Cron Check

Cron is where agents make confident mistakes: 0 0 13 * 5 runs every Friday and every 13th, not only on Friday the 13th; */7 in the minute field leaves a 4-minute gap at the top of each hour; Quartz and AWS count Sunday as 1 where Unix cron counts it as 0; GitHub Actions never runs a schedule more often than every 5 minutes. Cron Check answers with the rules of the scheduler in question:

• Explain and check up to 20 expressions: plain English, the dialect (detected or given), errors with the field that is wrong, the gotchas above, schedules that never fire (February 30), and the next runs.

• Next runs in any time zone, local and UTC, with runs that fall into a daylight-saving gap or repeat flagged, because schedulers treat them differently.

• Convert between Unix cron, GitHub Actions, Kubernetes, AWS EventBridge, Quartz and Spring: seconds and year fields, the ? rule, weekdays written as names so the numbering cannot bite. What a target cannot express (Unix's day-of-month OR day-of-week in AWS) is refused with the fix.

Runs offline. No key needed.

Built and maintained by Arhan Canli.

Install

Needs Node.js 20 or newer. No account or key.

Claude Code

claude mcp add cron-check -- npx -y cron-check-mcp

Claude Desktop: download cron-check-mcp-<version>.mcpb from the latest release and open it. The bundle is signed; verify it with gh attestation verify <file> --repo arhancanli/cron-check-mcp.

Any other client (Windsurf, Zed, Cline, Continue and others), in its MCP config file:

{
  "mcpServers": {
    "cron-check": {
      "command": "npx",
      "args": [
        "-y",
        "cron-check-mcp"
      ]
    }
  }
}

Docker

docker build -t cron-check-mcp https://github.com/arhancanli/cron-check-mcp.git && docker run -i --rm cron-check-mcp

Hosted (Streamable HTTP): node src/server.mjs --http serves stateless MCP at POST /mcp (port from PORT, default 3000).

Related MCP server: cron-mcp

Example

An agent calls explain_cron with:

{
  "expressions": [
    "0 9 * * 1-5",
    "*/7 * * * *",
    "0 0 1 * 1",
    "0 0 30 2 *",
    "cron(0 12 ? * MON-FRI *)",
    "0 0/5 14 * * ?",
    "0 10 ? * 6#3",
    "@daily",
    "61 * * * *",
    "0 0 L * *"
  ]
}

and gets back (recorded from the live server on 2026-09-26):

{
  "results": [
    {
      "expression": "0 9 * * 1-5",
      "valid": true,
      "dialect": "unix",
      "description": "At 09:00, Monday through Friday",
      "next_utc": [
        "2026-09-28T09:00:00Z",
        "2026-09-29T09:00:00Z",
        "2026-09-30T09:00:00Z"
      ]
    },
    {
      "expression": "*/7 * * * *",
      "valid": true,
      "dialect": "unix",
      "description": "Every 7 minutes",
      "next_utc": [
        "2026-09-26T17:14:00Z",
        "2026-09-26T17:21:00Z",
        "2026-09-26T17:28:00Z"
      ],
      "warnings": [
        "*/7 in the minute field restarts at 0 each hour, so the last gap is shorter than 7."
      ]
    },
    {
      "expression": "0 0 1 * 1",
      "valid": true,
      "dialect": "unix",
      "description": "At 00:00, on day 1 of the month, and on Monday",
      "next_utc": [
        "2026-09-28T00:00:00Z",
        "2026-10-01T00:00:00Z",
        "2026-10-05T00:00:00Z"
      ],
      "warnings": [
        "Both day fields are restricted, so it runs on days matching either one (day-of-month OR day-of-week), not only days matching both."
      ]
    },
    {
      "expression": "0 0 30 2 *",
      "valid": false,
      "dialect": "unix",
      "description": "At 00:00, on day 30 of the month, only in February",
      "error": "It never fires: no date in the next six years matches (for example February 30)."
    },
    {
      "expression": "cron(0 12 ? * MON-FRI *)",
      "valid": true,
      "dialect": "aws",
      "description": "At 12:00, Monday through Friday",
      "next_utc": [
        "2026-09-28T12:00:00Z",
        "2026-09-29T12:00:00Z",
        "2026-09-30T12:00:00Z"
      ],
      "warnings": [
        "EventBridge cron runs in UTC unless the schedule sets a time zone (EventBridge Scheduler)."
... (44 more lines)

Tools

How it behaves

• Offline and read-only: no network call at all (factory.allowHosts is empty). Descriptions come from cronstrue (MIT); parsing, matching and next runs from this server's own engine, time zones from Node's ICU database.

• Dialects: unix (Vixie cron: two restricted day fields match either one), github (UTC, 5-minute floor), kubernetes (macros, ? read as *), aws (EventBridge: six fields with a year, Sunday is 1, ? required in one day field), quartz (seconds first, optional year, Sunday is 1), spring (seconds first, Sunday is 0 or 7). L, W and # are understood where the dialect has them.

• Results are compact JSON with a matching output schema.

Benchmark

Measured 2026-09-26 with gpt-5.4-mini, 10 fixed tasks graded by fixed checks (bench/tasks.json, raw results in bench/results/).

Performance

Measured 2026-09-26 from Dubai, home connection against the live upstream, Node 24.19.0 (bench/perf.json, scripts/perf.mjs in the factory).

First call: a fresh server process, including the TLS connection and the upstream's own time. Repeat: the same call again, answered from the in-process cache, so it shows this server's own overhead.

Tool definitions the model reads on every turn (name, description, input schema): 1,872 characters, against 834 for @pipeworx/mcp-crontab, a published cron MCP server. The full tool list, with the output schemas and annotations clients use to validate results, is 3,017 characters (832 for the alternative).

More MCP servers by Arhan Canli

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• Kube Check: Checks Kubernetes manifests for your version: removed APIs, unknown fields, Pod Security, risks.

• License Check: Open source license answers: SPDX ids, copyleft, and whether a dependency's license fits yours.

• The whole collection, 12 more

License

MIT, Copyright (c) 2026 Arhan Canli.