Watch something, and react with a decision, not a guess

examples/76-watch-and-react.halo policies |! people

halo serve checks a source on a schedule and starts this pipeline only when it changes (no model call while nothing changes). The pipeline gets what changed; an agent picks one action from a fixed list, as JSON the policy enforces; a branch routes the decision; and the risky action waits for a person. halo.json: { "files": ["examples/76-watch-and-react.halo"], "watches": [ { "pipeline": "OnStatusChange", "url": "https://www.githubstatus.com/api/v2/summary.json", "every": "2m" } ], "notify": { "slack": "Env.SLACK_WEBHOOK_URL" }, "slack": { "signingSecret": "Env.SLACK_SIGNING_SECRET" } }

Open in the playground →

halo test examples/76-watch-and-react.halo --args '{"source":"x","changes":[]}'

// ============================================================
// Watch something, and react with a decision, not a guess
// ============================================================
// `halo serve` checks a source on a schedule and starts this pipeline only
// when it changes (no model call while nothing changes). The pipeline gets
// what changed; an agent picks one action from a fixed list, as JSON the
// policy enforces; a branch routes the decision; and the risky action waits
// for a person.
//
// halo.json:
//   {
//     "files": ["examples/76-watch-and-react.halo"],
//     "watches": [
//       { "pipeline": "OnStatusChange", "url": "https://www.githubstatus.com/api/v2/summary.json", "every": "2m" }
//     ],
//     "notify": { "slack": "Env.SLACK_WEBHOOK_URL" },
//     "slack": { "signingSecret": "Env.SLACK_SIGNING_SECRET" }
//   }
//
// RUN:  halo serve           (approve or reject the page from Slack's buttons)
// TRY:  halo test examples/76-watch-and-react.halo --args '{"source":"x","changes":[]}'
// ============================================================

policy Decision : Enforce(JSON) & MatchRegex("\"action\"\\s*:\\s*\"(ignore|note|alert|page)\"") & MaxSpend(0.05)

schema Verdict {
  action: String,
  reason: String
}

tool Chat : Slack(Env.SLACK_BOT_TOKEN, channel: "#ops")
store Log : VectorStore(namespace: "status-changes")

@correct(1)
agent Triage [anthropic/claude-haiku-4-5, temp: 0.0]
  system """
    You decide what to do about a change in a service's status page.
    Reply with only JSON: {"action": "...", "reason": "<one sentence>"}
    where action is exactly one of:
      ignore: cosmetic or no real change
      note:   worth remembering, nobody needs to act
      alert:  a degradation people should know about
      page:   a major outage that needs someone now
  """
  requires Decision

pipeline OnStatusChange(source: String, changes: JSON) -> Verdict:
  changes
    |> Triage.decide("What changed at {{source}}, and what should we do?", changes) as verdict
    |> Log.log(content: context.data.verdict, tags: ["{{verdict.action}}"])
    |? {
         (context.data.verdict.action == "alert") => Chat.post(text: "Status change: {{verdict.reason}}"),
         else => System.Value(context.data.verdict)
       }
    |> System.Value(context.data.verdict)

    // Paging someone is the one action that waits for a person.
    |? (context.data.verdict.action == "page")
    |! yield(msg: "Page the on-call engineer? {{verdict.reason}}") as review
    |? (context.data.review.approved == true)
    |> Chat.post(text: "<!here> Paging on-call: {{verdict.reason}}")
    |> System.Value(context.data.verdict)