3 files changed, 15 insertions, 5 deletions
diff --git a/yt/yt/core/yson/protobuf_interop.cpp b/yt/yt/core/yson/protobuf_interop.cpp
index 5900fe24d23..59f62915874 100644
--- a/yt/yt/core/yson/protobuf_interop.cpp
+++ b/yt/yt/core/yson/protobuf_interop.cpp
@@ -132,14 +132,15 @@ bool IsMapKeyType(FieldDescriptor::Type type)
 TString ToUnderscoreCase(const TString& protobufName)
 {
     TStringBuilder builder;
-    for (auto ch : protobufName) {
-        if (isupper(ch)) {
-            if (builder.GetLength() > 0 && builder.GetBuffer()[builder.GetLength() - 1] != '_') {
+    for (size_t i = 0; i < protobufName.size(); ++i) {
+        if (isupper(protobufName[i])) {
+            size_t length = builder.GetLength();
+            if (length && builder.GetBuffer()[length - 1] != '_' && !isupper(protobufName[i - 1])) {
                 builder.AppendChar('_');
             }
-            builder.AppendChar(tolower(ch));
+            builder.AppendChar(tolower(protobufName[i]));
         } else {
-            builder.AppendChar(ch);
+            builder.AppendChar(protobufName[i]);
         }
     }
     return builder.Flush();
diff --git a/yt/yt/core/yson/unittests/proto/protobuf_yson_casing_ut.proto b/yt/yt/core/yson/unittests/proto/protobuf_yson_casing_ut.proto
index 627d6e23ad3..65fea7bc699 100644
--- a/yt/yt/core/yson/unittests/proto/protobuf_yson_casing_ut.proto
+++ b/yt/yt/core/yson/unittests/proto/protobuf_yson_casing_ut.proto
@@ -6,7 +6,14 @@ option (NYT.NYson.NProto.derive_underscore_case_names) = true;
 
 message TCamelCaseStyleMessage
 {
+    enum EEnum
+    {
+        VALUE_NONE = 0;
+        VALUE_FIRST = 1;
+    }
+
     optional int32 SomeField = 1;
     optional int32 AnotherField123 = 2;
     optional int32 Crazy_Field = 3;
+    optional EEnum EnumField = 4;
 }
diff --git a/yt/yt/core/yson/unittests/protobuf_yson_ut.cpp b/yt/yt/core/yson/unittests/protobuf_yson_ut.cpp
index ebc8afc360d..d6d97d6c1fe 100644
--- a/yt/yt/core/yson/unittests/protobuf_yson_ut.cpp
+++ b/yt/yt/core/yson/unittests/protobuf_yson_ut.cpp
@@ -1822,6 +1822,7 @@ TEST(TProtobufToYsonTest, Casing)
     message.set_somefield(1);
     message.set_anotherfield123(2);
     message.set_crazy_field(3);
+    message.set_enumfield(NYT::NYson::NProto::TCamelCaseStyleMessage::VALUE_FIRST);
 
     TEST_PROLOGUE()
     Y_PROTOBUF_SUPPRESS_NODISCARD message.SerializeToCodedStream(&codedStream);
@@ -1833,6 +1834,7 @@ TEST(TProtobufToYsonTest, Casing)
             .Item("some_field").Value(1)
             .Item("another_field123").Value(2)
             .Item("crazy_field").Value(3)
+            .Item("enum_field").Value("value_first")
         .EndMap();
     EXPECT_TRUE(AreNodesEqual(writtenNode, expectedNode));
 }