diff --git a/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/src/Extensions/StringExtensions.cs b/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/src/Extensions/StringExtensions.cs
index 5c754e5fe48..53fe46f08cd 100644
--- a/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/src/Extensions/StringExtensions.cs
+++ b/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/src/Extensions/StringExtensions.cs
@@ -9,10 +9,11 @@ namespace Microsoft.TypeSpec.Generator.Input.Extensions
 {
     public static class StringExtensions
     {
-        private static bool IsWordSeparator(char c) => !SyntaxFacts.IsIdentifierPartCharacter(c) || c == '_';
+        private static bool IsWordSeparator(char c, bool preserveUnderscores = false)
+            => !SyntaxFacts.IsIdentifierPartCharacter(c) || (!preserveUnderscores && c == '_');
 
         [return: NotNullIfNotNull("name")]
-        public static string ToIdentifierName(this string name, bool useCamelCase = false)
+        public static string ToIdentifierName(this string name, bool useCamelCase = false, bool preserveUnderscores = false)
         {
             if (string.IsNullOrEmpty(name))
             {
@@ -39,7 +40,7 @@ public static string ToIdentifierName(this string name, bool useCamelCase = fals
             for (; i < name.Length; i++)
             {
                 var c = name[i];
-                if (IsWordSeparator(c))
+                if (IsWordSeparator(c, preserveUnderscores))
                 {
                     upperCase = true;
                     continue;
@@ -56,7 +57,7 @@ public static string ToIdentifierName(this string name, bool useCamelCase = fals
                     upperCase = false;
                     // grow the first word length when this letter follows by two other upper case letters
                     // this happens in OSProfile, where OS is the first word
-                    if (i + 2 < name.Length && char.IsUpper(name[i + 1]) && (char.IsUpper(name[i + 2]) || IsWordSeparator(name[i + 2])))
+                    if (i + 2 < name.Length && char.IsUpper(name[i + 1]) && (char.IsUpper(name[i + 2]) || IsWordSeparator(name[i + 2], preserveUnderscores)))
                         firstWordLength++;
                     // grow the first word length when this letter follows by another upper case letter and an end of the string
                     // this happens when the string only has one word, like OS, DNS
@@ -77,6 +78,6 @@ public static string ToIdentifierName(this string name, bool useCamelCase = fals
         }
 
         [return: NotNullIfNotNull(nameof(name))]
-        public static string ToVariableName(this string name) => name.ToIdentifierName(useCamelCase: true);
+        public static string ToVariableName(this string name, bool preserveUnderscores = false) => name.ToIdentifierName(useCamelCase: true, preserveUnderscores: preserveUnderscores);
     }
 }
diff --git a/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/test/StringExtensionsTests.cs b/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/test/StringExtensionsTests.cs
new file mode 100644
index 00000000000..f6d04805531
--- /dev/null
+++ b/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.Input/test/StringExtensionsTests.cs
@@ -0,0 +1,51 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using Microsoft.TypeSpec.Generator.Input.Extensions;
+using NUnit.Framework;
+
+namespace Microsoft.TypeSpec.Generator.Input.Tests
+{
+    public class StringExtensionsTests
+    {
+        // Existing behavior without preserveUnderscores (default)
+        [TestCase("Tls_1_0", false, "Tls10")]
+        [TestCase("hello_world", false, "HelloWorld")]
+        [TestCase("_leading", false, "Leading")]
+        [TestCase("trailing_", false, "Trailing")]
+        [TestCase("UPPER_CASE", false, "UPPERCASE")]
+        [TestCase("simple", false, "Simple")]
+        [TestCase("", false, "")]
+        [TestCase(null, false, null)]
+        // New behavior with preserveUnderscores = true
+        [TestCase("Tls_1_0", true, "Tls_1_0")]
+        [TestCase("hello_world", true, "Hello_world")]
+        [TestCase("_leading", true, "_leading")]
+        [TestCase("trailing_", true, "Trailing_")]
+        [TestCase("UPPER_CASE", true, "UPPER_CASE")]
+        [TestCase("simple", true, "Simple")]
+        [TestCase("", true, "")]
+        [TestCase(null, true, null)]
+        [TestCase("TLS_1_0", true, "TLS_1_0")]
+        [TestCase("foo__bar", true, "Foo__bar")]
+        public void TestToIdentifierNamePreserveUnderscores(string name, bool preserveUnderscores, string expected)
+        {
+            var result = name.ToIdentifierName(preserveUnderscores: preserveUnderscores);
+            Assert.AreEqual(expected, result);
+        }
+
+        // Existing behavior of ToVariableName without preserveUnderscores
+        [TestCase("HelloWorld", false, "helloWorld")]
+        [TestCase("Tls_1_0", false, "tls10")]
+        [TestCase("UPPER_CASE", false, "upperCASE")]
+        // New behavior with preserveUnderscores = true
+        [TestCase("HelloWorld", true, "helloWorld")]
+        [TestCase("Tls_1_0", true, "tls_1_0")]
+        [TestCase("UPPER_CASE", true, "uppeR_CASE")]
+        public void TestToVariableNamePreserveUnderscores(string name, bool preserveUnderscores, string expected)
+        {
+            var result = name.ToVariableName(preserveUnderscores: preserveUnderscores);
+            Assert.AreEqual(expected, result);
+        }
+    }
+}