diff --git a/generator/.DevConfigs/aabf4e64-a7dd-4ee3-95a7-c8466c22a7e3.json b/generator/.DevConfigs/aabf4e64-a7dd-4ee3-95a7-c8466c22a7e3.json
new file mode 100644
index 000000000000..a0f861467e75
--- /dev/null
+++ b/generator/.DevConfigs/aabf4e64-a7dd-4ee3-95a7-c8466c22a7e3.json
@@ -0,0 +1,9 @@
+{
+  "core": {
+    "changeLogMessages": [
+      "Improve user agent string construction in Marshaller.cs"
+    ],
+    "type": "patch",
+    "updateMinimum": true
+  }
+}
\ No newline at end of file
diff --git a/sdk/src/Core/Amazon.Runtime/Pipeline/Handlers/Marshaller.cs b/sdk/src/Core/Amazon.Runtime/Pipeline/Handlers/Marshaller.cs
index 839d09fd4173..3b7fc0315665 100644
--- a/sdk/src/Core/Amazon.Runtime/Pipeline/Handlers/Marshaller.cs
+++ b/sdk/src/Core/Amazon.Runtime/Pipeline/Handlers/Marshaller.cs
@@ -15,6 +15,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Globalization;
 using System.Text;
 using Amazon.Runtime.Telemetry;
@@ -106,29 +107,28 @@ private static void SetRecursionDetectionHeader(IDictionary<string, string> head
 
         private static void SetUserAgentHeader(IRequestContext requestContext)
         {
-            var sb = new StringBuilder(requestContext.ClientConfig.UserAgent);
+            var sb = new StringBuilder(256);
+
+            sb.Append(InternalSDKUtils.ReplaceInvalidUserAgentCharacters(requestContext.ClientConfig.UserAgent));
 
             var clientAppId = requestContext.ClientConfig.ClientAppId;
             if (!string.IsNullOrEmpty(clientAppId))
-                sb.AppendFormat(" app/{0}", clientAppId);
+                sb.Append(" app/").Append(InternalSDKUtils.ReplaceInvalidUserAgentCharacters(clientAppId));
 
-            var retryMode = requestContext.ClientConfig.RetryMode.ToString().ToLower();
-            sb.AppendFormat(" cfg/retry-mode#{0}", retryMode);
+            sb.Append(" cfg/retry-mode#}").Append(ToUserAgentHeaderString(requestContext.ClientConfig.RetryMode));
 
-            sb.AppendFormat(" md/{0}", requestContext.IsAsync ? "ClientAsync" : "ClientSync");
+            sb.Append(" md/").Append(requestContext.IsAsync ? "ClientAsync" : "ClientSync");
 
-            sb.AppendFormat(" cfg/init-coll#{0}", AWSConfigs.InitializeCollections ? "1" : "0");
+            sb.Append(" cfg/init-coll#").Append(AWSConfigs.InitializeCollections ? '1' : '0');
 
             var userAgentAddition = requestContext.OriginalRequest.UserAgentAddition;
             if (!string.IsNullOrEmpty(userAgentAddition))
             {
-                sb.AppendFormat(" {0}", userAgentAddition);
+                sb.Append(' ').Append(InternalSDKUtils.ReplaceInvalidUserAgentCharacters(userAgentAddition));
             }
 
             var userAgent = sb.ToString();
 
