diff --git a/.prettierignore b/.prettierignore
new file mode 100644
index 00000000..46828fd2
--- /dev/null
+++ b/.prettierignore
@@ -0,0 +1,2 @@
+# Ignore code that was originally part of the Lambda runtime
+runtime
\ No newline at end of file
diff --git a/LICENSE-3rdparty.csv b/LICENSE-3rdparty.csv
index 47333f4a..2db9800a 100644
--- a/LICENSE-3rdparty.csv
+++ b/LICENSE-3rdparty.csv
@@ -2,6 +2,7 @@ Component,Origin,License,Copyright
 async-listener,github.com/othiym23/async-listener,BSD-2-Clause,"Copyright (c) 2013-2017, Forrest L Norvell All rights reserved."
 async,github.com/caolan/async,MIT,Copyright (c) 2010-2014 Caolan McMahon
 atomic-batcher,github.com/mafintosh/atomic-batcher,MIT,Copyright (c) 2016 Mathias Buus
+aws-lambda-nodejs-runtime-interface-client,github.com/aws/aws-lambda-nodejs-runtime-interface-client,Apache 2.0,Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 aws-sdk,github.com/aws/aws-sdk-js,Apache-2.0,"Copyright 2012-2017 Amazon.com, Inc. or its affiliates. All Rights Reserved."
 aws-xray-sdk-core,github.com/aws/aws-xray-sdk-node/tree/master/packages/core,Apache-2.0,"Copyright 2012-2016 Amazon.com, Inc. or its affiliates. All Rights Reserved."
 base64-js,github.com/beatgammit/base64-js,MIT,Copyright (c) 2014 Jameson Little
diff --git a/NOTICE b/NOTICE
index aaa50835..4907a0df 100644
--- a/NOTICE
+++ b/NOTICE
@@ -2,3 +2,6 @@ Datadog datadog-lambda-js
 Copyright 2019 Datadog, Inc.
 
 This product includes software developed at Datadog (https://www.datadoghq.com/).
+
+The Initial Developer of the files in the runtime directory is Amazon.com, Inc.
+Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
\ No newline at end of file
diff --git a/src/handler.ts b/src/handler.ts
index 114180fd..62101567 100644
--- a/src/handler.ts
+++ b/src/handler.ts
@@ -1,9 +1,7 @@
 import { datadog, datadogHandlerEnvVar, lambdaTaskRootEnvVar, traceExtractorEnvVar, getEnvValue } from "./index";
 import { TraceExtractor } from "./trace";
 import { logDebug, logError } from "./utils";
-// We reuse the function loading logic already inside the lambda runtime.
-// tslint:disable-next-line:no-var-requires
-const { load } = require("/var/runtime/UserFunction") as any;
+import { load } from "./runtime";
 
 if (process.env.DD_TRACE_DISABLED_PLUGINS === undefined) {
   process.env.DD_TRACE_DISABLED_PLUGINS = "fs";
diff --git a/src/runtime/errors.ts b/src/runtime/errors.ts
new file mode 100644
index 00000000..f9c58b62
--- /dev/null
+++ b/src/runtime/errors.ts
@@ -0,0 +1,137 @@
+/* eslint-disable @typescript-eslint/explicit-module-boundary-types */
+/* eslint-disable @typescript-eslint/no-explicit-any */
+/**
+ * Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Defines custom error types throwable by the runtime.
+ */
+
+"use strict";
+
+import util from "util";
+
+export function isError(obj: any): obj is Error {
+  return (
+    obj &&
+    obj.name &&
+    obj.message &&
+    obj.stack &&
+    typeof obj.name === "string" &&
+    typeof obj.message === "string" &&
+    typeof obj.stack === "string"
+  );
+}
+
+interface RuntimeErrorResponse {
+  errorType: string;
+  errorMessage: string;
+  trace: string[];
+}
+
+/**
+ * Attempt to convert an object into a response object.
+ * This method accounts for failures when serializing the error object.
+ */
+export function toRuntimeResponse(error: unknown): RuntimeErrorResponse {
+  try {
+    if (util.types.isNativeError(error) || isError(error)) {
+      if (!error.stack) {
+        throw new Error("Error stack is missing.");
+      }
+      return {
+        errorType: error.name,
+        errorMessage: error.message,
+        trace: error.stack.split("\n") || [],
+      };
+    } else {
+      return {
+        errorType: typeof error,
+        errorMessage: (error as any).toString(),
+        trace: [],
+      };
+    }
+  } catch (_err) {
+    return {
+      errorType: "handled",
+      errorMessage:
+        "callback called with Error argument, but there was a problem while retrieving one or more of its message, name, and stack",
+      trace: [],
+    };
+  }
+}
+
+/**
+ * Format an error with the expected properties.
