FastHarness

FastHarness exposes agents through Google’s A2A (Agent-to-Agent) protocol. Define agents with decorators, pick a runtime backend, and FastHarness handles protocol compliance, message conversion, task lifecycle, and multi-turn conversations.

Key Features

• Pluggable Runtime Backends: Swap between Claude Agent SDK, OpenHands, and Pydantic DeepAgents without changing agent definitions
• Executor Protocols: Decoupled AgentRuntime / AgentRuntimeFactory system separating agent logic from execution
• A2A Streaming: Server-sent events for real-time token streaming via message/sendStream
• Multi-turn Conversations: Runtime sessions maintain conversation history across A2A requests
• Python Client: FastHarnessClient with async send, streaming, and context management
• CLI: Command-line interface for running and interacting with agents
• Zero-Config Protocol Bridge: Decorator API handles all A2A protocol machinery, agent card generation, and JSON-RPC endpoints
• Cost Tracking & Step Logging: Built-in telemetry callbacks for monitoring API usage and debugging tool calls

Quick Start

from fastharness import FastHarness, Skill

harness = FastHarness(name="my-agent")

harness.agent(
    name="assistant",
    description="A helpful assistant",
    skills=[Skill(id="help", name="Help", description="Answer questions")],
    system_prompt="You are helpful.",
    tools=["Read", "Grep"],
)

app = harness.app  # Ready to deploy

# Install with your preferred backend
uv add fastharness                    # Claude (default)
uv add fastharness[openhands]         # OpenHands
uv add fastharness[deepagents]        # Pydantic DeepAgents