Cron expressions for scheduled tasks

[thymikee]

Problem or use case

Fixed-time and interval schedules can't express common recurring patterns in a single task:

• Several clock times a day: [Feature]: Add model options and custom recurrence to Scheduled Tasks #15565 asks for 09:00 and 19:00, and a commenter there created four identical tasks for 00:00, 06:00, 12:00 and 18:00.
• Monthly runs: Monthly option for fixed-time scheduled tasks #19009.
• Working-hours patterns, such as hourly from 08:00 to 20:00 plus two overnight runs. In Claude Code I schedule this as 17 0,4,8-20 * * *.

Today these need one task per time, all sharing a prompt, or an interval that drifts away from the clock times.

Proposed solution

• A Custom schedule that takes one standard five-field cron expression (minute, hour, day of month, month, weekday). It is evaluated in the environment's time zone, like fixed-time tasks, and validated on save.
• Agents can create and edit it through schedule_task and update_scheduled_task with { "type": "cron", "expression": "..." }.
• Optional: after saving, the thread-naming model writes a plain-language label for the task list, such as "Hourly at :17 from 08:17 to 20:17, plus 00:17 and 04:17". Until it arrives, or if generation fails, the list shows the expression.

Smallest useful scope

The cron schedule type, its validation and next-run calculation, the Custom option in the web and mobile editors, and the MCP schema. Effect already ships effect/Cron, so this needs no new dependency. Existing schedules are unchanged. The generated label can be split out or dropped.

How a task behaves when its previous run is still busy is a separate problem, tracked in #16818. It is not part of this proposal.

Questions

1. Is cron the shape you want? The alternative is to extend fixed-time schedules separately: a list of times (suggested in [Feature]: Add model options and custom recurrence to Scheduled Tasks #15565) and a monthly option (Monthly option for fixed-time scheduled tasks #19009). Cron covers both, plus ranges and steps.
2. Do you want the generated label, or should the list show the raw expression only?

Prior work

I have a working implementation in #19381, which was closed for missing approval. If you approve a direction, I'll cut it down to the approved scope. I won't ask for reconsideration until then.

Before	After

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Posted by Claude (Opus 5.5, Claude Code via T3 Code) on behalf of @thymikee.

[manvankaliber]

Codex migration use case: I migrated an hourly automation configured to run at minute :10 of each hour. An interval schedule preserves the one-hour cadence but anchors it to task creation time, so creating it at 09:37 shifts runs to 10:37, 11:37, and so on. A cron expression such as 10 * * * * preserves the intended clock alignment and avoids drifting away from reporting windows and other hourly work.

[uwe-schwarz]

I would propose a custom field with cron and rrule (https://datatracker.ietf.org/doc/html/rfc5545). This gives the maximum flexibility and the agents can fill those fields very easily. Also cron/rrule can be distinguished very easily from another.

[nacholibre]

For the monthly case listed above (#19009), there is now a PR that works without cron: #26001 adds days of the month (1–31) and an optional set of months to the existing At a time schedule, such as the 5th of January, April, July and October. It doesn't cover several times a day or hour ranges, so it doesn't replace this proposal. #25479 also adds a single monthly date.

Written with Claude Opus 5.5 (claude-opus-5-5) in Claude Code, running inside T3 Code.