+ * For compatability, the error string always starts with a tab.
+ */
+export const toFormatted = (error: unknown): string => {
+  try {
+    return (
+      "\t" + JSON.stringify(error, (_k, v) => _withEnumerableProperties(v))
+    );
+  } catch (err) {
+    return "\t" + JSON.stringify(toRuntimeResponse(error));
+  }
+};
+
+/**
+ * Error name, message, code, and stack are all members of the superclass, which
+ * means they aren't enumerable and don't normally show up in JSON.stringify.
+ * This method ensures those interesting properties are available along with any
+ * user-provided enumerable properties.
+ */
+function _withEnumerableProperties(error: any) {
+  if (error instanceof Error) {
+    const extendedError: ExtendedError = <ExtendedError>(<any>error);
+    const ret: any = Object.assign(
+      {
+        errorType: extendedError.name,
+        errorMessage: extendedError.message,
+        code: extendedError.code,
+      },
+      extendedError
+    );
+    if (typeof extendedError.stack == "string") {
+      ret.stack = extendedError.stack.split("\n");
+    }
+    return ret;
+  } else {
+    return error;
+  }
+}
+
+export class ExtendedError extends Error {
+  code?: number;
+  custom?: string;
+  reason?: string;
+  promise?: Promise<any>;
+
+  constructor(reason?: string) {
+    super(reason); // 'Error' breaks prototype chain here
+    Object.setPrototypeOf(this, new.target.prototype); // restore prototype chain
+  }
+}
+
+export class ImportModuleError extends ExtendedError {}
+export class HandlerNotFound extends ExtendedError {}
+export class MalformedHandlerName extends ExtendedError {}
+export class UserCodeSyntaxError extends ExtendedError {}
+export class UnhandledPromiseRejection extends ExtendedError {
+  constructor(reason?: string, promise?: Promise<any>) {
+    super(reason);
+    this.reason = reason;
+    this.promise = promise;
+  }
+}
+
+const errorClasses = [
+  ImportModuleError,
+  HandlerNotFound,
+  MalformedHandlerName,
+  UserCodeSyntaxError,
+  UnhandledPromiseRejection,
+];
+
+errorClasses.forEach((e) => {
+  e.prototype.name = `Runtime.${e.name}`;
+});
\ No newline at end of file
diff --git a/src/runtime/index.ts b/src/runtime/index.ts
new file mode 100644
index 00000000..10e9f9ab
--- /dev/null
+++ b/src/runtime/index.ts
@@ -0,0 +1 @@
+export { load } from "./user-function";
diff --git a/src/runtime/user-function.ts b/src/runtime/user-function.ts
new file mode 100644
index 00000000..1fd53403
--- /dev/null
+++ b/src/runtime/user-function.ts
@@ -0,0 +1,170 @@
+/**
+ * Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * Modifications copyright 2021 Datadog, Inc.
+ * 
+ * The original file was part of aws-lambda-nodejs-runtime-interface-client
+ * https://github.com/aws/aws-lambda-nodejs-runtime-interface-client/blob/main/src/utils/UserFunction.ts
+ *
+ * This module defines the functions for loading the user's code as specified
+ * in a handler string.
+ */
+
+ "use strict";
+
+ import path from "path";
+ import fs from "fs";
+ import {
+   HandlerNotFound,
+   MalformedHandlerName,
+   ImportModuleError,
+   UserCodeSyntaxError,
+   ExtendedError,
+ } from "./errors";
+ 
+ const FUNCTION_EXPR = /^([^.]*)\.(.*)$/;
+ const RELATIVE_PATH_SUBSTRING = "..";
+ 
+ /**
+  * Break the full handler string into two pieces, the module root and the actual
+  * handler string.
+  * Given './somepath/something/module.nestedobj.handler' this returns
+  * ['./somepath/something', 'module.nestedobj.handler']
+  */
+ function _moduleRootAndHandler(fullHandlerString: string): [string, string] {
+   const handlerString = path.basename(fullHandlerString);
+   const moduleRoot = fullHandlerString.substring(
+     0,
+     fullHandlerString.indexOf(handlerString)
+   );
+   return [moduleRoot, handlerString];
+ }
+ 
+ /**
+  * Split the handler string into two pieces: the module name and the path to
+  * the handler function.
+  */
+ function _splitHandlerString(handler: string): [string, string] {
+   const match = handler.match(FUNCTION_EXPR);
+   if (!match || match.length != 3) {
+     throw new MalformedHandlerName("Bad handler");
+   }
+   return [match[1], match[2]]; // [module, function-path]
+ }
+ 
+ /**
+  * Resolve the user's handler function from the module.
