diff --git a/packages/middleware-flexible-checksums/src/middleware-flexible-checksums.integ.spec.ts b/packages/middleware-flexible-checksums/src/middleware-flexible-checksums.integ.spec.ts
index 027e8ad74b1f..40b0d1a045d2 100644
--- a/packages/middleware-flexible-checksums/src/middleware-flexible-checksums.integ.spec.ts
+++ b/packages/middleware-flexible-checksums/src/middleware-flexible-checksums.integ.spec.ts
@@ -137,7 +137,7 @@ describe("middleware-flexible-checksums", () => {
 
           requireRequestsFrom(client).toMatch({
             headers: {
-              "user-agent": new RegExp(`(.*?) m\/${id}$`),
+              "user-agent": new RegExp(`(.*?) m\/${id},E$`),
             },
           });
 
diff --git a/packages/middleware-sdk-s3/src/s3-express/middleware-s3-express.integ.spec.ts b/packages/middleware-sdk-s3/src/s3-express/middleware-s3-express.integ.spec.ts
index a73facfe5d8c..e969678effe7 100644
--- a/packages/middleware-sdk-s3/src/s3-express/middleware-s3-express.integ.spec.ts
+++ b/packages/middleware-sdk-s3/src/s3-express/middleware-s3-express.integ.spec.ts
@@ -93,7 +93,7 @@ describe("middleware-s3-express", () => {
 
       requireRequestsFrom(client).toMatch({
         headers: {
-          "user-agent": /(.*?) m\/J$/,
+          "user-agent": /(.*?) m\/J,E$/,
         },
       });
 
diff --git a/packages/middleware-user-agent/src/check-features.ts b/packages/middleware-user-agent/src/check-features.ts
index 18b41d24ac4e..6272b3085d78 100644
--- a/packages/middleware-user-agent/src/check-features.ts
+++ b/packages/middleware-user-agent/src/check-features.ts
@@ -6,7 +6,13 @@ import type {
   AwsSdkCredentialsFeatures,
 } from "@aws-sdk/types";
 import type { IHttpRequest } from "@smithy/protocol-http";
-import type { AwsCredentialIdentityProvider, BuildHandlerArguments, Provider } from "@smithy/types";
+import type {
+  AwsCredentialIdentityProvider,
+  BuildHandlerArguments,
+  Provider,
+  RetryStrategy,
+  RetryStrategyV2,
+} from "@smithy/types";
 
 /**
  * @internal
@@ -14,8 +20,14 @@ import type { AwsCredentialIdentityProvider, BuildHandlerArguments, Provider } f
 type PreviouslyResolved = Partial<{
   credentials?: AwsCredentialIdentityProvider;
   accountIdEndpointMode?: Provider<AccountIdEndpointMode>;
+  retryStrategy?: Provider<RetryStrategy | RetryStrategyV2>;
 }>;
 
+/**
+ * @internal
+ */
+const ACCOUNT_ID_ENDPOINT_REGEX = /\d{12}\.ddb/;
+
 /**
  * @internal
  * Check for features that don't have a middleware activation site but
@@ -26,9 +38,30 @@ export async function checkFeatures(
   config: PreviouslyResolved,
   args: BuildHandlerArguments<any>
 ): Promise<void> {
-  // eslint-disable-next-line
   const request = args.request as IHttpRequest;
+
+  if (request?.headers?.["smithy-protocol"] === "rpc-v2-cbor") {
+    setFeature(context, "PROTOCOL_RPC_V2_CBOR", "M");
+  }
+
+  if (typeof config.retryStrategy === "function") {
+    const retryStrategy = await config.retryStrategy();
+    if (typeof (retryStrategy as RetryStrategyV2).acquireInitialRetryToken === "function") {
+      if (retryStrategy.constructor?.name?.includes("Adaptive")) {
+        setFeature(context, "RETRY_MODE_ADAPTIVE", "F");
+      } else {
+        setFeature(context, "RETRY_MODE_STANDARD", "E");
+      }
+    } else {
+      setFeature(context, "RETRY_MODE_LEGACY", "D");
+    }
+  }
+
   if (typeof config.accountIdEndpointMode === "function") {
+    const endpointV2 = context.endpointV2;
+    if (String(endpointV2?.url?.hostname).match(ACCOUNT_ID_ENDPOINT_REGEX)) {
+      setFeature(context, "ACCOUNT_ID_ENDPOINT", "O");
+    }
     switch (await config.accountIdEndpointMode?.()) {
       case "disabled":
         setFeature(context, "ACCOUNT_ID_MODE_DISABLED", "Q");
diff --git a/packages/middleware-user-agent/src/middleware-user-agent.integ.spec.ts b/packages/middleware-user-agent/src/middleware-user-agent.integ.spec.ts
index ca98caeec642..2ee442d5bf28 100644
--- a/packages/middleware-user-agent/src/middleware-user-agent.integ.spec.ts
+++ b/packages/middleware-user-agent/src/middleware-user-agent.integ.spec.ts
@@ -27,24 +27,25 @@ describe("middleware-user-agent", () => {
   });
 
   describe("features", () => {
-    it("should detect DDB mapper, and account id mode", async () => {
+    it("should detect DDB mapper, account id, and account id mode", async () => {
       const client = new DynamoDB({
-        credentials: {
-          accessKeyId: "",
-          secretAccessKey: "",
-          accountId: "123",
-        },
-        accountIdEndpointMode: async () => "preferred" as const,
+        accountIdEndpointMode: async () => "required" as const,
       });
 
       const doc = DynamoDBDocument.from(client);
 
       requireRequestsFrom(doc).toMatch({
         headers: {
-          "user-agent": /(.*?) m\/d,P$/,
+          "user-agent": /(.*?) m\/d,E,O,R$/,
         },
       });
 
+      client.config.credentials = async () => ({
+        accessKeyId: "",
+        secretAccessKey: "",
+        accountId: "123456789012",
+      });
+
       await doc.get({
         TableName: "table",
         Key: {