diff --git a/.eslintrc.js b/.eslintrc.js
index 9b75c36c95abdb..3161a25b708708 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -305,6 +305,8 @@ module.exports = {
                   '!src/core/server/mocks{,.ts}',
                   '!src/core/server/types{,.ts}',
                   '!src/core/server/test_utils{,.ts}',
+                  '!src/core/server/utils', // ts alias
+                  '!src/core/server/utils/**/*',
                   // for absolute imports until fixed in
                   // https://github.com/elastic/kibana/issues/36096
                   '!src/core/server/*.test.mocks{,.ts}',
diff --git a/src/cli_keystore/add.js b/src/cli_keystore/add.js
index 44737e387c2d29..462259ec942dd5 100644
--- a/src/cli_keystore/add.js
+++ b/src/cli_keystore/add.js
@@ -19,7 +19,7 @@
 
 import { Logger } from '../cli_plugin/lib/logger';
 import { confirm, question } from '../legacy/server/utils';
-import { createPromiseFromStreams, createConcatStream } from '../legacy/utils';
+import { createPromiseFromStreams, createConcatStream } from '../core/server/utils';
 
 /**
  * @param {Keystore} keystore
diff --git a/src/core/server/saved_objects/export/get_sorted_objects_for_export.test.ts b/src/core/server/saved_objects/export/get_sorted_objects_for_export.test.ts
index 85b3a281aef7f6..c084125f43127a 100644
--- a/src/core/server/saved_objects/export/get_sorted_objects_for_export.test.ts
+++ b/src/core/server/saved_objects/export/get_sorted_objects_for_export.test.ts
@@ -20,7 +20,7 @@
 import { exportSavedObjectsToStream } from './get_sorted_objects_for_export';
 import { savedObjectsClientMock } from '../service/saved_objects_client.mock';
 import { Readable } from 'stream';
-import { createPromiseFromStreams, createConcatStream } from '../../../../legacy/utils/streams';
+import { createPromiseFromStreams, createConcatStream } from '../../utils/streams';
 
 async function readStreamToCompletion(stream: Readable) {
   return createPromiseFromStreams([stream, createConcatStream([])]);
diff --git a/src/core/server/saved_objects/export/get_sorted_objects_for_export.ts b/src/core/server/saved_objects/export/get_sorted_objects_for_export.ts
index 94f727e238ecf3..214b51db7dd6bb 100644
--- a/src/core/server/saved_objects/export/get_sorted_objects_for_export.ts
+++ b/src/core/server/saved_objects/export/get_sorted_objects_for_export.ts
@@ -18,7 +18,7 @@
  */
 
 import Boom from 'boom';
-import { createListStream } from '../../../../legacy/utils/streams';
+import { createListStream } from '../../utils/streams';
 import { SavedObjectsClientContract, SavedObject } from '../types';
 import { fetchNestedDependencies } from './inject_nested_depdendencies';
 import { sortObjects } from './sort_objects';
diff --git a/src/core/server/saved_objects/import/collect_saved_objects.ts b/src/core/server/saved_objects/import/collect_saved_objects.ts
index f55e6bf0d2af4c..8e84f864cf4490 100644
--- a/src/core/server/saved_objects/import/collect_saved_objects.ts
+++ b/src/core/server/saved_objects/import/collect_saved_objects.ts
@@ -23,7 +23,7 @@ import {
   createFilterStream,
   createMapStream,
   createPromiseFromStreams,
-} from '../../../../legacy/utils/streams';
+} from '../../utils/streams';
 import { SavedObject } from '../types';
 import { createLimitStream } from './create_limit_stream';
 import { SavedObjectsImportError } from './types';
diff --git a/src/core/server/saved_objects/import/create_limit_stream.test.ts b/src/core/server/saved_objects/import/create_limit_stream.test.ts
index 736cfadcb62228..a7e689710a564f 100644
--- a/src/core/server/saved_objects/import/create_limit_stream.test.ts
+++ b/src/core/server/saved_objects/import/create_limit_stream.test.ts
@@ -21,7 +21,7 @@ import {
   createConcatStream,
   createListStream,
   createPromiseFromStreams,
-} from '../../../../legacy/utils/streams';
+} from '../../utils/streams';
 import { createLimitStream } from './create_limit_stream';
 
 describe('createLimitStream()', () => {
diff --git a/src/core/server/saved_objects/routes/export.ts b/src/core/server/saved_objects/routes/export.ts
index 9445c144ecda45..35a65d8d9651fe 100644
--- a/src/core/server/saved_objects/routes/export.ts
+++ b/src/core/server/saved_objects/routes/export.ts
@@ -19,11 +19,7 @@
 
 import { schema } from '@kbn/config-schema';
 import stringify from 'json-stable-stringify';
-import {
-  createPromiseFromStreams,
-  createMapStream,
-  createConcatStream,
-} from '../../../../legacy/utils/streams';
+import { createPromiseFromStreams, createMapStream, createConcatStream } from '../../utils/streams';
 import { IRouter } from '../../http';
 import { SavedObjectConfig } from '../saved_objects_config';
 import { exportSavedObjectsToStream } from '../export';
diff --git a/src/core/server/saved_objects/routes/integration_tests/export.test.ts b/src/core/server/saved_objects/routes/integration_tests/export.test.ts
index d47f7c6050d8f7..a3891712fd22b7 100644
--- a/src/core/server/saved_objects/routes/integration_tests/export.test.ts
+++ b/src/core/server/saved_objects/routes/integration_tests/export.test.ts
@@ -22,7 +22,7 @@ jest.mock('../../export', () => ({
 }));
 
