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# A2A: Agent-to-Agent Communication Server
A lightweight, transport-agnostic framework for agent-to-agent communication based on JSON-RPC, implementing the [A2A Protocol](https://github.com/a2a-proto/a2a-protocol).
## Server
The A2A server provides a flexible JSON-RPC interface for agent-to-agent communication with support for multiple transport protocols.
### Features
- **Multiple Transport Protocols**:
- HTTP JSON-RPC endpoint (`POST /rpc`)
- WebSocket for bidirectional communication (`/ws`)
- Server-Sent Events (SSE) for real-time updates (`/events`)
- Standard I/O mode for CLI applications (`--stdio`)
- **Task-Based Workflow**:
- Create and manage asynchronous tasks
- Monitor task status through state transitions
- Receive artifacts produced during task execution
- **Simple Event System**:
- Real-time notifications for status changes
- Artifact update events
- Event replay for reconnecting clients
- **Extensible Handler System**:
- Automatic handler discovery
- Plugin system via entry points (`a2a.task_handlers`)
- Custom handler development via subclassing `TaskHandler`
- **Intuitive URL Structure**:
- Direct handler mounting at `/{handler_name}/rpc`, `/{handler_name}/ws`, and `/{handler_name}/events`
- Default handler accessible at root paths (`/rpc`, `/ws`, `/events`)
- Handler health checks at `/{handler_name}`
- **A2A Protocol Compliant**:
- Agent Cards at `/.well-known/agent.json` and `/{handler_name}/.well-known/agent.json`
- Standard Task/Message/Artifact structure
- Streaming support
### Running the Server
```bash
# Basic usage (HTTP, WS, SSE on port 8000)
uv run a2a-server
# Specify host and port
uv run a2a-server --host 0.0.0.0 --port 8000
# Enable detailed logging
uv run a2a-server --log-level debug
# Run in stdio JSON-RPC mode
uv run a2a-server --stdio
# List all available task handlers
uv run a2a-server --list-handlers
# List all registered routes (useful for debugging)
uv run a2a-server --list-routes
# Register additional handler packages
uv run a2a-server --handler-package my_custom_module.handlers
# Disable automatic handler discovery
uv run a2a-server --no-discovery
```
### Example: Configuring Agents via YAML
You can configure multiple agents with their capabilities and agent cards in YAML:
```yaml
server:
port: 8000
handlers:
use_discovery: false
default: chef_agent
pirate_agent:
type: a2a.server.tasks.handlers.google_adk_handler.GoogleADKHandler
agent: a2a.server.sample_agents.pirate_agent.pirate_agent
name: pirate_agent
agent_card:
name: Pirate Agent
description: "Converts your text into salty pirate‑speak"
url: "https://pirate.example.com"
version: "0.1.0"
documentationUrl: "https://pirate.example.com/docs"
provider:
organization: "Acme"
url: "https://acme.example.com"
capabilities:
streaming: true
pushNotifications: false
authentication:
schemes:
- "Bearer"
defaultInputModes:
- "text/plain"
defaultOutputModes:
- "text/plain"
skills:
- id: pirate-talk
name: Pirate Talk
description: "Turn any message into pirate lingo"
tags:
- pirate
- fun
examples:
- "Arrr! Give me yer loot!"
chef_agent:
type: a2a.server.tasks.handlers.google_adk_handler.GoogleADKHandler
agent: a2a.server.sample_agents.chef_agent.chef_agent
name: chef_agent
agent_card:
name: Chef Agent
description: "Suggests delicious recipes from your ingredients"
# Other fields can be automatically derived...
```
Then launch:
```bash
uv run a2a-server --config agent.yaml --log-level debug
```
### Interacting with the Server
```bash
# Create a task with default handler
curl -N -X POST http://127.0.0.1:8000/rpc \
-H 'Content-Type: application/json' \
-d '{
"jsonrpc":"2.0",
"id":1,
"method":"tasks/send",
"params":{
"message":{
"role":"user",
"parts":[{ "type":"text","text":"What can I make with chicken?" }]
}
}
}'
# Create a task with specific handler
curl -N -X POST http://127.0.0.1:8000/pirate_agent/rpc \
-H 'Content-Type: application/json' \
-d '{
"jsonrpc":"2.0",
"id":1,
"method":"tasks/send",
"params":{
"message":{
"role":"user",
"parts":[{ "type":"text","text":"Tell me about the sea" }]
}
}
}'
# Stream events from default handler
curl -N http://127.0.0.1:8000/events
# Stream events from specific handler
curl -N http://127.0.0.1:8000/pirate_agent/events
# Get the default agent card (A2A Protocol compliant)
curl http://127.0.0.1:8000/.well-known/agent.json
# Get a specific handler's agent card
curl http://127.0.0.1:8000/pirate_agent/.well-known/agent.json
# Check handler health
curl http://127.0.0.1:8000/pirate_agent
```
## URL Structure
The server provides a consistent URL structure:
### Default Handler
- `/rpc` - JSON-RPC endpoint for the default handler
- `/ws` - WebSocket endpoint for the default handler
- `/events` - SSE endpoint for the default handler
- `/.well-known/agent.json` - Agent Card for the default handler (A2A Protocol compliant)
### Specific Handlers
- `/{handler_name}/rpc` - JSON-RPC endpoint for a specific handler
- `/{handler_name}/ws` - WebSocket endpoint for a specific handler
- `/{handler_name}/events` - SSE endpoint for a specific handler
- `/{handler_name}/.well-known/agent.json` - Agent Card for a specific handler (A2A Protocol compliant)
### Health Checks
- `/` - Root health check with information about all handlers
- `/{handler_name}` - Handler-specific health check
## Agent Cards
The server implements the A2A Protocol's Agent Card specification. Agent cards contain information about the agent's:
- Name, description, and version
- Provider information
- Capabilities (streaming, push notifications)
- Authentication requirements
- Supported input/output modes
- Skills (capabilities the agent can perform)
Agent cards are generated from:
1. YAML configuration in the `agent_card` section
2. Dynamically detected capabilities (streaming support, etc.)
3. Reasonable defaults for missing fields
## Handler Details
- **`google_adk_handler`** wraps raw Google ADK `Agent` instances via `ADKAgentAdapter` so `.invoke()`/`.stream()` always exist.
- **`prepare_handler_params`** treats the `name` parameter as a literal, allowing YAML overrides without import errors.
### Custom Handler Development
Subclass `TaskHandler`, implement `process_task`, and register via:
- **Automatic discovery** (`--handler-package`)
- **Entry points** in `setup.py` under `a2a.task_handlers`
### Installation
Clone the repo and install the core library:
```bash
git clone https://github.com/yourusername/a2a.git
cd a2a
pip install -e .
```
Install optional extras as needed:
```bash
pip install -e ".[jsonrpc]" # core JSON-RPC only
pip install -e ".[server]" # HTTP, WS, SSE server
pip install -e ".[client]" # CLI client
pip install -e ".[adk]" # Google ADK agent support
pip install -e ".[full]" # All features
```
After installation, run the server or client using `uv run`:
```bash
uv run a2a-server --host 0.0.0.0 --port 8000 --log-level info
uv run a2a-client --help
```
### Requirements
- Python 3.9+
- HTTPX, WebSockets (for client)
- FastAPI, Uvicorn (for server)
- Pydantic v2+ (for JSON-RPC)
### Testing
```bash
pytest
```