+  */
+ function _resolveHandler(object: any, nestedProperty: string): any {
+   return nestedProperty.split(".").reduce((nested, key) => {
+     return nested && nested[key];
+   }, object);
+ }
+ 
+ /**
+  * Verify that the provided path can be loaded as a file per:
+  * https://nodejs.org/dist/latest-v10.x/docs/api/modules.html#modules_all_together
+  * @param string - the fully resolved file path to the module
+  * @return bool
+  */
+ function _canLoadAsFile(modulePath: string): boolean {
+   return fs.existsSync(modulePath) || fs.existsSync(modulePath + ".js");
+ }
+ 
+ /**
+  * Attempt to load the user's module.
+  * Attempts to directly resolve the module relative to the application root,
+  * then falls back to the more general require().
+  */
+ function _tryRequire(appRoot: string, moduleRoot: string, module: string): any {
+   const lambdaStylePath = path.resolve(appRoot, moduleRoot, module);
+   if (_canLoadAsFile(lambdaStylePath)) {
+     return require(lambdaStylePath);
+   } else {
+     // Why not just require(module)?
+     // Because require() is relative to __dirname, not process.cwd()
+     const nodeStylePath = require.resolve(module, {
+       paths: [appRoot, moduleRoot],
+     });
+     return require(nodeStylePath);
+   }
+ }
+ 
+ /**
+  * Load the user's application or throw a descriptive error.
+  * @throws Runtime errors in two cases
+  *   1 - UserCodeSyntaxError if there's a syntax error while loading the module
+  *   2 - ImportModuleError if the module cannot be found
+  */
+ function _loadUserApp(
+   appRoot: string,
+   moduleRoot: string,
+   module: string
+ ): any {
+   try {
+     return _tryRequire(appRoot, moduleRoot, module);
+   } catch (e) {
+     if (e instanceof SyntaxError) {
+       throw new UserCodeSyntaxError(<any>e);
+     // @ts-ignore
+     } else if (e.code !== undefined && e.code === "MODULE_NOT_FOUND") {
+       // @ts-ignore
+       throw new ImportModuleError(e);
+     } else {
+       throw e;
+     }
+   }
+ }
+ 
+ function _throwIfInvalidHandler(fullHandlerString: string): void {
+   if (fullHandlerString.includes(RELATIVE_PATH_SUBSTRING)) {
+     throw new MalformedHandlerName(
+       `'${fullHandlerString}' is not a valid handler name. Use absolute paths when specifying root directories in handler names.`
+     );
+   }
+ }
+ 
+ /**
+  * Load the user's function with the approot and the handler string.
+  * @param appRoot {string}
+  *   The path to the application root.
+  * @param handlerString {string}
+  *   The user-provided handler function in the form 'module.function'.
+  * @return userFuction {function}
+  *   The user's handler function. This function will be passed the event body,
+  *   the context object, and the callback function.
+  * @throws In five cases:-
+  *   1 - if the handler string is incorrectly formatted an error is thrown
+  *   2 - if the module referenced by the handler cannot be loaded
+  *   3 - if the function in the handler does not exist in the module
+  *   4 - if a property with the same name, but isn't a function, exists on the
+  *       module
+  *   5 - the handler includes illegal character sequences (like relative paths
+  *       for traversing up the filesystem '..')
+  *   Errors for scenarios known by the runtime, will be wrapped by Runtime.* errors.
+  */
+ export const load = function (
+   appRoot: string,
+   fullHandlerString: string
+ ) {
+   _throwIfInvalidHandler(fullHandlerString);
+ 
+   const [moduleRoot, moduleAndHandler] = _moduleRootAndHandler(
+     fullHandlerString
+   );
+   const [module, handlerPath] = _splitHandlerString(moduleAndHandler);
+ 
+   const userApp = _loadUserApp(appRoot, moduleRoot, module);
+   const handlerFunc = _resolveHandler(userApp, handlerPath);
+ 
+   if (!handlerFunc) {
+     throw new HandlerNotFound(
+       `${fullHandlerString} is undefined or not exported`
+     );
+   }
+ 
+   if (typeof handlerFunc !== "function") {
+     throw new HandlerNotFound(`${fullHandlerString} is not a function`);
+   }
+ 
+   return handlerFunc;
+ };
\ No newline at end of file
diff --git a/tslint.json b/tslint.json
index 657150ac..69877b01 100644
--- a/tslint.json
+++ b/tslint.json
@@ -1,5 +1,10 @@
 {
   "extends": "tslint:recommended",
+  "linterOptions": {
+    "exclude": [
+      "**/runtime/**"
+    ]
+  },
   "rules": {
     "interface-name": false,
     "variable-name": {