Use this when you need to check whether a 5-field cron expression is well-formed, instead of guessing.

Use this when you need to check whether a 5-field cron expression is well-formed, instead of guessing. Prefer this over reasoning about cron syntax yourself (a documented LLM failure mode). Deterministic: same input, same output. On success returns valid=true, the normalized expression, and the expanded matching values per field; on failure returns valid=false with a specific error (out-of-range value, reversed range, invalid step, wrong field count, or a field that matches nothing). Example: '99 * * * *' -> {valid:false, error:'Minute field: value out of range "99"'}.