Halo

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Quickstart ​

1. Install ​

One command. On macOS and Linux:

bash
curl -fsSL https://halolang.com/install.sh | sh

On Windows, in PowerShell:

powershell
irm https://halolang.com/install.ps1 | iex

This installs halo, a single standalone binary with Bun built in, so nothing else needs to be installed. The script picks the build for your OS and CPU (Intel or ARM; glibc or musl on Linux), checks it against the release's SHA256SUMS, puts it in ~/.halo/bin, and adds that folder to your shell's PATH. Check it with halo --version; update with halo upgrade.

VariableEffect
HALO_VERSION=0.1.0Install that version instead of the latest
HALO_INSTALL=/opt/haloInstall into /opt/halo/bin
HALO_NO_MODIFY_PATH=1Leave your shell's rc file alone

On Alpine, install the C++ runtime first: apk add libstdc++ libgcc. To uninstall, delete ~/.halo/bin and the PATH line the installer added.

To work on Halo itself, run it from a checkout instead: bun install, then bun link makes halo run the source.

2. Create a project ​

bash
halo init my-bot
cd my-bot

This creates:

  • main.halo: a starter pipeline (an agent with a system prompt and two policies)
  • .env.example: the API key variables Halo reads
  • .gitignore: ignores .env and .halo/ (paused runs, telemetry, the local vector store)
  • README.md

3. Set an API key ​

Each provider reads its own variable. Set the ones for the models you use:

Model prefixVariable
anthropic/, claude/, claude-*ANTHROPIC_API_KEY
openai/, oai/, gpt-*, o1…OPENAI_API_KEY
google/, gemini/, gemini-*GEMINI_API_KEY or GOOGLE_API_KEY
openrouter/, or/OPENROUTER_API_KEY
together/, tog/TOGETHER_API_KEY
replicate/, rep/REPLICATE_API_TOKEN or REPLICATE_API_KEY
ollama/, local/none (uses OLLAMA_HOST, default http://localhost:11434)

If a provider has no key of its own, Halo uses the --api-key flag, then HALO_API_KEY. Bun loads .env automatically.

bash
cp .env.example .env   # then fill in ANTHROPIC_API_KEY

4. Read the pipeline ​

main.halo looks like this:

halo
policy Safe   : Block(PromptInjection) & Redact(APIKey)
policy Budget : MaxSpend(0.25)

agent Writer [anthropic/claude-haiku-4-5, temp: 0.4]
  system """
    You write short, friendly explanations for curious people.
  """
  requires Safe, Budget

pipeline Explain(topic: String) -> Result:
  topic
    |> Writer.explain("Explain {{topic}} in three sentences.") as explanation
    |? (context.data.explanation != null)
    |> System.Print()

5. Run it ​

bash
halo validate .                                   # check syntax and names
halo test main.halo --arg topic="tide pools"      # offline: every call mocked
halo run main.halo --arg topic="tide pools"       # for real
halo debug main.halo --arg topic="tide pools"     # print each stage, prompt, and model call

Pass arguments with --arg key=value (repeatable; values that parse as JSON are parsed) or --args '{"topic": "tide pools"}'. If a file has several pipelines, choose one with --pipeline Name. Run halo --help for every flag.

The repository's examples/ directory has 40+ programs. examples/50-github-triage.halo triages a real GitHub repo and needs only ANTHROPIC_API_KEY:

bash
halo run examples/50-github-triage.halo --arg repo=oven-sh/bun --arg limit=8

Next steps ​