Agent Harness

Agent Harness

Agent Harness is a small Python adapter for running the same tool registry through OpenAI Agents SDK or Anthropic Claude Agent SDK provider implementations. The core package has no runtime SDK dependency; provider SDKs are imported lazily only when that provider is used.

[](https://github.com/evalops/agent-harness/actions/workflows/ci.yml) [](https://github.com/evalops/agent-harness/actions/workflows/bazel-rbe.yml) [](https://www.python.org/downloads/)

What It Provides

• A thread-safe global tool registry via @register_tool.
• JSON Schema generation from Python type hints, including Optional, T | None,

list[T], dict, Literal, and multi-type unions.

• A common HarnessConfig and AgentResponse shape across providers.
• Lazy OpenAI and Claude provider adapters.
• Provider comparison helpers for running the same prompt against multiple adapters.
• A built-in optional You.com search tool adapter.

The registry and schema generation work without OpenAI or Anthropic packages installed. Actual model runs require the provider extras and API credentials.

Install

pip install -e .
pip install -e ".[openai]"
pip install -e ".[anthropic]"
pip install -e ".[all]"
pip install -e ".[dev]"

Provider credentials:

export OPENAI_API_KEY="sk-..."
export ANTHROPIC_API_KEY="sk-ant-..."

Quick Start

import asyncio

from agent_harness import AgentHarness, HarnessConfig, register_tool

@register_tool(description="Get weather for a city")
def get_weather(city: str) -> str:
    return f"Weather in {city}: sunny, 72F"

async def main() -> None:
    config = HarnessConfig(
        system_prompt="You are a helpful assistant.",
        tool_names=["get_weather"],
        max_turns=5,
        timeout_sec=30.0,
    )

    async with AgentHarness(provider="openai", config=config) as harness:
        response = await harness.run("What is the weather in Tokyo?")
        print(response.final_output)

        await harness.switch_provider("claude")
        response = await harness.run("What is the weather in Tokyo?")
        print(response.final_output)

asyncio.run(main())

Tool Registry

Register a tool once and let each provider adapter wrap it for its SDK:

from typing import Literal

from agent_harness import get_registry, register_tool

@register_tool(description="Score a deployment risk")
def score_risk(service: str, mode: Literal["fast", "safe"], retries: int | None = None) -> str:
    return f"{service}: {mode}, retries={retries}"

tool = get_registry().get("score_risk")
print(tool.json_schema)

Default values are not required in the generated schema. Parameters without annotations are treated as strings.

Configuration

from agent_harness import HarnessConfig

config = HarnessConfig(
    system_prompt="You are an expert operator.",
    model="gpt-4o",
    max_turns=10,
    temperature=0.7,
    timeout_sec=30.0,
    max_output_tokens=1000,
    top_p=0.9,
    stop_sequences=["STOP"],
    tool_names=["score_risk"],
    retry_attempts=3,
    retry_backoff=1.0,
    request_id="optional-stable-id",
    provider_options={"permission_mode": "acceptEdits"},
)

Validation rejects non-positive turns, timeout, retry backoff, and output token limits; temperatures outside 0.0..2.0; top_p outside 0.0..1.0; and negative retry counts.

Optional You.com Search Tool

import os

from agent_harness import HarnessConfig, register_you_com_search_tool

os.environ["YDC_API_KEY"] = "your_ydc_api_key"
register_you_com_search_tool(name="you_search")

config = HarnessConfig(
    system_prompt="Use web search when fresh context matters.",
    tool_names=["you_search"],
)

The adapter is intentionally non-fatal:

• Missing API key returns a setup message to the agent.
• HTTP or JSON failures return a concise error string.
• Empty result sets return No web results found.

Provider Notes

OpenAI provider:

• Installs through .[openai].
• Lazily imports agents.
• Wraps registered tools with function_tool.
• Runs through Runner.run() and streams through Runner.run_streamed().

Claude provider:

• Installs through .[anthropic].
• Lazily imports claude_agent_sdk.
• Exposes registered tools through an in-process MCP server.
• Accepts Claude-specific options through provider_options.

Development

python3 -m venv .venv
source .venv/bin/activate
pip install -e ".[dev]"
python3 -m pytest -q
make bazel-check

Remote execution smoke:

make bazel-rbe-smoke

The Bazel RBE GitHub Actions workflow runs on the EvalOps bazel-rbe-dev farm when BAZEL_RBE_ENABLED=true is set for the repository. It uses the evalops-agent-harness-rbe and bazel-rbe self-hosted labels.

Repository Layout

agent_harness.py          core registry, config, provider adapters
tests/                    pytest coverage
example_usage.py          provider examples
BUILD.bazel               Bazel pytest target
MODULE.bazel              Bazel module dependencies

Source & license

This open-source MCP server is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: evalops
• Source: evalops/agent-harness
• License: MIT

Install and usage instructions live in the source repository linked above.