 import * as exportMock from '../../export';
-import { createListStream } from '../../../../../legacy/utils/streams';
+import { createListStream } from '../../../utils/streams';
 import supertest from 'supertest';
 import { UnwrapPromise } from '@kbn/utility-types';
 import { SavedObjectConfig } from '../../saved_objects_config';
diff --git a/src/core/server/saved_objects/routes/utils.test.ts b/src/core/server/saved_objects/routes/utils.test.ts
index 24719724785af1..fd3bdad8606ed5 100644
--- a/src/core/server/saved_objects/routes/utils.test.ts
+++ b/src/core/server/saved_objects/routes/utils.test.ts
@@ -19,7 +19,7 @@
 
 import { createSavedObjectsStreamFromNdJson, validateTypes, validateObjects } from './utils';
 import { Readable } from 'stream';
-import { createPromiseFromStreams, createConcatStream } from '../../../../legacy/utils/streams';
+import { createPromiseFromStreams, createConcatStream } from '../../utils/streams';
 
 async function readStreamToCompletion(stream: Readable) {
   return createPromiseFromStreams([stream, createConcatStream([])]);
diff --git a/src/core/server/saved_objects/routes/utils.ts b/src/core/server/saved_objects/routes/utils.ts
index 3963833a9c718b..f16a6e471257d0 100644
--- a/src/core/server/saved_objects/routes/utils.ts
+++ b/src/core/server/saved_objects/routes/utils.ts
@@ -19,11 +19,7 @@
 
 import { Readable } from 'stream';
 import { SavedObject, SavedObjectsExportResultDetails } from 'src/core/server';
-import {
-  createSplitStream,
-  createMapStream,
-  createFilterStream,
-} from '../../../../legacy/utils/streams';
+import { createSplitStream, createMapStream, createFilterStream } from '../../utils/streams';
 
 export function createSavedObjectsStreamFromNdJson(ndJsonStream: Readable) {
   return ndJsonStream
diff --git a/src/core/server/utils/index.ts b/src/core/server/utils/index.ts
index b01a4c4e048998..d9c4217c4117f7 100644
--- a/src/core/server/utils/index.ts
+++ b/src/core/server/utils/index.ts
@@ -20,3 +20,4 @@
 export * from './crypto';
 export * from './from_root';
 export * from './package_json';
+export * from './streams';
diff --git a/src/legacy/utils/streams/concat_stream.test.js b/src/core/server/utils/streams/concat_stream.test.ts
similarity index 98%
rename from src/legacy/utils/streams/concat_stream.test.js
rename to src/core/server/utils/streams/concat_stream.test.ts
index 1498334013d1ab..e964ab2a7a97ed 100644
--- a/src/legacy/utils/streams/concat_stream.test.js
+++ b/src/core/server/utils/streams/concat_stream.test.ts
@@ -17,7 +17,7 @@
  * under the License.
  */
 
-import { createListStream, createPromiseFromStreams, createConcatStream } from './';
+import { createListStream, createPromiseFromStreams, createConcatStream } from './index';
 
 describe('concatStream', () => {
   test('accepts an initial value', async () => {
diff --git a/src/legacy/utils/streams/concat_stream.js b/src/core/server/utils/streams/concat_stream.ts
similarity index 96%
rename from src/legacy/utils/streams/concat_stream.js
rename to src/core/server/utils/streams/concat_stream.ts
index e3f8f7261d2b7a..03450cb51b8322 100644
--- a/src/legacy/utils/streams/concat_stream.js
+++ b/src/core/server/utils/streams/concat_stream.ts
@@ -41,6 +41,6 @@ import { createReduceStream } from './reduce_stream';
  *                       items will concat with
  *  @return {Transform}
  */
-export function createConcatStream(initial) {
+export function createConcatStream<T>(initial?: T) {
   return createReduceStream((acc, chunk) => acc.concat(chunk), initial);
 }
diff --git a/src/legacy/utils/streams/concat_stream_providers.test.js b/src/core/server/utils/streams/concat_stream_providers.test.ts
similarity index 100%
rename from src/legacy/utils/streams/concat_stream_providers.test.js
rename to src/core/server/utils/streams/concat_stream_providers.test.ts
diff --git a/src/legacy/utils/streams/concat_stream_providers.js b/src/core/server/utils/streams/concat_stream_providers.ts
similarity index 91%
rename from src/legacy/utils/streams/concat_stream_providers.js
rename to src/core/server/utils/streams/concat_stream_providers.ts
index 11dfb84284df38..bb836e3d737872 100644
--- a/src/legacy/utils/streams/concat_stream_providers.js
+++ b/src/core/server/utils/streams/concat_stream_providers.ts
@@ -17,7 +17,7 @@
  * under the License.
  */
 
-import { PassThrough } from 'stream';
+import { Readable, PassThrough, TransformOptions } from 'stream';
 
 /**
  *  Write the data and errors from a list of stream providers
@@ -29,7 +29,10 @@ import { PassThrough } from 'stream';
  *  @param {PassThroughOptions} options options passed to the PassThrough constructor
  *  @return {WritableStream} combined stream
  */
-export function concatStreamProviders(sourceProviders, options = {}) {
+export function concatStreamProviders(
+  sourceProviders: Array<() => Readable>,
+  options?: TransformOptions
+) {
   const destination = new PassThrough(options);
   const queue = sourceProviders.slice();
 
diff --git a/src/legacy/utils/streams/filter_stream.test.ts b/src/core/server/utils/streams/filter_stream.test.ts
similarity index 99%
rename from src/legacy/utils/streams/filter_stream.test.ts
rename to src/core/server/utils/streams/filter_stream.test.ts
index 28b7f2588628e7..41073e54b0a842 100644
--- a/src/legacy/utils/streams/filter_stream.test.ts
+++ b/src/core/server/utils/streams/filter_stream.test.ts
@@ -22,7 +22,7 @@ import {
   createFilterStream,
   createListStream,
   createPromiseFromStreams,
-} from './';
+} from './index';
 
