harness

Run any coding agent from a single CLI. Harness spawns the selected agent as a subprocess, translates its native streaming output into a unified NDJSON event stream, and outputs it to stdout.

Write one integration, and it works with any supported agent backend.

Supported agents

Agent	Binary	Status
Claude Code	claude	Supported
OpenAI Codex	codex	Supported
OpenCode	opencode	Supported
Cursor	agent	Supported

Install

Recommended — curl installer:

curl -fsSL https://harness.lol/install.sh | sh

Via crates.io:

cargo install harnesscli

Build from source:

git clone https://github.com/ayshptk/harness-cli.git
cd harness
cargo build --release
# Binary at target/release/harness

Quick start

# Run Claude Code with a prompt
harness run --agent claude --prompt "explain this codebase"

# Use a model alias
harness run --agent claude --model sonnet --prompt "fix the bug"

# Dry-run — see the resolved command without executing
harness run --agent claude --model sonnet --prompt "hello" --dry-run

# Pipe prompt from stdin
echo "explain this codebase" | harness run --agent claude

# List available agents
harness list

# Check if an agent is installed
harness check claude --capabilities

Model registry

Harness maps human-friendly model names to the exact IDs each agent expects:

# List all known models
harness models list

# Resolve an alias for a specific agent
harness models resolve opus --agent claude
# → claude-opus-4-6

# Update the registry cache
harness models update

Built-in aliases include opus. You can add your own in harness.toml, and the cached registry at ~/.harness/models.toml is auto-updated from GitHub.

Configuration

Place a harness.toml in your project root (or any parent directory):

default_agent = "claude"
default_model = "sonnet"
default_permissions = "full-access"
default_timeout_secs = 300

[agents.claude]
model = "opus"
extra_args = ["--verbose"]

[models.my-model]
description = "My custom model"
provider = "anthropic"
claude = "my-custom-model-id"

Unified event stream

Every agent's output is translated into a common NDJSON format with 8 event types:

• SessionStart — session initialized
• TextDelta — streaming text chunk
• Message — complete message
• ToolStart — tool invocation beginning
• ToolEnd — tool invocation complete
• UsageDelta — incremental token usage and cost update
• Result — run finished
• Error — error occurred

Documentation

Full docs at harness.lol

License

MIT