Getting started

This page gets you from zero to running your first Harn program.

Prerequisites

Rust 1.70 or later – install with curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
Git

Installation

From crates.io

cargo install harn-cli

From source

git clone https://github.com/burin-labs/harn
cd harn
./scripts/dev_setup.sh   # installs dev tooling, portal deps/build, git hooks, sccache
cargo build --release
cp target/release/harn ~/.local/bin/

Verify the installation:

harn version

Your first program

Create a file called hello.harn:

println("Hello, world!")

Run it:

harn run hello.harn

That’s it. Harn files can contain top-level code without any boilerplate. The above is an implicit pipeline – the runtime wraps your top-level statements automatically.

Adding a pipeline

For larger programs, organize code into named pipelines. The runtime executes the default pipeline (or the first one declared):

pipeline default(task) {
  let name = "Harn"
  println("Hello from ${name}!")
}

The task parameter is injected by the host runtime. It carries the user’s request when Harn is used as an agent backend.

Calling an LLM

Harn has native LLM support. Set your API key and call a model directly:

export ANTHROPIC_API_KEY=sk-ant-...

let response = llm_call(
  "Explain quicksort in two sentences.",
  "You are a computer science tutor."
)
println(response)

No imports, no SDK initialization, no response parsing. Harn ships with built-in configs for Anthropic, OpenAI, OpenRouter, Ollama, HuggingFace, and local OpenAI-compatible servers.

The REPL

Start an interactive session:

harn repl

The REPL evaluates expressions as you type and displays results immediately. It keeps a persistent history in ~/.harn/repl_history and supports multi-line blocks until delimiters are balanced, which makes it useful for experimenting with builtins and small snippets.

Project setup

Scaffold a new project with harn init or pick a starter with harn new:

harn new my-agent --template agent
cd my-agent
harn doctor --no-network

This creates a directory with harn.toml (project config) and starter files for the selected template. Run it with:

harn run main.harn

Remote MCP quick start

If you want to use a cloud MCP server such as Notion, authorize it once with the CLI and then reference it from harn.toml:

harn mcp redirect-uri
harn mcp login https://mcp.notion.com/mcp --scope "read write"

Next steps

Why Harn? – What problems Harn solves
Language basics – Syntax, types, control flow
LLM calls and agent loops – Calling models and building agents
Cookbook – Practical recipes and patterns

Keyboard shortcuts

Harn Documentation