Cron Triggers

triggers.crons in the wrangler file runs the Worker’s scheduled(controller, env, ctx) handler on a schedule, in celld’s dialect, which is Cloudflare’s: five fields (minute, hour, day of month, month, day of week), UTC, one-minute resolution, with L, W and # forms and month and weekday names.

{
  "name": "cron",
  "main": "index.js",
  "compatibility_date": "2026-01-01",
  "triggers": { "crons": ["* * * * *"] }
}

export default {
  async fetch(request, env) {
    return new Response("The cron runs once a minute.\n");
  },
  async scheduled(controller, env, ctx) {
    console.log("cron", controller.scheduledTime);
  },
};

This is celld’s cron example (conformance/celld-examples/cron), copied unchanged and run by the conformance suite on Node and in Chromium (it waits for a real minute boundary).

Each runtime runs the triggers itself

The runtime that loads the project schedules its triggers, as celld does on a node:

• one invocation at a time, with its waitUntil() work finished before the next;
• a handler that throws is retried after 4 s, doubling, until it has failed six times, controller.noRetry() is called, or the next occurrence comes; scheduledTime names the occurrence, not the attempt;
• the last occurrence that fired is kept in the runtime’s SQLite, and after downtime only the latest missed occurrence runs.

What breaks

• A trigger fires once per runtime that loads the project. Nothing coordinates triggers between runtimes: with startProjectShards, every shard worker builds the project and runs its triggers, so with four shards a trigger fires four times.
• In a browser, triggers run only while a page runs the project. No page means no worker and nothing scheduled.
• triggers.crons without a scheduled() handler is reported in unsupported, not ignored.
• An expression celld refuses is refused: a descending range, a list containing *, a step wider than its field.