diff --git a/cmd/tool/cmd.go b/cmd/tool/cmd.go
index 36953343..b201c37c 100644
--- a/cmd/tool/cmd.go
+++ b/cmd/tool/cmd.go
@@ -29,7 +29,7 @@ func Run(name string, args []string) error {
 func usage(name string) string {
 	return fmt.Sprintf(`Usage: %s COMMAND [flags]
 
-Commands: slowest
+Commands: slowest, ci-matrix
 
 Use '%s COMMAND --help' for command specific help.
 `, name, name)
diff --git a/cmd/tool/matrix/matrix.go b/cmd/tool/matrix/matrix.go
index 4fb874bd..9ee07995 100644
--- a/cmd/tool/matrix/matrix.go
+++ b/cmd/tool/matrix/matrix.go
@@ -235,9 +235,9 @@ type bucket struct {
 }
 
 func writeBuckets(buckets []bucket) error {
-	out := make(map[int][]string)
+	out := make(map[int]string)
 	for i, bucket := range buckets {
-		out[i] = bucket.Packages
+		out[i] = strings.Join(bucket.Packages, " ")
 	}
 
 	raw, err := json.Marshal(out)