The Keel Language

Latest: v0.1.33 — All stdlib errors are now typed — distinguish causes in try/catch blocks.

Alpha. Breaking changes expected between 0.x releases. See versioning.

Keel is a small, statically-typed language for building AI agents. The actor model is its one concurrency primitive. Everything else — AI calls, scheduling, human-in-the-loop, HTTP, email, memory — lives in a standard library that is auto-imported into every program.

You never write use keel/ai. You write Ai.classify(...) and the prelude makes it work.

type Urgency = low | medium | high | critical

agent EmailAssistant {
  @role "Professional email assistant"
  @tools [Email]

  on message(msg: Message) {
    urgency = Ai.classify(msg.body, as: Urgency) ?? Urgency.medium

    when urgency {
      low, medium => {
        reply = Ai.draft("response to {msg.body}", tone: "friendly")
        if Io.confirm(reply) { Email.send(reply, to: msg.from) }
      }
      high, critical => {
        Io.notify("{urgency}: {msg.subject}")
        guidance = Io.ask("How should I respond?")
        reply = Ai.draft("response to {msg.body}", guidance: guidance)
        if Io.confirm(reply) { Email.send(reply, to: msg.from) }
      }
    }
  }

  @on_start {
    Schedule.every(5.minutes, () => {
      for email in Email.fetch(unread: true) {
        Agent.send(self, email.as_message())
      }
    })
  }
}

run(EmailAssistant)

Zero imports. Namespaces like Ai, Io, Email, and Schedule are in scope from the start.

Design Principles

1. Small core, deep stdlib. If a feature can be a library, it is one. The core language has a small reserved keyword set.
2. Agents are primitives. agent is the only concurrency model. Per-agent serial mailboxes with isolated mutable state via self.
3. Prelude-as-stdlib. The standard library is auto-imported. Users get keyword-feel ergonomics without the compiler having to know about every feature.
4. Interfaces everywhere. LLM providers, memory stores, HTTP clients, loggers — all behind interfaces. Users swap implementations without leaving the language.
5. Statically typed. Full inference. Exhaustive pattern matching. Nullable safety. No implicit any.
6. No silent fallbacks. Misconfiguration fails loud at startup, not with plausible-looking nonsense at runtime.

Try It

Install and run your first agent:

curl https://keel-lang.dev/install.sh | sh
keel run examples/showcase.keel

→ Installation

Versioning and Breaking Changes

Keel is in alpha. Semver is not respected between 0.x minor versions.

• 0.1.x — current alpha. Complete reset: new design + migrated implementation. Breaking changes in patch releases are allowed.
• 0.2.x / 0.3.x — deliberately unplanned until v0.1 ships.
• 1.0.x — first API-stable release. Semver begins.

See SPEC.md for the authoritative design and ROADMAP.md for the plan.