diff --git a/packages/middleware-user-agent/src/encode-metrics.spec.ts b/packages/middleware-user-agent/src/encode-metrics.spec.ts
new file mode 100644
index 0000000000000..0b1ceb7e9de2f
--- /dev/null
+++ b/packages/middleware-user-agent/src/encode-metrics.spec.ts
@@ -0,0 +1,26 @@
+import { encodeMetrics } from "./encode-metrics";
+
+describe(encodeMetrics.name, () => {
+  it("encodes empty metrics", () => {
+    expect(encodeMetrics({})).toEqual("");
+  });
+
+  it("encodes metrics", () => {
+    expect(
+      encodeMetrics({
+        A: "A",
+        z: "z",
+      } as any)
+    ).toEqual("A,z");
+  });
+
+  it("drops values that would exceed 1024 bytes", () => {
+    expect(
+      encodeMetrics({
+        A: "A".repeat(512),
+        B: "B".repeat(511),
+        z: "z",
+      } as any)
+    ).toEqual("A".repeat(512) + "," + "B".repeat(511));
+  });
+});
diff --git a/packages/middleware-user-agent/src/encode-metrics.ts b/packages/middleware-user-agent/src/encode-metrics.ts
new file mode 100644
index 0000000000000..13c2e3973c4ab
--- /dev/null
+++ b/packages/middleware-user-agent/src/encode-metrics.ts
@@ -0,0 +1,25 @@
+import type { AwsSdkFeatures } from "@aws-sdk/types";
+
+const BYTE_LIMIT = 1024;
+
+export function encodeMetrics(metrics: AwsSdkFeatures): string {
+  let buffer = "";
+
+  // currently all possible values are 1 byte,
+  // so string length is used.
+
+  for (const key in metrics) {
+    const val = metrics[key as keyof typeof metrics]!;
+    if (buffer.length + val!.length + 1 <= BYTE_LIMIT) {
+      if (buffer.length) {
+        buffer += "," + val;
+      } else {
+        buffer += val;
+      }
+      continue;
+    }
+    break;
+  }
+
+  return buffer;
+}
diff --git a/packages/types/src/middleware.ts b/packages/types/src/middleware.ts
index 3ae51bd9a83d0..f8134a5ddd7d4 100644
--- a/packages/types/src/middleware.ts
+++ b/packages/types/src/middleware.ts
@@ -1,3 +1,5 @@
+import { HandlerExecutionContext } from "@smithy/types";
+
 export {
   AbsoluteLocation,
   BuildHandler,
@@ -38,3 +40,68 @@ export {
   Step,
   Terminalware,
 } from "@smithy/types";
+
+/**
+ * @internal
+ * Contains reserved keys for AWS SDK internal usage of the
+ * handler execution context object.
+ */
+export interface AwsHandlerExecutionContext extends HandlerExecutionContext {
+  __aws_sdk_context?: {
+    features?: Partial<{
+      RESOURCE_MODEL: "A";
+      WAITER: "B";
+      PAGINATOR: "C";
+      RETRY_MODE_LEGACY: "D";
+      RETRY_MODE_STANDARD: "E";
+      RETRY_MODE_ADAPTIVE: "F";
+      // S3_TRANSFER: "G"; // not applicable.
+      // S3_CRYPTO_V1N: "H"; // not applicable.
+      // S3_CRYPTO_V2: "I"; // not applicable.
+      S3_EXPRESS_BUCKET: "J";
+      S3_ACCESS_GRANTS: "K";
+      GZIP_REQUEST_COMPRESSION: "L";
+      PROTOCOL_RPC_V2_CBOR: "M";
+      ENDPOINT_OVERRIDE: "N";
+      ACCOUNT_ID_ENDPOINT: "O";
+      ACCOUNT_ID_MODE_PREFERRED: "P";
+      ACCOUNT_ID_MODE_DISABLED: "Q";
+      ACCOUNT_ID_MODE_REQUIRED: "R";
+      SIGV4A_SIGNING: "S";
+      RESOLVED_ACCOUNT_ID: "T";
+      FLEXIBLE_CHECKSUMS_REQ_CRC32: "U";
+      FLEXIBLE_CHECKSUMS_REQ_CRC32C: "V";
+      FLEXIBLE_CHECKSUMS_REQ_CRC64: "W";
+      FLEXIBLE_CHECKSUMS_REQ_SHA1: "X";
+      FLEXIBLE_CHECKSUMS_REQ_SHA256: "Y";
+      FLEXIBLE_CHECKSUMS_REQ_WHEN_SUPPORTED: "Z";
+      FLEXIBLE_CHECKSUMS_REQ_WHEN_REQUIRED: "a";
+      FLEXIBLE_CHECKSUMS_RES_WHEN_SUPPORTED: "b";
+      FLEXIBLE_CHECKSUMS_RES_WHEN_REQUIRED: "c";
+      DDB_MAPPER: "d";
+      CREDENTIALS_CODE: "e";
+      // CREDENTIALS_JVM_SYSTEM_PROPERTIES: "f"; // not applicable.
+      CREDENTIALS_ENV_VARS: "g";
+      CREDENTIALS_ENV_VARS_STS_WEB_ID_TOKEN: "h";
+      CREDENTIALS_STS_ASSUME_ROLE: "i";
+      CREDENTIALS_STS_ASSUME_ROLE_SAML: "j";
+      CREDENTIALS_STS_ASSUME_ROLE_WEB_ID: "k";
+      CREDENTIALS_STS_FEDERATION_TOKEN: "l";
+      CREDENTIALS_STS_SESSION_TOKEN: "m";
+      CREDENTIALS_PROFILE: "n";
+      CREDENTIALS_PROFILE_SOURCE_PROFILE: "o";
+      CREDENTIALS_PROFILE_NAMED_PROVIDER: "p";
+      CREDENTIALS_PROFILE_STS_WEB_ID_TOKEN: "q";
+      CREDENTIALS_PROFILE_SSO: "r";
+      CREDENTIALS_SSO: "s";
+      CREDENTIALS_PROFILE_SSO_LEGACY: "t";
+      CREDENTIALS_SSO_LEGACY: "u";
+      CREDENTIALS_PROFILE_PROCESS: "v";
+      CREDENTIALS_PROCESS: "w";
+      CREDENTIALS_BOTO2_CONFIG_FILE: "x";
+      CREDENTIALS_AWS_SDK_STORE: "y";
+      CREDENTIALS_HTTP: "z";
+      CREDENTIALS_IMDS: "0";
+    }>;
+  };
+}