diff --git a/cli/src/__tests__/utils/getBenchClientId.test.ts b/cli/src/__tests__/utils/getBenchClientId.test.ts
new file mode 100644
index 000000000..f9cee9045
--- /dev/null
+++ b/cli/src/__tests__/utils/getBenchClientId.test.ts
@@ -0,0 +1,38 @@
+import { expect, describe, it } from '@jest/globals'
+import getBenchClientId from '../../utils/getBenchClientId'
+
+describe('getBenchClientId', () => {
+  describe('single connection (count = 1)', () => {
+    it('should return original clientId when no placeholder', () => {
+      expect(getBenchClientId('mqtt_client', 1, 1)).toBe('mqtt_client')
+    })
+
+    it('should replace %i when has placeholder', () => {
+      expect(getBenchClientId('mqtt_client_%i', 1, 1)).toBe('mqtt_client_1')
+    })
+  })
+
+  describe('multiple connections (count > 1)', () => {
+    it('should append index when no placeholder', () => {
+      expect(getBenchClientId('mqtt_client', 1, 5)).toBe('mqtt_client_1')
+    })
+
+    it('should replace %i when has placeholder', () => {
+      expect(getBenchClientId('mqtt_client_%i', 2, 5)).toBe('mqtt_client_2')
+    })
+
+    it('should handle multiple %i placeholders', () => {
+      expect(getBenchClientId('mqtt_%i_client_%i', 3, 5)).toBe('mqtt_3_client_3')
+    })
+  })
+
+  describe('edge cases', () => {
+    it('should handle empty clientId', () => {
+      expect(getBenchClientId('', 1, 5)).toBe('_1')
+    })
+
+    it('should handle clientId with only %i', () => {
+      expect(getBenchClientId('%i', 1, 5)).toBe('1')
+    })
+  })
+})
diff --git a/cli/src/lib/conn.ts b/cli/src/lib/conn.ts
index 7bc3f1f10..fd2a9777d 100644
--- a/cli/src/lib/conn.ts
+++ b/cli/src/lib/conn.ts
@@ -5,6 +5,7 @@ import delay from '../utils/delay'
 import { handleSaveOptions, handleLoadOptions } from '../utils/options'
 import * as Debug from 'debug'
 import { triggerExitInfo } from '../utils/exitInfo'
+import getBenchClientId from '../utils/getBenchClientId'
 
 const conn = (options: ConnectOptions) => {
   const { debug, saveOptions, loadOptions } = options
@@ -83,7 +84,7 @@ const benchConn = async (options: BenchConnectOptions) => {
     ;((i: number, connOpts: mqtt.IClientOptions) => {
       const opts = { ...connOpts }
 
-      opts.clientId = clientId.includes('%i') ? clientId.replaceAll('%i', i.toString()) : `${clientId}_${i}`
+      opts.clientId = getBenchClientId(clientId, i, count)
 
       const client = mqtt.connect(opts)
 
diff --git a/cli/src/lib/pub.ts b/cli/src/lib/pub.ts
index 85bc44e41..ae99f8423 100644
--- a/cli/src/lib/pub.ts
+++ b/cli/src/lib/pub.ts
@@ -16,6 +16,7 @@ import { serializeProtobufToBuffer } from '../utils/protobuf'
 import { serializeAvroToBuffer } from '../utils/avro'
 import { loadSimulator } from '../utils/simulate'
 import { triggerExitInfo } from '../utils/exitInfo'
+import getBenchClientId from '../utils/getBenchClientId'
 
 /**
  * Processes the outgoing message through two potential stages:
@@ -390,7 +391,7 @@ const multiPub = async (commandType: CommandType, options: BenchPublishOptions |
     ;((i: number, connOpts: mqtt.IClientOptions) => {
       const opts = { ...connOpts }
 
-      opts.clientId = clientId.includes('%i') ? clientId.replaceAll('%i', i.toString()) : `${clientId}_${i}`
+      opts.clientId = getBenchClientId(clientId, i, count)
 
       let topicName = topic.replaceAll('%i', i.toString()).replaceAll('%c', clientId)
       username && (topicName = topicName.replaceAll('%u', username))
diff --git a/cli/src/lib/sub.ts b/cli/src/lib/sub.ts
index ee968c731..8f47e493d 100644
--- a/cli/src/lib/sub.ts
+++ b/cli/src/lib/sub.ts
@@ -9,6 +9,7 @@ import { deserializeBufferToProtobuf } from '../utils/protobuf'
 import isSupportedBinaryFormatForMQTT from '../utils/binaryFormats'
 import * as Debug from 'debug'
 import { deserializeBufferToAvro } from '../utils/avro'
+import getBenchClientId from '../utils/getBenchClientId'
 
 /**
  *
@@ -266,7 +267,7 @@ const benchSub = async (options: BenchSubscribeOptions) => {
     ;((i: number, connOpts: mqtt.IClientOptions) => {
       const opts = { ...connOpts }
 
-      opts.clientId = clientId.includes('%i') ? clientId.replaceAll('%i', i.toString()) : `${clientId}_${i}`
+      opts.clientId = getBenchClientId(clientId, i, count)
 
       const client = mqtt.connect(opts)
 
diff --git a/cli/src/utils/getBenchClientId.ts b/cli/src/utils/getBenchClientId.ts
new file mode 100644
index 000000000..2a23dcb48
--- /dev/null
+++ b/cli/src/utils/getBenchClientId.ts
@@ -0,0 +1,18 @@
+/**
+ * Generate a unique client ID for benchmarking purposes
+ * @param clientId - Base client ID string that may contain '%i' placeholder
+ * @param index - Index number to replace placeholder or append to client ID
+ * @param count - Total count of clients being generated
+ * @returns Modified client ID string with index incorporated
+ *
+ * If clientId contains '%i', replaces all instances with index.
+ * If count > 1 and no '%i' placeholder exists, appends '_index' to clientId.
+ * Otherwise returns clientId unchanged.
+ */
+const getBenchClientId = (clientId: string, index: number, count: number) => {
+  const hasPlaceholder = clientId.includes('%i')
+  const baseClientId = hasPlaceholder ? clientId.replaceAll('%i', index.toString()) : clientId
+  return count > 1 && !hasPlaceholder ? `${baseClientId}_${index}` : baseClientId
+}
+
+export default getBenchClientId