diff --git a/cmd/simdata/main.go b/cmd/simdata/main.go
new file mode 100644
index 0000000..d188d3f
--- /dev/null
+++ b/cmd/simdata/main.go
@@ -0,0 +1,59 @@
+package main
+
+import (
+	"encoding/json"
+	"flag"
+	"fmt"
+	"os"
+
+	"git.sunturtle.xyz/zephyr/horse/umadump/replay"
+)
+
+func main() {
+	var out string
+	flag.StringVar(&out, "o", "", "output `file`")
+	flag.Parse()
+
+	r := os.Stdin
+	if flag.NArg() > 0 {
+		f, err := os.Open(flag.Arg(0))
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(2)
+		}
+		r = f
+		defer f.Close()
+	}
+
+	w := os.Stdout
+	if out != "" {
+		f, err := os.Create(out)
+		if err != nil {
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(2)
+		}
+		w = f
+		defer f.Close()
+	}
+
+	var race replay.Race
+	d := json.NewDecoder(r)
+	d.DisallowUnknownFields()
+	if err := d.Decode(&race); err != nil {
+		fmt.Fprintln(os.Stderr, "replay:", err)
+		os.Exit(1)
+	}
+	simdata, err := replay.DecodeSim(race.SimDataBase64)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "simdata:", err)
+	}
+
+	b, err := json.Marshal(simdata)
+	if err != nil {
+		panic(err)
+	}
+	_, err = w.Write(b)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+	}
+}