diff --git a/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/IFormatterResolver.cs b/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/IFormatterResolver.cs
index 51a8e14e4..fbf3cf9be 100644
--- a/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/IFormatterResolver.cs
+++ b/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/IFormatterResolver.cs
@@ -116,6 +116,7 @@ public FormatterNotRegisteredException(string message)
         {
         }
 
+        [Obsolete("This API supports obsolete formatter-based serialization. It should not be called or extended by application code.")]
         protected FormatterNotRegisteredException(SerializationInfo info, StreamingContext context)
             : base(info, context)
         {
diff --git a/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/Internal/TinyJsonReader.cs b/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/Internal/TinyJsonReader.cs
index 02229621b..88c73de28 100644
--- a/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/Internal/TinyJsonReader.cs
+++ b/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/Internal/TinyJsonReader.cs
@@ -48,6 +48,7 @@ public TinyJsonException(string message)
         {
         }
 
+        [Obsolete("This API supports obsolete formatter-based serialization. It should not be called or extended by application code.")]
         protected TinyJsonException(SerializationInfo info, StreamingContext context)
             : base(info, context)
         {
diff --git a/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/MessagePackSerializationException.cs b/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/MessagePackSerializationException.cs
index 85aa1ef7e..a849480a7 100644
--- a/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/MessagePackSerializationException.cs
+++ b/src/MessagePack.UnityClient/Assets/Scripts/MessagePack/MessagePackSerializationException.cs
@@ -47,6 +47,7 @@ public MessagePackSerializationException(string message, Exception inner)
         /// </summary>
         /// <param name="info">Serialization info.</param>
         /// <param name="context">Serialization context.</param>
+        [Obsolete("This API supports obsolete formatter-based serialization. It should not be called or extended by application code.")]
         protected MessagePackSerializationException(
           System.Runtime.Serialization.SerializationInfo info,
           System.Runtime.Serialization.StreamingContext context)