Desktop harness for Claude Code, Codex, and ACP agents

[!CAUTION] Harnss is currently pending a large rewrite to improve usability, performance and overall codebase into a more production ready app. I will share more info about it soon.

[!WARNING] Harnss is in early development and issues are to be expected. Please feel free to report bugs and issues in the Issues section.

Harness your AI coding agents.

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Harnss is a cross-platform desktop app that gives you one interface to run, manage, and switch between AI coding agents — Claude Code, Codex, and any ACP-compatible agent — without losing context, sessions, or tool state.

Why Harnss?

• One app, every agent. Run Claude Code, Codex, and custom ACP agents side by side. No more juggling terminals or losing context when switching tools.
• See what your AI is actually doing. Tool calls render as interactive cards with word-level diffs, syntax highlighting, and inline bash output — not raw JSON.
• Your workspace, your way. Built-in terminal, browser, git, MCP servers, and file panels — all scoped per project, all staying open while you work.

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Screenshots

Terminal, browser, git, and MCP integrations — all in one window.

One UI - all agents. Browse entire ACP registry.

Click to reveal other screenshots

https://github.com/user-attachments/assets/41f5bcbc-c141-4a16-b430-79f8e01d107e

Organize your projects in spaces.

Every tool call beautifully visualised - even those from popular MCPs.

Run multiple agent sessions side by side — switch instantly without losing progress.

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Features

Multi-engine sessions

Run Claude Code (via the Anthropic SDK), Codex, and ACP-compatible agents in parallel. Each session has its own state, history, and context. Switch between them instantly.

Rich tool visualization

Every tool call renders as an interactive card. File edits show word-level diffs with syntax highlighting. Bash output appears inline. Subagent tasks nest with step-by-step progress tracking. File changes are summarized per turn with a dedicated Changes panel.

MCP server management

Connect any MCP server per project via stdio, SSE, or HTTP transport. OAuth flows are handled automatically. Server status and available tool counts are visible at a glance. Jira, Confluence, and other integrations render with dedicated UIs rather than raw JSON.

Git integration

Stage, unstage, commit, and push without leaving the app. Browse branches, view commit history, and manage git worktrees. AI-generated commit messages are available from the staged diff.

Built-in terminal & browser

Multi-tab PTY terminal backed by native shell processes. An embedded browser for opening URLs inline and providing additional context to the agent. Both panels stay mounted while you work.

Project workspaces & spaces

Projects map to folders on disk. Spaces let you organize projects into named groups with custom icons and colors. Sessions, history, and panel settings are all scoped per project.

Agent Store

Browse and install agents from the ACP community registry directly in the app. Add custom agents by specifying a command, arguments, environment variables, and an icon. All configuration is managed through Settings — no config files.

Plan mode & permission control

Work in plan mode to have the agent draft a plan before making any changes. Three permission levels — Ask First, Accept Edits, Allow All — control how much autonomy the agent has. Switch modes at any point without interrupting context.

Background task agents

Task agents spawned during a session continue running in the background and are tracked in a dedicated panel. Keep working in other sessions while long-running tasks complete.

Image attachments & annotation

Attach screenshots or images directly in the chat. An built-in annotation tool lets you draw, highlight, and mark up images with freehand strokes before sending them to the agent.

Voice input & notifications

Voice input via native macOS dictation or an on-device Whisper model (no API key required). Configurable OS notifications for plan approval requests, permission prompts, agent questions, and session completion.

Session search & history

Full-text search across session titles and message content. Import and resume conversations previously started in the Claude Code CLI.

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Quick Start

1. Download the latest release for your platform from the Releases page
2. Open a project — point Harnss at any folder on disk
3. Choose an engine — Claude Code, Codex, or any installed ACP agent — and start working

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Engines & Agents

Harnss supports three execution engines out of the box:

Engine	Protocol	Requirements
Claude Code	Anthropic Agent SDK	Claude account (subscription or API key)
Codex	JSON-RPC app-server	Codex CLI in PATH + OpenAI API key or ChatGPT account
ACP agents	Agent Client Protocol	Agent-specific (see registry)

Claude Code and Codex are built-in. ACP agents can be installed from the ACP Agent Registry inside the app, or configured manually.

Examples of installable ACP-compatible agents:

Agent	Command	Notes
Gemini CLI	gemini --experimental-acp	Experimental ACP flag
Goose	goose acp
Docker cagent	cagent acp agent.yml	Container-based agents

Adding an agent

Open Settings → ACP Agents. The Agent Store tab lets you browse and install agents from the community registry. The My Agents tab lets you create custom agents — set the binary command, arguments, environment variables, and icon, or paste a JSON definition to auto-fill the form.

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MCP Servers

MCP servers are configured per project through the MCP Servers panel in the right-side toolbar. Supported transports: stdio, SSE, and HTTP. OAuth authentication is handled in-app with token persistence across sessions.

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Install

[!NOTE] Pre-built binaries are currently unsigned. On macOS, right-click the app and choose Open to bypass the Gatekeeper warning on first launch. On Windows, click More info → Run anyway if Windows Defender flags the installer.

Platform	Download
macOS (Apple Silicon)	.dmg (arm64)
macOS (Intel)	.dmg (x64)
Windows (x64)	.exe installer
Windows (ARM64)	.exe installer
Linux	.AppImage / .deb

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Development

git clone https://github.com/OpenSource03/harnss.git
cd harnss
pnpm install
pnpm dev

Build installers

pnpm dist:mac      # macOS DMG (arm64 + x64)
pnpm dist:win      # Windows NSIS installer (x64 + ARM64)
pnpm dist:linux    # Linux AppImage + deb

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Contributing

1. Fork the repo and create a feature branch
2. Follow the conventions in CLAUDE.md
3. Test with pnpm dev
4. Open a pull request

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License

MIT

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Built on the Agent Client Protocol