 describe('createFilterStream()', () => {
   test('calls the function with each item in the source stream', async () => {
diff --git a/src/legacy/utils/streams/filter_stream.ts b/src/core/server/utils/streams/filter_stream.ts
similarity index 100%
rename from src/legacy/utils/streams/filter_stream.ts
rename to src/core/server/utils/streams/filter_stream.ts
diff --git a/src/legacy/utils/streams/index.js b/src/core/server/utils/streams/index.ts
similarity index 100%
rename from src/legacy/utils/streams/index.js
rename to src/core/server/utils/streams/index.ts
diff --git a/src/legacy/utils/streams/intersperse_stream.test.js b/src/core/server/utils/streams/intersperse_stream.test.ts
similarity index 99%
rename from src/legacy/utils/streams/intersperse_stream.test.js
rename to src/core/server/utils/streams/intersperse_stream.test.ts
index e11b36d77106a6..9aa15035d2a1cc 100644
--- a/src/legacy/utils/streams/intersperse_stream.test.js
+++ b/src/core/server/utils/streams/intersperse_stream.test.ts
@@ -22,7 +22,7 @@ import {
   createListStream,
   createIntersperseStream,
   createConcatStream,
-} from './';
+} from './index';
 
 describe('intersperseStream', () => {
   test('places the intersperse value between each provided value', async () => {
diff --git a/src/legacy/utils/streams/intersperse_stream.js b/src/core/server/utils/streams/intersperse_stream.ts
similarity index 95%
rename from src/legacy/utils/streams/intersperse_stream.js
rename to src/core/server/utils/streams/intersperse_stream.ts
index 5f9f0b03cd7eb8..272507221caff3 100644
--- a/src/legacy/utils/streams/intersperse_stream.js
+++ b/src/core/server/utils/streams/intersperse_stream.ts
@@ -40,7 +40,7 @@ import { Transform } from 'stream';
  *  @param  {String|Buffer} intersperseChunk
  *  @return {Transform}
  */
-export function createIntersperseStream(intersperseChunk) {
+export function createIntersperseStream(intersperseChunk: string | Buffer) {
   let first = true;
 
   return new Transform({
@@ -55,7 +55,7 @@ export function createIntersperseStream(intersperseChunk) {
         }
 
         this.push(chunk);
-        callback(null);
+        callback();
       } catch (err) {
         callback(err);
       }
diff --git a/src/legacy/utils/streams/list_stream.test.js b/src/core/server/utils/streams/list_stream.test.ts
similarity index 97%
rename from src/legacy/utils/streams/list_stream.test.js
rename to src/core/server/utils/streams/list_stream.test.ts
index 12e20696b05101..2a20c929db6b97 100644
--- a/src/legacy/utils/streams/list_stream.test.js
+++ b/src/core/server/utils/streams/list_stream.test.ts
@@ -17,7 +17,7 @@
  * under the License.
  */
 
-import { createListStream } from './';
+import { createListStream } from './index';
 
 describe('listStream', () => {
   test('provides the values in the initial list', async () => {
diff --git a/src/legacy/utils/streams/list_stream.js b/src/core/server/utils/streams/list_stream.ts
similarity index 90%
rename from src/legacy/utils/streams/list_stream.js
rename to src/core/server/utils/streams/list_stream.ts
index a614620b054b7f..e62f6d3fa930bb 100644
--- a/src/legacy/utils/streams/list_stream.js
+++ b/src/core/server/utils/streams/list_stream.ts
@@ -26,8 +26,8 @@ import { Readable } from 'stream';
  *  @param  {Array<any>} items - the list of items to provide
  *  @return {Readable}
  */
-export function createListStream(items = []) {
-  const queue = [].concat(items);
+export function createListStream<T = any>(items: T | T[] = []) {
+  const queue = Array.isArray(items) ? [...items] : [items];
 
   return new Readable({
     objectMode: true,
diff --git a/src/legacy/utils/streams/map_stream.test.js b/src/core/server/utils/streams/map_stream.test.ts
similarity index 95%
rename from src/legacy/utils/streams/map_stream.test.js
rename to src/core/server/utils/streams/map_stream.test.ts
index d86da178f0c1b3..bf0cab39c21f40 100644
--- a/src/legacy/utils/streams/map_stream.test.js
+++ b/src/core/server/utils/streams/map_stream.test.ts
@@ -39,7 +39,7 @@ describe('createMapStream()', () => {
   test('send the return value from the mapper on the output stream', async () => {
     const result = await createPromiseFromStreams([
       createListStream([1, 2, 3]),
-      createMapStream((n) => n * 100),
+      createMapStream((n: number) => n * 100),
       createConcatStream([]),
     ]);
 
@@ -49,7 +49,7 @@ describe('createMapStream()', () => {
   test('supports async mappers', async () => {
     const result = await createPromiseFromStreams([
       createListStream([1, 2, 3]),
-      createMapStream(async (n, i) => {
+      createMapStream(async (n: number, i: number) => {
         await delay(n);
         return n * i;
       }),
diff --git a/src/legacy/utils/streams/map_stream.js b/src/core/server/utils/streams/map_stream.ts
similarity index 93%
rename from src/legacy/utils/streams/map_stream.js
rename to src/core/server/utils/streams/map_stream.ts
index 4e906471330f1d..aad53cc5266260 100644
--- a/src/legacy/utils/streams/map_stream.js
+++ b/src/core/server/utils/streams/map_stream.ts
@@ -19,7 +19,7 @@
 
 import { Transform } from 'stream';
 
