diff --git a/schema/message_parser.go b/schema/message_parser.go
index 7bd95b81c..b0597a46f 100644
--- a/schema/message_parser.go
+++ b/schema/message_parser.go
@@ -50,17 +50,18 @@ type MessageJSONParseConfig struct {
 
 // NewMessageJSONParser creates a new MessageJSONParser.
 func NewMessageJSONParser[T any](config *MessageJSONParseConfig) MessageParser[T] {
-	if config == nil {
-		config = &MessageJSONParseConfig{}
+	var parseConfig MessageJSONParseConfig
+	if config != nil {
+		parseConfig = *config
 	}
 
-	if config.ParseFrom == "" {
-		config.ParseFrom = MessageParseFromContent
+	if parseConfig.ParseFrom == "" {
+		parseConfig.ParseFrom = MessageParseFromContent
 	}
 
 	return &MessageJSONParser[T]{
-		ParseFrom:    config.ParseFrom,
-		ParseKeyPath: config.ParseKeyPath,
+		ParseFrom:    parseConfig.ParseFrom,
+		ParseKeyPath: parseConfig.ParseKeyPath,
 	}
 }
 
diff --git a/schema/message_parser_test.go b/schema/message_parser_test.go
index e645f8793..fe1f1c0cd 100644
--- a/schema/message_parser_test.go
+++ b/schema/message_parser_test.go
@@ -46,6 +46,16 @@ func TestMessageJSONParser(t *testing.T) {
 		assert.Equal(t, 1, parsed.ID)
 	})
 
+	t.Run("default source leaves the config unchanged", func(t *testing.T) {
+		config := &MessageJSONParseConfig{ParseKeyPath: "id"}
+		parser := NewMessageJSONParser[int](config)
+
+		parsed, err := parser.Parse(ctx, &Message{Content: `{"id": 7}`})
+		assert.NoError(t, err)
+		assert.Equal(t, 7, parsed)
+		assert.Equal(t, &MessageJSONParseConfig{ParseKeyPath: "id"}, config)
+	})
+
 	t.Run("parse from tool call", func(t *testing.T) {
 		t.Run("only one tool call, default use first tool call", func(t *testing.T) {
 			parser := NewMessageJSONParser[TestStructForParse](&MessageJSONParseConfig{