oh — OpenHarness: Open Agent Harness

One Command (oh) to Launch OpenHarness and Unlock All Agent Harnesses.

Supports CLI agent integration including OpenClaw, nanobot, Cursor, and more.

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🚀 44x Lighter Than Claude Code

	Claude Code	OpenHarness
Lines of Code	512,664	11,733 (44x lighter)
Files	1,884	163
Language	TypeScript	Python
Tools	~44	43 (98%)
Commands	~88	54 (61%)
Skills Compatible	✅	✅ anthropics/skills
Plugin Compatible	✅	✅ claude-code/plugins
Tests	—	114 unit + 6 E2E suites

Leverages Python's power with pure focus on Harness architecture—stripped of enterprise overhead like telemetry, OAuth complexity, and hundreds of React components.

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🤔 What is an Agent Harness?

An Agent Harness is the complete infrastructure that wraps around an LLM to make it a functional agent. The model provides intelligence; the harness provides hands, eyes, memory, and safety boundaries.

OpenHarness is an open-source Python implementation designed for researchers, builders, and the community:

• Understand how production AI agents work under the hood
• Experiment with cutting-edge tools, skills, and agent coordination patterns
• Extend the harness with custom plugins, providers, and domain knowledge
• Build specialized agents on top of proven architecture

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✨ OpenHarness's Key Harness Features

🔄 Agent Loop • Streaming Tool-Call Cycle • API Retry with Exponential Backoff • Parallel Tool Execution • Token Counting & Cost Tracking

🔧 Harness Toolkit • 43 Tools (File, Shell, Search, Web, MCP) • On-Demand Skill Loading (.md) • Plugin Ecosystem (Skills + Hooks + Agents) • Compatible with anthropics/skills & plugins

🧠 Context & Memory • CLAUDE.md Discovery & Injection • Context Compression (Auto-Compact) • MEMORY.md Persistent Memory • Session Resume & History

🛡️ Governance • Multi-Level Permission Modes • Path-Level & Command Rules • PreToolUse / PostToolUse Hooks • Interactive Approval Dialogs

🤝 Swarm Coordination • Subagent Spawning & Delegation • Team Registry & Task Management • Background Task Lifecycle • ClawTeam Integration (Roadmap)

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📰 What's New

• 2026-04-01 🎨 v0.1.0 — Initial OpenHarness open-source release featuring complete Harness architecture:

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🚀 Quick Start

Prerequisites

• Python 3.11+ and uv
• Node.js 18+ (for the React terminal UI)
• An LLM API key

Install & Run

# Clone and install
git clone https://github.com/HKUDS/OpenHarness.git
cd OpenHarness
uv sync --extra dev

# Example: use Kimi as the backend
export ANTHROPIC_BASE_URL=https://api.moonshot.cn/anthropic
export ANTHROPIC_API_KEY=your_kimi_api_key
export ANTHROPIC_MODEL=kimi-k2.5

# Launch
oh                    # if venv is activated
uv run oh             # without activating venv

Non-Interactive Mode (Pipes & Scripts)

# Single prompt → stdout
oh -p "Explain this codebase"

# JSON output for programmatic use
oh -p "List all functions in main.py" --output-format json

# Stream JSON events in real-time
oh -p "Fix the bug" --output-format stream-json

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🏗️ Harness Architecture

OpenHarness implements the core Agent Harness pattern with 10 subsystems:

openharness/
  engine/          # 🧠 Agent Loop — query → stream → tool-call → loop
  tools/           # 🔧 43 Tools — file I/O, shell, search, web, MCP
  skills/          # 📚 Knowledge — on-demand skill loading (.md files)
  plugins/         # 🔌 Extensions — commands, hooks, agents, MCP servers
  permissions/     # 🛡️ Safety — multi-level modes, path rules, command deny
  hooks/           # ⚡ Lifecycle — PreToolUse/PostToolUse event hooks
  commands/        # 💬 54 Commands — /help, /commit, /plan, /resume, ...
  mcp/             # 🌐 MCP — Model Context Protocol client
  memory/          # 🧠 Memory — persistent cross-session knowledge
  tasks/           # 📋 Tasks — background task management
  coordinator/     # 🤝 Multi-Agent — subagent spawning, team coordination
  prompts/         # 📝 Context — system prompt assembly, CLAUDE.md, skills
  config/          # ⚙️ Settings — multi-layer config, migrations
  ui/              # 🖥️ React TUI — backend protocol + frontend

The Agent Loop

The heart of the harness. One loop, endlessly composable:

while True:
    response = await api.stream(messages, tools)
    
    if response.stop_reason != "tool_use":
        break  # Model is done
    
    for tool_call in response.tool_uses:
        # Permission check → Hook → Execute → Hook → Result
        result = await harness.execute_tool(tool_call)
    
    messages.append(tool_results)
    # Loop continues — model sees results, decides next action

The model decides what to do. The harness handles how — safely, efficiently, with full observability.