-export function createMapStream(fn) {
+export function createMapStream<T>(fn: (value: T, i: number) => void) {
   let i = 0;
 
   return new Transform({
diff --git a/src/legacy/utils/streams/promise_from_streams.test.js b/src/core/server/utils/streams/promise_from_streams.test.ts
similarity index 96%
rename from src/legacy/utils/streams/promise_from_streams.test.js
rename to src/core/server/utils/streams/promise_from_streams.test.ts
index e4d9835106f12c..1f2596c16a6fa3 100644
--- a/src/legacy/utils/streams/promise_from_streams.test.js
+++ b/src/core/server/utils/streams/promise_from_streams.test.ts
@@ -19,7 +19,7 @@
 
 import { Readable, Writable, Duplex, Transform } from 'stream';
 
-import { createListStream, createPromiseFromStreams, createReduceStream } from './';
+import { createListStream, createPromiseFromStreams, createReduceStream } from './index';
 
 describe('promiseFromStreams', () => {
   test('pipes together an array of streams', async () => {
@@ -76,14 +76,13 @@ describe('promiseFromStreams', () => {
     test('waits for writing and resolves to final value', async () => {
       let written = '';
 
-      const duplexReadQueue = [];
+      const duplexReadQueue: Array<Promise<unknown>> = [];
       const duplexItemsToPush = ['foo', 'bar', null];
       const result = await createPromiseFromStreams([
         createListStream(['a', 'b', 'c']),
         new Duplex({
           async read() {
-            const result = await duplexReadQueue.shift();
-            this.push(result);
+            this.push(await duplexReadQueue.shift());
           },
 
           write(chunk, enc, cb) {
diff --git a/src/legacy/utils/streams/promise_from_streams.js b/src/core/server/utils/streams/promise_from_streams.ts
similarity index 80%
rename from src/legacy/utils/streams/promise_from_streams.js
rename to src/core/server/utils/streams/promise_from_streams.ts
index 05f6a08aa1a09b..f5fc4af62bc838 100644
--- a/src/legacy/utils/streams/promise_from_streams.js
+++ b/src/core/server/utils/streams/promise_from_streams.ts
@@ -34,16 +34,20 @@
  *  @return {Promise<any>}
  */
 
-import { pipeline, Writable } from 'stream';
+import { pipeline, Writable, Readable } from 'stream';
 
-export async function createPromiseFromStreams(streams) {
-  let finalChunk;
+function isReadable(stream: Readable | Writable): stream is Readable {
+  return 'read' in stream && typeof stream.read === 'function';
+}
+
+export async function createPromiseFromStreams<T>(streams: [Readable, ...Writable[]]): Promise<T> {
+  let finalChunk: any;
   const last = streams[streams.length - 1];
-  if (typeof last.read !== 'function' && streams.length === 1) {
+  if (!isReadable(last) && streams.length === 1) {
     // For a nicer error than what stream.pipeline throws
     throw new Error('A minimum of 2 streams is required when a non-readable stream is given');
   }
-  if (typeof last.read === 'function') {
+  if (isReadable(last)) {
     // We are pushing a writable stream to capture the last chunk
     streams.push(
       new Writable({
@@ -57,7 +61,9 @@ export async function createPromiseFromStreams(streams) {
       })
     );
   }
+
   return new Promise((resolve, reject) => {
+    // @ts-expect-error 'pipeline' doesn't support variable length of arguments
     pipeline(...streams, (err) => {
       if (err) return reject(err);
       resolve(finalChunk);
diff --git a/src/legacy/utils/streams/reduce_stream.test.js b/src/core/server/utils/streams/reduce_stream.test.ts
similarity index 92%
rename from src/legacy/utils/streams/reduce_stream.test.js
rename to src/core/server/utils/streams/reduce_stream.test.ts
index 2c073f67f82a89..e4a7dc1cef4912 100644
--- a/src/legacy/utils/streams/reduce_stream.test.js
+++ b/src/core/server/utils/streams/reduce_stream.test.ts
@@ -16,10 +16,10 @@
  * specific language governing permissions and limitations
  * under the License.
  */
+import { Transform } from 'stream';
+import { createReduceStream, createPromiseFromStreams, createListStream } from './index';
 
-import { createReduceStream, createPromiseFromStreams, createListStream } from './';
-
-const promiseFromEvent = (name, emitter) =>
+const promiseFromEvent = (name: string, emitter: Transform) =>
   new Promise((resolve) => emitter.on(name, () => resolve(name)));
 
 describe('reduceStream', () => {
@@ -47,7 +47,10 @@ describe('reduceStream', () => {
   });
 
   test('emits an error if an iteration fails', async () => {
-    const reduce = createReduceStream((acc, i) => expect(i).toBe(1), 0);
+    const reduce = createReduceStream((acc, i) => {
+      expect(i).toBe(1);
+      return acc;
+    }, 0);
     const errorEvent = promiseFromEvent('error', reduce);
 
     reduce.write(1);
diff --git a/src/legacy/utils/streams/reduce_stream.js b/src/core/server/utils/streams/reduce_stream.ts
similarity index 95%
rename from src/legacy/utils/streams/reduce_stream.js
rename to src/core/server/utils/streams/reduce_stream.ts
index d66b0124d1dab9..9129df096ad13d 100644
--- a/src/legacy/utils/streams/reduce_stream.js
+++ b/src/core/server/utils/streams/reduce_stream.ts
@@ -32,7 +32,10 @@ import { Transform } from 'stream';
  *                        initial value.
  *  @return {Transform}
  */
-export function createReduceStream(reducer, initial) {
+export function createReduceStream<T>(
+  reducer: (value: any, chunk: T, enc: string) => T,
+  initial?: T
+) {
   let i = -1;
   let value = initial;
 
diff --git a/src/legacy/utils/streams/replace_stream.test.js b/src/core/server/utils/streams/replace_stream.test.ts
similarity index 91%
rename from src/legacy/utils/streams/replace_stream.test.js
rename to src/core/server/utils/streams/replace_stream.test.ts
index 01b89f93e5af06..c9da42395fb85c 100644
--- a/src/legacy/utils/streams/replace_stream.test.js
+++ b/src/core/server/utils/streams/replace_stream.test.ts
@@ -16,6 +16,7 @@
  * specific language governing permissions and limitations
  * under the License.
  */
+import { Writable, Readable } from 'stream';
 
 import {
   createReplaceStream,
@@ -23,19 +24,19 @@ import {
   createPromiseFromStreams,
   createListStream,
   createMapStream,
-} from './';
+} from './index';
 