-            userAgent = InternalSDKUtils.ReplaceInvalidUserAgentCharacters(userAgent);
-
             if (requestContext.ClientConfig.UseAlternateUserAgentHeader)
             {
                 requestContext.Request.Headers[HeaderKeys.XAmzUserAgentHeader] = userAgent;
@@ -138,5 +138,18 @@ private static void SetUserAgentHeader(IRequestContext requestContext)
                 requestContext.Request.Headers[HeaderKeys.UserAgentHeader] = userAgent;
             }
         }
+
+        private static string ToUserAgentHeaderString(RequestRetryMode requestRetryMode)
+        {
+            switch (requestRetryMode)
+            {
+                case RequestRetryMode.Standard:
+                    return "standard";
+                case RequestRetryMode.Adaptive:
+                    return "adaptive";
+                default:
+                    return requestRetryMode.ToString().ToLowerInvariant();
+            }
+        }
     }
 }
diff --git a/sdk/src/Core/GlobalSuppressions.cs b/sdk/src/Core/GlobalSuppressions.cs
index 476b7e93d6d5..359bd1bc2926 100644
--- a/sdk/src/Core/GlobalSuppressions.cs
+++ b/sdk/src/Core/GlobalSuppressions.cs
@@ -191,9 +191,9 @@
 [module: SuppressMessage("Microsoft.Globalization", "CA1308:NormalizeStringsToUppercase", Scope = "member", Target = "Amazon.Runtime.Internal.Auth.AWS4Signer.#CanonicalizeHeaders(System.Collections.Generic.ICollection`1<System.Collections.Generic.KeyValuePair`2<System.String,System.String>>)")]
 [module: SuppressMessage("Microsoft.Globalization", "CA1308:NormalizeStringsToUppercase", Scope = "member", Target = "Amazon.Runtime.Internal.Auth.S3Signer.#BuildCanonicalizedHeaders(System.Collections.Generic.IDictionary`2<System.String,System.String>)")]
 [module: SuppressMessage("Microsoft.Globalization", "CA1308:NormalizeStringsToUppercase", Scope = "member", Target = "Amazon.Runtime.Internal.Auth.AWS4Signer.#DetermineSigningRegion(Amazon.Runtime.IClientConfig,System.String,Amazon.RegionEndpoint,Amazon.Runtime.Internal.IRequest)")]
-
 [module: SuppressMessage("Microsoft.Globalization", "CA1308:NormalizeStringsToUppercase", Scope = "member", Target = "Amazon.Runtime.Internal.Auth.AWS4Signer.#CanonicalizeHeaders(System.Collections.Generic.IDictionary`2<System.String,System.String>)")]
 [module: SuppressMessage("Microsoft.Globalization", "CA1308:NormalizeStringsToUppercase", Scope = "member", Target = "Amazon.Runtime.Internal.Auth.AWS4Signer.#CanonicalizeHeaderNames(System.Collections.Generic.IDictionary`2<System.String,System.String>)")]
+[module: SuppressMessage("Microsoft.Globalization", "CA1308:NormalizeStringsToUppercase", Scope = "member", Target = "Amazon.Runtime.Internal.Marshaller.#ToUserAgentHeaderString(Amazon.Runtime.RequestRetryMode)")]
 
 // Types names matching namespaces
 [module: SuppressMessage("Microsoft.Naming", "CA1724:TypeNamesShouldNotMatchNamespaces", Scope = "type", Target = "Amazon.Auth.AccessControlPolicy.Policy")]
diff --git a/sdk/test/Services/SecurityToken/UnitTests/Custom/StaticCheckTests.cs b/sdk/test/Services/SecurityToken/UnitTests/Custom/StaticCheckTests.cs
index 1f516a944cfd..1b576b6ae127 100644
--- a/sdk/test/Services/SecurityToken/UnitTests/Custom/StaticCheckTests.cs
+++ b/sdk/test/Services/SecurityToken/UnitTests/Custom/StaticCheckTests.cs
@@ -83,5 +83,15 @@ public void EnsureCredentialProfileDetectorSetup()
             // the properties are used at least once.
             Assert.IsTrue(profileOptionsProperties.SetEquals(referencedProfileOptionsProperties));
         }
+
+        [TestMethod]
+        public void LookForRequestRetryModeChanges()
+        {
+            var expectedHash = "2FC699DB6A4284C5D53F3B7FA4E64165576FD1F6AE458BC737AE503F06DFBE4C";
+            AssertExtensions.AssertEnumUnchanged(
+                typeof(RequestRetryMode),
+                expectedHash,
+                "The Amazon.Runtime.Internal.Marshaller.ToUserAgentHeaderString method implementation may need to be updated.");
+        }
     }
 }
\ No newline at end of file