The Keel Language

Keel is a programming language where AI agents are first-class citizens.

Building an AI agent that monitors your email, classifies messages, and drafts replies takes ~180 lines of Python with LangChain. In Keel, it takes 40:

type Urgency = low | medium | high | critical

agent EmailAssistant {
  role "Professional email assistant"
  model "claude-sonnet"
  tools [email]

  task triage(email: {body: str}) -> Urgency {
    classify email.body as Urgency fallback medium
  }

  task handle(email: {body: str, from: str, subject: str}) {
    urgency = triage(email)

    when urgency {
      low, medium => {
        reply = draft "response to {email}" { tone: "friendly" }
        confirm user reply then send reply to email
      }
      high, critical => {
        notify user "{urgency}: {email.subject}"
        guidance = ask user "How to respond?"
        reply = draft "response to {email}" { guidance: guidance }
        confirm user reply then send reply to email
      }
    }
  }

  every 5.minutes {
    for email in fetch email where unread {
      handle(email)
    }
  }
}

run EmailAssistant

Design Principles

1. Agents are primitives — agent is a keyword, not a class pattern
2. Intent over implementation — classify, draft, summarize are built-in keywords, not library calls
3. Statically typed — full inference, every expression has a known type, mismatches are compile errors
4. Humans in the loop — ask, confirm, notify are first-class
5. Web is native — fetch, search, send need no imports
6. Time is built in — every, after, at for scheduling

What Keel Looks Like

Classify with AI

urgency = classify email.body as Urgency considering [
  "mentions a deadline" => high,
  "newsletter"          => low
] fallback medium using "claude-haiku"

Pattern matching

when urgency {
  low      => archive email
  medium   => auto_reply(email)
  high     => escalate(email)
  critical => page_oncall(email)
}

Collections with lambdas

urgent = emails.filter(e => triage(e) == critical)
names = contacts.map(c => c.name).sort_by(n => n)

Retry with backoff

retry 3 times with backoff { send email }

Getting Started

Install Keel and build your first agent in 5 minutes: Installation →