-async function concatToString(streams) {
+async function concatToString(streams: [Readable, ...Writable[]]) {
   return await createPromiseFromStreams([
     ...streams,
-    createMapStream((buff) => buff.toString('utf8')),
+    createMapStream((buff: Buffer) => buff.toString('utf8')),
     createConcatStream(''),
   ]);
 }
 
 describe('replaceStream', () => {
   test('produces buffers when it receives buffers', async () => {
-    const chunks = await createPromiseFromStreams([
+    const chunks = await createPromiseFromStreams<Buffer[]>([
       createListStream([Buffer.from('foo'), Buffer.from('bar')]),
       createReplaceStream('o', '0'),
       createConcatStream([]),
@@ -47,7 +48,7 @@ describe('replaceStream', () => {
   });
 
   test('produces buffers when it receives strings', async () => {
-    const chunks = await createPromiseFromStreams([
+    const chunks = await createPromiseFromStreams<string[]>([
       createListStream(['foo', 'bar']),
       createReplaceStream('o', '0'),
       createConcatStream([]),
@@ -59,6 +60,7 @@ describe('replaceStream', () => {
   });
 
   test('expects toReplace to be a string', () => {
+    // @ts-expect-error
     expect(() => createReplaceStream(Buffer.from('foo'))).toThrowError(/be a string/);
   });
 
diff --git a/src/legacy/utils/streams/replace_stream.js b/src/core/server/utils/streams/replace_stream.ts
similarity index 96%
rename from src/legacy/utils/streams/replace_stream.js
rename to src/core/server/utils/streams/replace_stream.ts
index 7309bd241fa521..05391bb3341c24 100644
--- a/src/legacy/utils/streams/replace_stream.js
+++ b/src/core/server/utils/streams/replace_stream.ts
@@ -19,7 +19,7 @@
 
 import { Transform } from 'stream';
 
-export function createReplaceStream(toReplace, replacement) {
+export function createReplaceStream(toReplace: string, replacement: string | Buffer) {
   if (typeof toReplace !== 'string') {
     throw new TypeError('toReplace must be a string');
   }
@@ -78,6 +78,7 @@ export function createReplaceStream(toReplace, replacement) {
         this.push(buffer);
       }
 
+      // @ts-expect-error
       buffer = null;
       callback();
     },
diff --git a/src/legacy/utils/streams/split_stream.test.js b/src/core/server/utils/streams/split_stream.test.ts
similarity index 93%
rename from src/legacy/utils/streams/split_stream.test.js
rename to src/core/server/utils/streams/split_stream.test.ts
index e0736d220ba5ca..f131bd0661e54f 100644
--- a/src/legacy/utils/streams/split_stream.test.js
+++ b/src/core/server/utils/streams/split_stream.test.ts
@@ -16,16 +16,16 @@
  * specific language governing permissions and limitations
  * under the License.
  */
+import { Transform } from 'stream';
+import { createSplitStream, createConcatStream, createPromiseFromStreams } from './index';
 
-import { createSplitStream, createConcatStream, createPromiseFromStreams } from './';
-
-async function split(stream, input) {
+async function split(stream: Transform, input: Array<Buffer | string>) {
   const concat = createConcatStream();
   concat.write([]);
   stream.pipe(concat);
   const output = createPromiseFromStreams([concat]);
 
-  input.forEach((i) => {
+  input.forEach((i: any) => {
     stream.write(i);
   });
   stream.end();
diff --git a/src/legacy/utils/streams/split_stream.js b/src/core/server/utils/streams/split_stream.ts
similarity index 95%
rename from src/legacy/utils/streams/split_stream.js
rename to src/core/server/utils/streams/split_stream.ts
index f55cbc7bd290d7..ae820f60abbf67 100644
--- a/src/legacy/utils/streams/split_stream.js
+++ b/src/core/server/utils/streams/split_stream.ts
@@ -38,7 +38,7 @@ import { Transform } from 'stream';
  *  @param  {String} splitChunk
  *  @return {Transform}
  */
-export function createSplitStream(splitChunk) {
+export function createSplitStream(splitChunk: string | Uint8Array) {
   let unsplitBuffer = Buffer.alloc(0);
 
   return new Transform({
@@ -55,7 +55,7 @@ export function createSplitStream(splitChunk) {
         }
 
         unsplitBuffer = toSplit;
-        callback(null);
+        callback();
       } catch (err) {
         callback(err);
       }
@@ -65,7 +65,7 @@ export function createSplitStream(splitChunk) {
       try {
         this.push(unsplitBuffer.toString('utf8'));
 
-        callback(null);
+        callback();
       } catch (err) {
         callback(err);
       }
diff --git a/src/dev/build/lib/watch_stdio_for_line.ts b/src/dev/build/lib/watch_stdio_for_line.ts
index 2322d017abc61b..3d7929ccfc33ac 100644
--- a/src/dev/build/lib/watch_stdio_for_line.ts
+++ b/src/dev/build/lib/watch_stdio_for_line.ts
@@ -24,7 +24,7 @@ import {
   createPromiseFromStreams,
   createSplitStream,
   createMapStream,
-} from '../../../legacy/utils/streams';
+} from '../../../core/server/utils';
 
