diff --git a/src/Microsoft.ML.Tokenizers/Model/TiktokenTokenizer.cs b/src/Microsoft.ML.Tokenizers/Model/TiktokenTokenizer.cs
index a460135c22..69f2af814f 100644
--- a/src/Microsoft.ML.Tokenizers/Model/TiktokenTokenizer.cs
+++ b/src/Microsoft.ML.Tokenizers/Model/TiktokenTokenizer.cs
@@ -1044,6 +1044,8 @@ private static readonly (string Prefix, ModelEncoding Encoding)[] _modelPrefixTo
                                                                 ( "o4-mini-", ModelEncoding.O200kBase ),  // e.g. o4-mini
 
                                                                 // chat
+                                                                ( "gpt-5.6-", ModelEncoding.O200kBase ),
+                                                                ( "gpt-5.5-", ModelEncoding.O200kBase ),
                                                                 ( "gpt-5.4-", ModelEncoding.O200kBase ),
                                                                 ( "gpt-5.3-", ModelEncoding.O200kBase ),
                                                                 ( "gpt-5.2-", ModelEncoding.O200kBase ),
@@ -1075,6 +1077,8 @@ private static readonly (string Prefix, ModelEncoding Encoding)[] _modelPrefixTo
                                                                 { "o4-mini", ModelEncoding.O200kBase },
 
                                                                 // chat
+                                                                { "gpt-5.6", ModelEncoding.O200kBase },
+                                                                { "gpt-5.5", ModelEncoding.O200kBase },
                                                                 { "gpt-5.4", ModelEncoding.O200kBase },
                                                                 { "gpt-5.3", ModelEncoding.O200kBase },
                                                                 { "gpt-5.2", ModelEncoding.O200kBase },
diff --git a/test/Microsoft.ML.Tokenizers.Tests/TiktokenTests.cs b/test/Microsoft.ML.Tokenizers.Tests/TiktokenTests.cs
index cc3814143b..7af5732509 100644
--- a/test/Microsoft.ML.Tokenizers.Tests/TiktokenTests.cs
+++ b/test/Microsoft.ML.Tokenizers.Tests/TiktokenTests.cs
@@ -40,6 +40,8 @@ public class TiktokenTests
         public static Tokenizer GPT5_2 { get; } = TiktokenTokenizer.CreateForModel("gpt-5.2");
         public static Tokenizer GPT5_3 { get; } = TiktokenTokenizer.CreateForModel("gpt-5.3");
         public static Tokenizer GPT5_4 { get; } = TiktokenTokenizer.CreateForModel("gpt-5.4");
+        public static Tokenizer GPT5_5 { get; } = TiktokenTokenizer.CreateForModel("gpt-5.5");
+        public static Tokenizer GPT5_6 { get; } = TiktokenTokenizer.CreateForModel("gpt-5.6");
         public static Tokenizer Phi4 { get; } = TiktokenTokenizer.CreateForModel("phi-4");
         public static TiktokenTokenizer GptOss { get; } = TiktokenTokenizer.CreateForModel("gpt-oss-20b");
 
@@ -427,6 +429,10 @@ public void TestEncodeR50kBase()
         [InlineData("gpt-5.3-mini")]
         [InlineData("gpt-5.4")]
         [InlineData("gpt-5.4-nano")]
+        [InlineData("gpt-5.5")]
+        [InlineData("gpt-5.5-mini")]
+        [InlineData("gpt-5.6")]
+        [InlineData("gpt-5.6-nano")]
         [InlineData("chatgpt-4o-")]
         [InlineData("gpt-4")]
         [InlineData("gpt-4-")]
@@ -549,6 +555,8 @@ public void TestEncodingNamesNegativeCases()
         [InlineData("gpt-5.2")]
         [InlineData("gpt-5.3")]
         [InlineData("gpt-5.4")]
+        [InlineData("gpt-5.5")]
+        [InlineData("gpt-5.6")]
         [InlineData("o1")]
         [InlineData("o3")]
         [InlineData("o4-mini")]