diff --git a/packages/kbn-test/src/ci_parallel_process_prefix.ts b/packages/kbn-test/src/ci_parallel_process_prefix.ts
new file mode 100644
index 000000000000..67dafc7e8532
--- /dev/null
+++ b/packages/kbn-test/src/ci_parallel_process_prefix.ts
@@ -0,0 +1,30 @@
+/*
+ * 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.
+ */
+
+const job = process.env.JOB ? `job-${process.env.JOB}-` : '';
+const num = process.env.CI_PARALLEL_PROCESS_NUMBER
+  ? `worker-${process.env.CI_PARALLEL_PROCESS_NUMBER}-`
+  : '';
+
+/**
+ * A prefix string that is unique for each parallel CI process that
+ * is an empty string outside of CI, so it can be safely injected
+ * into strings as a prefix
+ */
+export const CI_PARALLEL_PROCESS_PREFIX = `${job}${num}`;
diff --git a/packages/kbn-test/src/functional_tests/lib/run_elasticsearch.js b/packages/kbn-test/src/functional_tests/lib/run_elasticsearch.js
index 136704a639bf..5f58190078f0 100644
--- a/packages/kbn-test/src/functional_tests/lib/run_elasticsearch.js
+++ b/packages/kbn-test/src/functional_tests/lib/run_elasticsearch.js
@@ -42,10 +42,11 @@ export async function runElasticsearch({ config, options }) {
     esFrom: esFrom || config.get('esTestCluster.from'),
     dataArchive: config.get('esTestCluster.dataArchive'),
     esArgs,
+    esEnvVars,
     ssl,
   });
 
-  await cluster.start(esArgs, esEnvVars);
+  await cluster.start();
 
   if (isSecurityEnabled) {
     await setupUsers({
diff --git a/packages/kbn-test/src/junit_report_path.ts b/packages/kbn-test/src/junit_report_path.ts
index d46c9455dcff..90405d7a89c0 100644
--- a/packages/kbn-test/src/junit_report_path.ts
+++ b/packages/kbn-test/src/junit_report_path.ts
@@ -18,17 +18,13 @@
  */
 
 import { resolve } from 'path';
-
-const job = process.env.JOB ? `job-${process.env.JOB}-` : '';
-const num = process.env.CI_PARALLEL_PROCESS_NUMBER
-  ? `worker-${process.env.CI_PARALLEL_PROCESS_NUMBER}-`
-  : '';
+import { CI_PARALLEL_PROCESS_PREFIX } from './ci_parallel_process_prefix';
 
 export function makeJunitReportPath(rootDirectory: string, reportName: string) {
   return resolve(
     rootDirectory,
     'target/junit',
     process.env.JOB || '.',
-    `TEST-${job}${num}${reportName}.xml`
+    `TEST-${CI_PARALLEL_PROCESS_PREFIX}${reportName}.xml`
   );
 }
diff --git a/packages/kbn-test/src/legacy_es/legacy_es_test_cluster.js b/packages/kbn-test/src/legacy_es/legacy_es_test_cluster.js
index 355304c86a3c..f795b32d78b8 100644
--- a/packages/kbn-test/src/legacy_es/legacy_es_test_cluster.js
+++ b/packages/kbn-test/src/legacy_es/legacy_es_test_cluster.js
@@ -22,6 +22,7 @@ import { format } from 'url';
 import { get } from 'lodash';
 import toPath from 'lodash/internal/toPath';
 import { Cluster } from '@kbn/es';
+import { CI_PARALLEL_PROCESS_PREFIX } from '../ci_parallel_process_prefix';
 import { esTestConfig } from './es_test_config';
 
 import { KIBANA_ROOT } from '../';
@@ -38,14 +39,22 @@ export function createLegacyEsTestCluster(options = {}) {
     basePath = resolve(KIBANA_ROOT, '.es'),
     esFrom = esTestConfig.getBuildFrom(),
     dataArchive,
-    esArgs,
+    esArgs: customEsArgs = [],
+    esEnvVars,
+    clusterName: customClusterName = 'es-test-cluster',
     ssl,
   } = options;
 
-  const randomHash = Math.random()
-    .toString(36)
-    .substring(2);
-  const clusterName = `test-${randomHash}`;
+  const clusterName = `${CI_PARALLEL_PROCESS_PREFIX}${customClusterName}`;
+
+  const esArgs = [
+    `cluster.name=${clusterName}`,
+    `http.port=${port}`,
+    'discovery.type=single-node',
+    `transport.port=${esTestConfig.getTransportPort()}`,
+    ...customEsArgs,
+  ];
+
   const config = {
     version: esTestConfig.getVersion(),
     installPath: resolve(basePath, clusterName),
@@ -55,7 +64,6 @@ export function createLegacyEsTestCluster(options = {}) {
     basePath,
     esArgs,
   };
-  const transportPort = esTestConfig.getTransportPort();
 
   const cluster = new Cluster({ log, ssl });
 
@@ -67,7 +75,7 @@ export function createLegacyEsTestCluster(options = {}) {
       return esFrom === 'snapshot' ? 3 * minute : 6 * minute;
     }
 
-    async start(esArgs = [], esEnvVars) {
+    async start() {
       let installPath;
 
       if (esFrom === 'source') {
@@ -86,13 +94,7 @@ export function createLegacyEsTestCluster(options = {}) {
 
       await cluster.start(installPath, {
         password: config.password,
-        esArgs: [
-          `cluster.name=${clusterName}`,
-          `http.port=${port}`,
-          'discovery.type=single-node',
-          `transport.port=${transportPort}`,
-          ...esArgs,
-        ],
+        esArgs,
         esEnvVars,
       });
     }