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Monitor Deno cron jobs: Deno.cron native API and built-in fetch, zero dependencies

Deno.cron is a first-party cron API, stable in Deno 2.x. Wrap the handler in try/catch: await fetch(PING) on success; await fetch(PING + '/fail', ...) with the error body on failure; re-throw so Deno marks the run as failed. Built-in fetch, no npm, no dependencies. Absence detection covers the process that never starts or is killed. Free, no monitor cap.

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On this page
  1. The snippets
  2. What this catches
  3. Questions people ask

The snippets

Create a monitor in the console, set the schedule plus a grace window, copy the ping URL, and add it to your script.

Deno.cron: basic try/catch (Deno 2.x)

Deno.cron accepts a name, a cron expression, and an async handler. Re-throw after the /fail ping so Deno can apply its own failure handling: 

// job.ts — Deno 2.x (Deno.cron is stable, no flags needed)
const PING = Deno.env.get("LASTPING_URL")
  ?? "https://ping.lastping.dev/<your-monitor-id>";

async function runJob(): Promise<void> {
  // your job
}

Deno.cron("nightly-etl", "0 2 * * *", async () => {
  try {
    await runJob();
    await fetch(PING);
  } catch (err) {
    const msg = String(err instanceof Error ? err.stack : err);
    await fetch(`${PING}/fail`, {
      method: "POST",
      body: msg.slice(0, 10_000),
      headers: { "Content-Type": "text/plain" },
    });
    throw err; // re-throw so Deno marks the run as failed
  }
});

With /start ping for overrun detection

A /start ping arms the hung-job alarm. A rid pairs start with finish so each run's duration is recorded in the event trail: 

Deno.cron("nightly-etl", "0 2 * * *", async () => {
  const rid = Date.now().toString();
  await fetch(`${PING}/start?rid=${rid}`);

  try {
    await runJob();
    await fetch(`${PING}?rid=${rid}`);
  } catch (err) {
    const msg = String(err instanceof Error ? err.stack : err);
    await fetch(`${PING}/fail`, {
      method: "POST",
      body: msg.slice(0, 10_000),
    });
    throw err;
  }
});

Deno 1.x: with --unstable-cron flag

On Deno 1.38–1.x, pass --unstable-cron at startup. The API is identical; only the startup flag differs: 

# deno run --unstable-cron --allow-net --allow-env job.ts

// job.ts (same API as Deno 2.x)
const PING = Deno.env.get("LASTPING_URL")
  ?? "https://ping.lastping.dev/<your-monitor-id>";

Deno.cron("nightly-etl", "0 2 * * *", async () => {
  try {
    await runJob();
    await fetch(PING);
  } catch (err) {
    await fetch(`${PING}/fail`, {
      method: "POST",
      body: String(err).slice(0, 10_000),
    });
    throw err;
  }
});

Deno Deploy and external cron vs. Deno.cron

On Deno Deploy, Deno.cron runs inside the isolate and is the recommended approach. For scripts invoked externally by a system cron, use the same try/catch pattern but wrap the top-level await call rather than a Deno.cron handler. See also the Node.js guide for a comparison and the LastPing MCP server for AI-agent provisioning of monitors.

What this catches

The catch reports what it can; the silence reports the rest.

  • A thrown error: the /fail ping opens an incident immediately with the stack trace stored on the ping event.
  • An unhandled rejection: if the outer try/catch doesn't catch it, Deno marks the cron run as failed; the absence of a success ping opens the incident.
  • Process kill: no ping fires; the missing ping opens the incident after the grace window.
  • Deploy failure: new deploy that crashes on startup never registers the cron handler; absence → incident.
  • Recovery: the next successful run closes the incident and sends a recovery notice automatically.

Questions people ask

Front-loaded answers: the most important fact first.

  • How do I monitor a Deno cron job?

    Use Deno.cron to register your task. Inside the async handler, wrap your job in try/catch: on success await fetch('https://ping.lastping.dev/<your-monitor-id>'); on error await fetch('/fail', { method: 'POST', body: String(err).slice(0, 10000) }) and re-throw.

  • Does Deno.cron require any flags in Deno 2?

    No. Deno.cron is stable in Deno 2.x and available without flags. In Deno 1.x (1.38+) it was unstable and required --unstable-cron. Check your Deno version with deno --version.

  • Do I need npm or a third-party package?

    No. Deno ships fetch and the native Deno.cron API built in. The entire integration is zero-dependency: no npm, no JSR package, no import map entry.

  • Does this work on Deno Deploy?

    Yes. Deno.cron is supported on Deno Deploy and runs inside the isolate. Each invocation gets its own execution context, so the try/catch wraps the full body of each tick.

  • What if the Deno process is killed?

    A killed process sends no ping. Absence-detection monitoring handles this: the missing ping opens an incident after the grace window without any code running on the failure side.

  • Is Deno cron monitoring free?

    Yes. LastPing is free for individuals with no monitor cap. See how it compares to Healthchecks.io and Cronitor.

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Built-in fetch, native cron, zero setup: just add the ping.

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