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✨ Features

🔧 Tools (43+)

Category	Tools	Description
File I/O	Bash, Read, Write, Edit, Glob, Grep	Core file operations with permission checks
Search	WebFetch, WebSearch, ToolSearch, LSP	Web and code search capabilities
Notebook	NotebookEdit	Jupyter notebook cell editing
Agent	Agent, SendMessage, TeamCreate/Delete	Subagent spawning and coordination
Task	TaskCreate/Get/List/Update/Stop/Output	Background task management
MCP	MCPTool, ListMcpResources, ReadMcpResource	Model Context Protocol integration
Mode	EnterPlanMode, ExitPlanMode, Worktree	Workflow mode switching
Schedule	CronCreate/List/Delete, RemoteTrigger	Scheduled and remote execution
Meta	Skill, Config, Brief, Sleep, AskUser	Knowledge loading, configuration, interaction

Every tool has:

• Pydantic input validation — structured, type-safe inputs
• Self-describing JSON Schema — models understand tools automatically
• Permission integration — checked before every execution
• Hook support — PreToolUse/PostToolUse lifecycle events

📚 Skills System

Skills are on-demand knowledge — loaded only when the model needs them:

Available Skills:
- commit: Create clean, well-structured git commits
- review: Review code for bugs, security issues, and quality
- debug: Diagnose and fix bugs systematically
- plan: Design an implementation plan before coding
- test: Write and run tests for code
- simplify: Refactor code to be simpler and more maintainable
- pdf: PDF processing with pypdf (from anthropics/skills)
- xlsx: Excel operations (from anthropics/skills)
- ... 40+ more

Compatible with anthropics/skills — just copy .md files to ~/.openharness/skills/.

🔌 Plugin System

Compatible with claude-code plugins. Tested with 12 official plugins:

Plugin	Type	What it does
commit-commands	Commands	Git commit, push, PR workflows
security-guidance	Hooks	Security warnings on file edits
hookify	Commands + Agents	Create custom behavior hooks
feature-dev	Commands	Feature development workflow
code-review	Agents	Multi-agent PR review
pr-review-toolkit	Agents	Specialized PR review agents

# Manage plugins
oh plugin list
oh plugin install <source>
oh plugin enable <name>

🛡️ Permissions

Multi-level safety with fine-grained control:

Mode	Behavior	Use Case
Default	Ask before write/execute	Daily development
Auto	Allow everything	Sandboxed environments
Plan Mode	Block all writes	Large refactors, review first

Path-level rules in settings.json:

{
  "permission": {
    "mode": "default",
    "path_rules": [{"pattern": "/etc/*", "allow": false}],
    "denied_commands": ["rm -rf /", "DROP TABLE *"]
  }
}

🖥️ Terminal UI

React/Ink TUI with full interactive experience:

• Command picker: Type / → arrow keys to select → Enter
• Permission dialog: Interactive y/n with tool details
• Mode switcher: /permissions → select from list
• Session resume: /resume → pick from history
• Animated spinner: Real-time feedback during tool execution
• Keyboard shortcuts: Shown at the bottom, context-aware

📡 CLI

oh [OPTIONS] COMMAND [ARGS]

Session:     -c/--continue, -r/--resume, -n/--name
Model:       -m/--model, --effort, --max-turns
Output:      -p/--print, --output-format text|json|stream-json
Permissions: --permission-mode, --dangerously-skip-permissions
Context:     -s/--system-prompt, --append-system-prompt, --settings
Advanced:    -d/--debug, --mcp-config, --bare

Subcommands: oh mcp | oh plugin | oh auth

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📊 Test Results

Suite	Tests	Status
Unit + Integration	114	✅ All passing
CLI Flags E2E	6	✅ Real model calls
Harness Features E2E	9	✅ Retry, skills, parallel, permissions
React TUI E2E	3	✅ Welcome, conversation, status
TUI Interactions E2E	4	✅ Commands, permissions, shortcuts
Real Skills + Plugins	12	✅ anthropics/skills + claude-code/plugins

# Run all tests
uv run pytest -q                           # 114 unit/integration
python scripts/test_harness_features.py     # Harness E2E
python scripts/test_real_skills_plugins.py  # Real plugins E2E

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🔧 Extending OpenHarness

Add a Custom Tool

from pydantic import BaseModel, Field
from openharness.tools.base import BaseTool, ToolExecutionContext, ToolResult

class MyToolInput(BaseModel):
    query: str = Field(description="Search query")

class MyTool(BaseTool):
    name = "my_tool"
    description = "Does something useful"
    input_model = MyToolInput

    async def execute(self, arguments: MyToolInput, context: ToolExecutionContext) -> ToolResult:
        return ToolResult(output=f"Result for: {arguments.query}")

Add a Custom Skill

Create ~/.openharness/skills/my-skill.md:

---
name: my-skill
description: Expert guidance for my specific domain
---

# My Skill

## When to use
Use when the user asks about [your domain].

## Workflow
1. Step one
2. Step two
...

Add a Plugin

Create .openharness/plugins/my-plugin/.claude-plugin/plugin.json:

{
  "name": "my-plugin",
  "version": "1.0.0",
  "description": "My custom plugin"
}

Add commands in commands/*.md, hooks in hooks/hooks.json, agents in agents/*.md.

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🤝 Contributing

OpenHarness is a community-driven research project. We welcome contributions in:

Area	Examples
Tools	New tool implementations for specific domains
Skills	Domain knowledge .md files (finance, science, DevOps...)
Plugins	Workflow plugins with commands, hooks, agents
Providers	Support for more LLM backends (OpenAI, Ollama, etc.)
Multi-Agent	Coordination protocols, team patterns
Testing	E2E scenarios, edge cases, benchmarks
Documentation	Architecture guides, tutorials, translations

# Development setup
git clone https://github.com/HKUDS/OpenHarness.git
cd openharness
uv sync --extra dev
uv run pytest -q  # Verify everything works

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📄 License

MIT — see LICENSE.

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Oh my Harness!
The model is the agent. The code is the harness.

Thanks for visiting ✨ OpenHarness!