diff --git a/cmd/isprime.go b/cmd/isprime.go
new file mode 100644
index 0000000..7392ead
--- /dev/null
+++ b/cmd/isprime.go
@@ -0,0 +1,70 @@
+/*
+Copyright © 2025 filifa
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program. If not, see <http://www.gnu.org/licenses/>.
+*/
+package cmd
+
+import (
+	"fmt"
+	"log"
+	"math/big"
+
+	"github.com/spf13/cobra"
+)
+
+var rounds uint
+
+func isprime(cmd *cobra.Command, args []string) {
+	x := new(big.Int)
+
+	primes := make([]bool, len(args))
+	for i, s := range args {
+		_, ok := x.SetString(s, 10)
+		if !ok {
+			log.Fatal("invalid input " + s)
+		}
+
+		primes[i] = x.ProbablyPrime(int(rounds))
+	}
+
+	for _, b := range primes {
+		fmt.Println(b)
+	}
+}
+
+// isprimeCmd represents the isprime command
+var isprimeCmd = &cobra.Command{
+	Use:   "isprime",
+	Short: "Determine whether a number is prime",
+	Long:  `Determine whether a number is prime.`,
+	Args:  cobra.MinimumNArgs(1),
+	Run:   isprime,
+}
+
+func init() {
+	rootCmd.AddCommand(isprimeCmd)
+
+	// Here you will define your flags and configuration settings.
+
+	// Cobra supports Persistent Flags which will work for this command
+	// and all subcommands, e.g.:
+	// isprimeCmd.PersistentFlags().String("foo", "", "A help for foo")
+
+	// Cobra supports local flags which will only run when this command
+	// is called directly, e.g.:
+	// isprimeCmd.Flags().BoolP("toggle", "t", false, "Help message for toggle")
+
+	isprimeCmd.Flags().UintVarP(&rounds, "rounds", "r", 10, "number of iterations")
+}