diff --git a/src/encoding/json/tags.go b/src/encoding/json/tags.go
index b490328f4c4..8b7aa9eac7e 100644
--- a/src/encoding/json/tags.go
+++ b/src/encoding/json/tags.go
@@ -15,10 +15,17 @@ type tagOptions string
 // parseTag splits a struct field's json tag into its name and
 // comma-separated options.
 func parseTag(tag string) (string, tagOptions) {
-	tag, opt, _ := strings.Cut(tag, ",")
+	tag, opt := cutTag(tag)
 	return tag, tagOptions(opt)
 }
 
+func cutTag(tag string) (string, string) {
+	if i := strings.IndexByte(tag, ','); i >= 0 {
+		return tag[:i], tag[i+1:]
+	}
+	return tag, ""
+}
+
 // Contains reports whether a comma-separated list of options
 // contains a particular substr flag. substr must be surrounded by a
 // string boundary or commas.
@@ -29,7 +36,7 @@ func (o tagOptions) Contains(optionName string) bool {
 	s := string(o)
 	for s != "" {
 		var name string
-		name, s, _ = strings.Cut(s, ",")
+		name, s = cutTag(s)
 		if name == optionName {
 			return true
 		}