 // creates a stream that skips empty lines unless they are followed by
 // another line, preventing the empty lines produced by splitStream
diff --git a/src/legacy/server/i18n/index.ts b/src/legacy/server/i18n/index.ts
index 09f7022436049d..e895f83fe69017 100644
--- a/src/legacy/server/i18n/index.ts
+++ b/src/legacy/server/i18n/index.ts
@@ -20,7 +20,6 @@
 import { i18n, i18nLoader } from '@kbn/i18n';
 import { basename } from 'path';
 import { Server } from 'hapi';
-// eslint-disable-next-line @kbn/eslint/no-restricted-paths
 import { fromRoot } from '../../../core/server/utils';
 import { getTranslationPaths } from './get_translations_path';
 import { I18N_RC } from './constants';
diff --git a/src/legacy/server/logging/log_format_json.test.js b/src/legacy/server/logging/log_format_json.test.js
index 31e622ecae6113..f4fb939750566b 100644
--- a/src/legacy/server/logging/log_format_json.test.js
+++ b/src/legacy/server/logging/log_format_json.test.js
@@ -21,7 +21,7 @@ import moment from 'moment';
 
 // eslint-disable-next-line @kbn/eslint/no-restricted-paths
 import { attachMetaData } from '../../../../src/core/server/legacy/logging/legacy_logging_server';
-import { createListStream, createPromiseFromStreams } from '../../utils';
+import { createListStream, createPromiseFromStreams } from '../../../core/server/utils';
 
 import KbnLoggerJsonFormat from './log_format_json';
 
diff --git a/src/legacy/server/logging/log_format_string.test.js b/src/legacy/server/logging/log_format_string.test.js
index 067ad70380961e..842325865cce22 100644
--- a/src/legacy/server/logging/log_format_string.test.js
+++ b/src/legacy/server/logging/log_format_string.test.js
@@ -21,7 +21,7 @@ import moment from 'moment';
 // eslint-disable-next-line @kbn/eslint/no-restricted-paths
 import { attachMetaData } from '../../../../src/core/server/legacy/logging/legacy_logging_server';
 
-import { createListStream, createPromiseFromStreams } from '../../utils';
+import { createListStream, createPromiseFromStreams } from '../../../core/server/utils';
 
 import KbnLoggerStringFormat from './log_format_string';
 
diff --git a/src/legacy/utils/artifact_type.ts b/src/legacy/utils/artifact_type.ts
index 69f728e9e2220b..ef471ef8e050d6 100644
--- a/src/legacy/utils/artifact_type.ts
+++ b/src/legacy/utils/artifact_type.ts
@@ -17,7 +17,6 @@
  * under the License.
  */
 
-// eslint-disable-next-line @kbn/eslint/no-restricted-paths
 import { pkg } from '../../core/server/utils';
 export const IS_KIBANA_DISTRIBUTABLE = pkg.build && pkg.build.distributable === true;
 export const IS_KIBANA_RELEASE = pkg.build && pkg.build.release === true;
diff --git a/src/legacy/utils/index.d.ts b/src/legacy/utils/index.d.ts
index c294c79542bbe9..a57caad1d34bf8 100644
--- a/src/legacy/utils/index.d.ts
+++ b/src/legacy/utils/index.d.ts
@@ -18,16 +18,3 @@
  */
 
