What causes "pattern requires 6 fields, got %d" in nats-io/nats-server?

Add a seconds field so the pattern has exactly 6 fields: "0 30 2 * * *" (sec min hour dom mon dow)

pattern requires 6 fields, got %d

Error message

pattern requires 6 fields, got %d

What it means

parseCron validates that a cron pattern consists of exactly 6 whitespace-separated fields (sec min hour dom month dow). Patterns with fewer or more fields fail immediately before any field parsing. Message schedules (via parseMsgSchedule) funnel through this parser.

Source

Thrown at server/cron.go:52

// IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
// CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

package server

import (
	"errors"
	"fmt"
	"math"
	"strconv"
	"strings"
	"time"
)

// parseCron parses the given cron pattern and returns the next time it will fire based on the provided ts.
func parseCron(pattern string, loc *time.Location, ts int64) (time.Time, error) {
	fields := strings.Fields(pattern)
	if len(fields) != 6 {
		return time.Time{}, fmt.Errorf("pattern requires 6 fields, got %d", len(fields))
	}

	// If no time zone is passed, default to UTC.
	if loc == nil {
		loc = time.UTC
	}

	// Parse each field.
	var err error
	var second, minute, hour, dayOfMonth, month, dayOfWeek uint64
	if second, err = getField(fields[0], seconds); err != nil {
		return time.Time{}, err
	}
	if minute, err = getField(fields[1], minutes); err != nil {
		return time.Time{}, err
	}
	if hour, err = getField(fields[2], hours); err != nil {
		return time.Time{}, err

View on GitHub (pinned to 3a66a489d2)

Solutions

Add a seconds field so the pattern has exactly 6 fields: "0 30 2 * * *" (sec min hour dom mon dow)
Remove any extra fields such as year
Use the schedule format alternative if a cron expression is not required

Example fix

// before
 schedule := "30 2 * * *"
// after
 schedule := "0 30 2 * * *"

Defensive patterns

Strategy: validation

Validate before calling

func validateCron6(p string) error {
    if n := len(strings.Fields(p)); n != 6 {
        return fmt.Errorf("cron needs 6 fields, got %d", n)
    }
    return nil
}

Try / catch

if _, err := parseCron(pattern, loc, time.Now().Unix()); err != nil {
    log.Fatalf("invalid schedule %q: %v", pattern, err)
}

Prevention

Remember this parser requires seconds as the first field (6 fields total)
Test schedules with parseCron at startup, not at first use
Convert 5-field crontab entries by prefixing a seconds field

When it happens

Trigger: Scheduling a message (e.g. schedule-based message delivery in JetStream) with a cron string like "0 0 12 * *" (5 fields) or "0 0 12 * * * 2026" (7 fields).

Common situations: Copy-pasting a standard 5-field crontab entry, which omits the seconds field that this parser requires; appending a year field that is unsupported.

Related errors

AI-assisted analysis of nats-io/nats-server@3a66a489d2 (2026-09-02). Data as JSON: /api/errors/6351bd13244b18f9. Report an issue: GitHub.