diff --git a/lib/add-file-to-cache.js b/lib/add-file-to-cache.js
index 55c2736..44040fd 100644
--- a/lib/add-file-to-cache.js
+++ b/lib/add-file-to-cache.js
@@ -25,7 +25,7 @@ function getExtensionAndSampleRate(path) {
 }
 
 async function addFileToCache(client, logger, path,
-  {account_sid, vendor, language, voice, deploymentId, engine, text}) {
+  {account_sid, vendor, language, voice, deploymentId, engine, model, text}) {
   let key;
   logger = logger || noopLogger;
 
@@ -36,6 +36,7 @@ async function addFileToCache(client, logger, path,
       language: language || '',
       voice: voice || deploymentId,
       engine,
+      model,
       text,
     });
     const [extension, sampleRate] = getExtensionAndSampleRate(path);
diff --git a/lib/purge-tts-cache.js b/lib/purge-tts-cache.js
index 1b26ab6..86ba974 100644
--- a/lib/purge-tts-cache.js
+++ b/lib/purge-tts-cache.js
@@ -12,7 +12,7 @@ const debug = require('debug')('jambonz:realtimedb-helpers');
  * @returns {object} result - {error, purgedCount}
  */
 async function purgeTtsCache(client, logger, {all, account_sid, vendor,
-  language, voice, deploymentId, engine, text} = {all: true}) {
+  language, voice, deploymentId, engine, model, text} = {all: true}) {
   logger = logger || noopLogger;
 
   let purgedCount = 0, error;
@@ -33,6 +33,7 @@ async function purgeTtsCache(client, logger, {all, account_sid, vendor,
         language: language || '',
         voice: voice || deploymentId,
         engine,
+        model,
         text,
       });
       purgedCount = await client.del(key);
diff --git a/lib/synth-audio.js b/lib/synth-audio.js
index 72381a0..a8214f3 100644
--- a/lib/synth-audio.js
+++ b/lib/synth-audio.js
@@ -169,6 +169,8 @@ async function synthAudio(client, createHash, retrieveHash, logger, stats, { acc
     language: language || '',
     voice: voice || deploymentId,
     engine,
+    // model or model_id is used to identify the tts cache.
+    model: model || credentials.model_id,
     text
   });
 
diff --git a/lib/utils.js b/lib/utils.js
index a901d68..81047b5 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -17,9 +17,16 @@ const debug = require('debug')('jambonz:realtimedb-helpers');
 //const nuanceClientMap = new Map();
 
 function makeSynthKey({
-  account_sid = '', vendor, language, voice, engine = '', text}) {
+  account_sid = '',
+  vendor,
+  language,
+  voice,
+  engine = '',
+  model = '',
+  text
+}) {
   const hash = crypto.createHash('sha1');
-  hash.update(`${language}:${vendor}:${voice}:${engine}:${text}`);
+  hash.update(`${language}:${vendor}:${voice}:${engine}:${model}:${text}`);
   const hexHashKey = hash.digest('hex');
   const accountKey = account_sid ? `:${account_sid}` : '';
   const key = `tts${accountKey}:${hexHashKey}`;
diff --git a/test/synth.js b/test/synth.js
index 4e10893..bea43df 100644
--- a/test/synth.js
+++ b/test/synth.js
@@ -5,7 +5,7 @@ const fs = require('fs');
 const {makeSynthKey} = require('../lib/utils');
 const logger = require('pino')();
 const bent = require('bent');
-const getJSON = bent('json')
+const getJSON = bent('json');
 
 process.on('unhandledRejection', (reason, p) => {
   console.log('Unhandled Rejection at: Promise', p, 'reason:', reason);
@@ -853,11 +853,11 @@ test('TTS Cache tests', async(t) => {
     // save some random tts keys to cache
     const minRecords = 8;
     for (const i in Array(minRecords).fill(0)) {
-      await client.set(makeSynthKey({vendor: i, language: i, voice: i, engine: i, text: i}), i);
+      await client.set(makeSynthKey({vendor: i, language: i, voice: i, engine: i, model: i, text: i}), i);
     }
     const count = await getTtsSize();
     t.ok(count >= minRecords, 'getTtsSize worked.');
-    
+
     const {purgedCount} = await purgeTtsCache();
     t.ok(purgedCount >= minRecords, `successfully purged at least ${minRecords} tts records from cache`);