 export function unset(object: object, rawPath: string): void;
-
-export {
-  concatStreamProviders,
-  createConcatStream,
-  createFilterStream,
-  createIntersperseStream,
-  createListStream,
-  createMapStream,
-  createPromiseFromStreams,
-  createReduceStream,
-  createReplaceStream,
-  createSplitStream,
-} from './streams';
diff --git a/src/legacy/utils/index.js b/src/legacy/utils/index.js
index 4274fb2e4901a0..529b1ddfd8a4d9 100644
--- a/src/legacy/utils/index.js
+++ b/src/legacy/utils/index.js
@@ -23,15 +23,3 @@ export { deepCloneWithBuffers } from './deep_clone_with_buffers';
 export { unset } from './unset';
 export { IS_KIBANA_DISTRIBUTABLE } from './artifact_type';
 export { IS_KIBANA_RELEASE } from './artifact_type';
-
-export {
-  concatStreamProviders,
-  createConcatStream,
-  createIntersperseStream,
-  createListStream,
-  createPromiseFromStreams,
-  createReduceStream,
-  createSplitStream,
-  createMapStream,
-  createReplaceStream,
-} from './streams';
diff --git a/src/legacy/utils/streams/index.d.ts b/src/legacy/utils/streams/index.d.ts
deleted file mode 100644
index 470b5d9fa35051..00000000000000
--- a/src/legacy/utils/streams/index.d.ts
+++ /dev/null
@@ -1,36 +0,0 @@
-/*
- * Licensed to Elasticsearch B.V. under one or more contributor
- * license agreements. See the NOTICE file distributed with
- * this work for additional information regarding copyright
- * ownership. Elasticsearch B.V. licenses this file to you under
- * the Apache License, Version 2.0 (the "License"); you may
- * not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *    http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing,
- * software distributed under the License is distributed on an
- * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
- * KIND, either express or implied.  See the License for the
- * specific language governing permissions and limitations
- * under the License.
- */
-
-import { Readable, Writable, Transform, TransformOptions } from 'stream';
-
-export function concatStreamProviders(
-  sourceProviders: Array<() => Readable>,
-  options: TransformOptions
-): Transform;
-export function createIntersperseStream(intersperseChunk: string | Buffer): Transform;
-export function createSplitStream<T>(splitChunk: T): Transform;
-export function createListStream(items: any | any[]): Readable;
-export function createReduceStream<T>(reducer: (value: any, chunk: T, enc: string) => T): Transform;
-export function createPromiseFromStreams<T>([first, ...rest]: [Readable, ...Writable[]]): Promise<
-  T
->;
-export function createConcatStream(initial?: any): Transform;
-export function createMapStream<T>(fn: (value: T, i: number) => void): Transform;
-export function createReplaceStream(toReplace: string, replacement: string | Buffer): Transform;
-export function createFilterStream<T>(fn: (obj: T) => boolean): Transform;
diff --git a/x-pack/plugins/security_solution/server/lib/detection_engine/routes/rules/import_rules_route.ts b/x-pack/plugins/security_solution/server/lib/detection_engine/routes/rules/import_rules_route.ts
index 18eea7c45585fc..8a7215e5a5badd 100644
--- a/x-pack/plugins/security_solution/server/lib/detection_engine/routes/rules/import_rules_route.ts
+++ b/x-pack/plugins/security_solution/server/lib/detection_engine/routes/rules/import_rules_route.ts
@@ -20,7 +20,7 @@ import {
   importRulesSchema as importRulesResponseSchema,
 } from '../../../../../common/detection_engine/schemas/response/import_rules_schema';
 import { IRouter } from '../../../../../../../../src/core/server';
-import { createPromiseFromStreams } from '../../../../../../../../src/legacy/utils/streams';
+import { createPromiseFromStreams } from '../../../../../../../../src/core/server/utils/';
 import { DETECTION_ENGINE_RULES_URL } from '../../../../../common/constants';
 import { ConfigType } from '../../../../config';
 import { SetupPlugins } from '../../../../plugin';
diff --git a/x-pack/plugins/security_solution/server/lib/detection_engine/routes/rules/utils.test.ts b/x-pack/plugins/security_solution/server/lib/detection_engine/routes/rules/utils.test.ts
index 3122db1919c3c2..11f74c264ae0cf 100644
--- a/x-pack/plugins/security_solution/server/lib/detection_engine/routes/rules/utils.test.ts
+++ b/x-pack/plugins/security_solution/server/lib/detection_engine/routes/rules/utils.test.ts
@@ -22,7 +22,7 @@ import { INTERNAL_IDENTIFIER } from '../../../../../common/constants';
 import { RuleTypeParams } from '../../types';
 import { BulkError, ImportSuccessError } from '../utils';
 import { getOutputRuleAlertForRest } from '../__mocks__/utils';
-import { createPromiseFromStreams } from '../../../../../../../../src/legacy/utils/streams';
+import { createPromiseFromStreams } from '../../../../../../../../src/core/server/utils';
 import { PartialAlert } from '../../../../../../alerts/server';
 import { SanitizedAlert } from '../../../../../../alerts/server/types';
 import { createRulesStreamFromNdJson } from '../../rules/create_rules_stream_from_ndjson';
diff --git a/x-pack/plugins/security_solution/server/lib/detection_engine/rules/create_rules_stream_from_ndjson.test.ts b/x-pack/plugins/security_solution/server/lib/detection_engine/rules/create_rules_stream_from_ndjson.test.ts
index f2061ce1d36de5..60071bc2cef41e 100644
--- a/x-pack/plugins/security_solution/server/lib/detection_engine/rules/create_rules_stream_from_ndjson.test.ts
+++ b/x-pack/plugins/security_solution/server/lib/detection_engine/rules/create_rules_stream_from_ndjson.test.ts
@@ -5,7 +5,7 @@
  */
 import { Readable } from 'stream';
 import { createRulesStreamFromNdJson } from './create_rules_stream_from_ndjson';
-import { createPromiseFromStreams } from 'src/legacy/utils/streams';
+import { createPromiseFromStreams } from 'src/core/server/utils';
 import { BadRequestError } from '../errors/bad_request_error';
 import { ImportRulesSchemaDecoded } from '../../../../common/detection_engine/schemas/request/import_rules_schema';
 
diff --git a/x-pack/plugins/security_solution/server/lib/detection_engine/rules/create_rules_stream_from_ndjson.ts b/x-pack/plugins/security_solution/server/lib/detection_engine/rules/create_rules_stream_from_ndjson.ts
index d7723232ca921d..cd574a8d95615e 100644
--- a/x-pack/plugins/security_solution/server/lib/detection_engine/rules/create_rules_stream_from_ndjson.ts
+++ b/x-pack/plugins/security_solution/server/lib/detection_engine/rules/create_rules_stream_from_ndjson.ts
@@ -19,7 +19,7 @@ import {
   createSplitStream,
   createMapStream,
   createConcatStream,
-} from '../../../../../../../src/legacy/utils/streams';
+} from '../../../../../../../src/core/server/utils';
 import { BadRequestError } from '../errors/bad_request_error';
 import {
   parseNdjsonStrings,
diff --git a/x-pack/plugins/security_solution/server/lib/timeline/create_timelines_stream_from_ndjson.ts b/x-pack/plugins/security_solution/server/lib/timeline/create_timelines_stream_from_ndjson.ts
index 6b4017b5e4d5c7..2827cd373d5e75 100644
--- a/x-pack/plugins/security_solution/server/lib/timeline/create_timelines_stream_from_ndjson.ts
+++ b/x-pack/plugins/security_solution/server/lib/timeline/create_timelines_stream_from_ndjson.ts
@@ -13,7 +13,7 @@ import {
   createConcatStream,
   createSplitStream,
   createMapStream,
-} from '../../../../../../src/legacy/utils';
+} from '../../../../../../src/core/server/utils';
 import {
   parseNdjsonStrings,
   filterExportedCounts,
diff --git a/x-pack/plugins/security_solution/server/lib/timeline/routes/import_timelines_route.test.ts b/x-pack/plugins/security_solution/server/lib/timeline/routes/import_timelines_route.test.ts
index ff76045db90cb8..388ab5db3e852b 100644
--- a/x-pack/plugins/security_solution/server/lib/timeline/routes/import_timelines_route.test.ts
+++ b/x-pack/plugins/security_solution/server/lib/timeline/routes/import_timelines_route.test.ts
@@ -79,7 +79,7 @@ describe('import timelines', () => {
       };
     });
 
