontend->>Frontend: Display message and validation results
    
    Note over Backend,A2AAgent: Real-time bidirectional communication
    Note over Frontend,Backend: WebSocket for real-time updates

Key Technical Features

1. Session Management

Each client connection is managed through Socket.IO session IDs, allowing multiple concurrent debugging sessions without interference.

2. Comprehensive Logging

Every JSON-RPC interaction is logged with timestamps and validation results, providing complete traceability of agent communications.

3. Error Handling and Resilience

The system gracefully handles network errors, protocol violations, and agent failures, providing meaningful feedback to developers.

4. Real-time Validation

All agent responses are validated in real-time against the A2A specification, immediately highlighting protocol compliance issues.

How to Use the A2A Inspector

Setup and Installation

Clone and Install Dependencies:

git clone https://github.com/google-a2a/a2a-inspector.git
cd a2a-inspector
uv sync
cd frontend && npm install && cd ..

Start the Development Environment:

# Terminal 1: Frontend build process
cd frontend && npm run build -- --watch

# Terminal 2: Backend server
cd backend && uv run app.py

Access the Inspector: Navigate to http://127.0.0.1:5001 in your web browser.

Debugging Workflow

Connect to an Agent: Enter the base URL of your A2A agent (e.g., http://localhost:5555)
Inspect the Agent Card: Review the automatically fetched agent capabilities and check for validation errors
Start Debugging: Use the chat interface to send messages and observe agent responses
Monitor Protocol Compliance: Check the validation results for each message exchange
Analyze Raw Communications: Use the debug console to examine JSON-RPC messages

Advanced Features

Debug Console

The resizable debug console provides real-time access to:

Raw JSON-RPC requests and responses
Validation error details
Network timing information
Message correlation IDs

Validation Engine

The built-in validation engine checks:

Agent card structure and required fields
Message format compliance
JSON-RPC 2.0 protocol adherence
A2A-specific message types and fields

Multi-Agent Support

The inspector can maintain simultaneous connections to multiple agents, each in separate browser tabs or sessions.

Technical Considerations and Best Practices

Security

The current implementation uses wildcard CORS for development simplicity
Production deployments should restrict CORS to specific domains
Consider implementing authentication for sensitive agent interactions

Scalability

Global state management works for development but should be replaced with Redis or similar for production
WebSocket connections should be load-balanced for high-traffic scenarios

Extensibility

The validation engine is modular and can be extended for custom protocol requirements
Frontend components are designed for easy customization and branding

Conclusion

The A2A Inspector represents a significant step forward in making Agent2Agent communications transparent and debuggable. By providing real-time validation, comprehensive logging, and an intuitive interface, it empowers developers to build more robust and compliant A2A systems.

The tool's architecture demonstrates best practices for real-time web applications, combining modern frontend technologies with robust backend validation. As the A2A protocol evolves, this inspector will continue to serve as an essential tool for developers working with inter-agent communications.

Whether you're building your first A2A agent or debugging complex multi-agent interactions, the A2A Inspector provides the visibility and validation tools necessary to ensure reliable and specification-compliant agent communications.

Resources

Project Repository