-    jest.doMock('../../../../../../../src/legacy/utils', () => {
+    jest.doMock('../../../../../../../src/core/server/utils', () => {
       return {
         createPromiseFromStreams: jest.fn().mockReturnValue(mockParsedObjects),
       };
@@ -543,7 +543,7 @@ describe('import timeline templates', () => {
       };
     });
 
-    jest.doMock('../../../../../../../src/legacy/utils', () => {
+    jest.doMock('../../../../../../../src/core/server/utils', () => {
       return {
         createPromiseFromStreams: jest.fn().mockReturnValue(mockParsedTemplateTimelineObjects),
       };
diff --git a/x-pack/plugins/security_solution/server/lib/timeline/routes/utils/common.ts b/x-pack/plugins/security_solution/server/lib/timeline/routes/utils/common.ts
index fc25f1a48194e2..9a3dbf365e0266 100644
--- a/x-pack/plugins/security_solution/server/lib/timeline/routes/utils/common.ts
+++ b/x-pack/plugins/security_solution/server/lib/timeline/routes/utils/common.ts
@@ -10,7 +10,7 @@ import { Readable } from 'stream';
 
 import { KibanaRequest, RequestHandlerContext } from 'src/core/server';
 
-import { createListStream } from '../../../../../../../../src/legacy/utils';
+import { createListStream } from '../../../../../../../../src/core/server/utils';
 
 import { SetupPlugins } from '../../../../plugin';
 
diff --git a/x-pack/plugins/security_solution/server/lib/timeline/routes/utils/import_timelines.ts b/x-pack/plugins/security_solution/server/lib/timeline/routes/utils/import_timelines.ts
index f62f02cc7bba94..a19b18e7d89b1a 100644
--- a/x-pack/plugins/security_solution/server/lib/timeline/routes/utils/import_timelines.ts
+++ b/x-pack/plugins/security_solution/server/lib/timeline/routes/utils/import_timelines.ts
@@ -21,7 +21,7 @@ import { createBulkErrorObject, BulkError } from '../../../detection_engine/rout
 import { createTimelines } from './create_timelines';
 import { FrameworkRequest } from '../../../framework';
 import { createTimelinesStreamFromNdJson } from '../../create_timelines_stream_from_ndjson';
-import { createPromiseFromStreams } from '../../../../../../../../src/legacy/utils';
+import { createPromiseFromStreams } from '../../../../../../../../src/core/server/utils';
 
 import { getTupleDuplicateErrorsAndUniqueTimeline } from './get_timelines_from_stream';
 import { CompareTimelinesStatus } from './compare_timelines_status';
diff --git a/x-pack/plugins/security_solution/server/lib/timeline/routes/utils/install_prepacked_timelines.test.ts b/x-pack/plugins/security_solution/server/lib/timeline/routes/utils/install_prepacked_timelines.test.ts
index 66f16db01a508b..c63978a1f046e1 100644
--- a/x-pack/plugins/security_solution/server/lib/timeline/routes/utils/install_prepacked_timelines.test.ts
+++ b/x-pack/plugins/security_solution/server/lib/timeline/routes/utils/install_prepacked_timelines.test.ts
@@ -5,7 +5,7 @@
  */
 import { join, resolve } from 'path';
 
-import { createPromiseFromStreams } from '../../../../../../../../src/legacy/utils/streams';
+import { createPromiseFromStreams } from '../../../../../../../../src/core/server/utils';
 import { SecurityPluginSetup } from '../../../../../../security/server';
 
 import { FrameworkRequest } from '../../../framework';
diff --git a/x-pack/plugins/security_solution/server/utils/read_stream/create_stream_from_ndjson.ts b/x-pack/plugins/security_solution/server/utils/read_stream/create_stream_from_ndjson.ts
index 0eb021bfe2a83d..4446e82f99de46 100644
--- a/x-pack/plugins/security_solution/server/utils/read_stream/create_stream_from_ndjson.ts
+++ b/x-pack/plugins/security_solution/server/utils/read_stream/create_stream_from_ndjson.ts
@@ -16,7 +16,7 @@ import {
   ImportRulesSchema,
 } from '../../../common/detection_engine/schemas/request/import_rules_schema';
 import { exactCheck } from '../../../common/exact_check';
-import { createMapStream, createFilterStream } from '../../../../../../src/legacy/utils/streams';
+import { createMapStream, createFilterStream } from '../../../../../../src/core/server/utils';
 import { BadRequestError } from '../../lib/detection_engine/errors/bad_request_error';
 
 export interface RulesObjectsExportResultDetails {
diff --git a/x-pack/plugins/spaces/server/routes/api/__fixtures__/create_copy_to_space_mocks.ts b/x-pack/plugins/spaces/server/routes/api/__fixtures__/create_copy_to_space_mocks.ts
index 0e117b3f16e3f4..ef6f5e1541a469 100644
--- a/x-pack/plugins/spaces/server/routes/api/__fixtures__/create_copy_to_space_mocks.ts
+++ b/x-pack/plugins/spaces/server/routes/api/__fixtures__/create_copy_to_space_mocks.ts
@@ -5,7 +5,7 @@
  */
 
 import { Readable } from 'stream';
-import { createPromiseFromStreams, createConcatStream } from 'src/legacy/utils';
+import { createPromiseFromStreams, createConcatStream } from 'src/core/server/utils';
 
 async function readStreamToCompletion(stream: Readable) {
   return (await (createPromiseFromStreams([stream, createConcatStream([])]) as unknown)) as any[];