diff --git a/src/main/java/org/devlive/sdk/common/DefaultClient.java b/src/main/java/org/devlive/sdk/common/DefaultClient.java
index 0e01c2f..ae1c930 100644
--- a/src/main/java/org/devlive/sdk/common/DefaultClient.java
+++ b/src/main/java/org/devlive/sdk/common/DefaultClient.java
@@ -266,7 +266,7 @@ public AssistantsEntity createAssistants(AssistantsEntity configure)
     }
 
     public AssistantsFileEntity createAssistantsFile(String fileId,
-            String assistantId)
+                                                     String assistantId)
     {
         String url = String.format(ProviderUtils.getUrl(provider, UrlModel.FETCH_ASSISTANTS_FILES), assistantId);
         Map<String, String> configure = Maps.newHashMap();
diff --git a/src/main/java/org/devlive/sdk/openai/OpenAiClient.java b/src/main/java/org/devlive/sdk/openai/OpenAiClient.java
index ccb508b..6858ce5 100644
--- a/src/main/java/org/devlive/sdk/openai/OpenAiClient.java
+++ b/src/main/java/org/devlive/sdk/openai/OpenAiClient.java
@@ -185,6 +185,12 @@ public OpenAiClientBuilder model(CompletionModel model)
             return this;
         }
 
+        public OpenAiClientBuilder model(String model)
+        {
+            this.model = model;
+            return this;
+        }
+
         private String getDefaultHost()
         {
             if (ObjectUtils.isEmpty(this.provider)) {
diff --git a/src/main/java/org/devlive/sdk/openai/entity/ChatEntity.java b/src/main/java/org/devlive/sdk/openai/entity/ChatEntity.java
index 4bfd366..09787fb 100644
--- a/src/main/java/org/devlive/sdk/openai/entity/ChatEntity.java
+++ b/src/main/java/org/devlive/sdk/openai/entity/ChatEntity.java
@@ -13,6 +13,9 @@
 import org.devlive.sdk.openai.utils.EnumsUtils;
 
 import java.util.List;
+import java.util.Objects;
+
+import static org.devlive.sdk.openai.model.CompletionModel.GPT_35_TURBO;
 
 @Data
 @Builder
@@ -47,7 +50,7 @@ public class ChatEntity
     private ChatEntity(ChatEntityBuilder builder)
     {
         if (ObjectUtils.isEmpty(builder.model)) {
-            builder.model(CompletionModel.GPT_35_TURBO);
+            builder.model(GPT_35_TURBO);
         }
         this.model = builder.model;
         this.messages = builder.messages;
@@ -73,7 +76,7 @@ public static class ChatEntityBuilder
         public ChatEntityBuilder model(CompletionModel model)
         {
             if (ObjectUtils.isEmpty(model)) {
-                model = CompletionModel.GPT_35_TURBO;
+                model = GPT_35_TURBO;
             }
             switch (model) {
                 case GPT_35_TURBO:
@@ -96,6 +99,12 @@ public ChatEntityBuilder model(CompletionModel model)
             return this;
         }
 
+        public ChatEntityBuilder model(String model)
+        {
+            this.model = model;
+            return this;
+        }
+
         public ChatEntityBuilder temperature(Double temperature)
         {
             if (temperature < 0 || temperature > 2) {
@@ -108,11 +117,20 @@ public ChatEntityBuilder temperature(Double temperature)
         public ChatEntityBuilder maxTokens(Integer maxTokens)
         {
             CompletionModel completionModel = EnumsUtils.getCompleteModel(this.model);
-            if (ObjectUtils.isNotEmpty(this.model) && maxTokens > completionModel.getMaxTokens()) {
-                throw new ParamException(String.format("Invalid maxTokens: %s, Cannot be larger than the model default configuration %s", maxTokens, completionModel.getMaxTokens()));
+            if (Objects.isNull(completionModel)) {
+                this.maxTokens = maxTokens;
+                return this;
+            }
+            else {
+                if (ObjectUtils.isNotEmpty(this.model)
+                        && maxTokens > completionModel.getMaxTokens()) {
+                    throw new ParamException(String.format(
+                            "Invalid maxTokens: %s, Cannot be larger than the model default configuration %s",
+                            maxTokens, completionModel.getMaxTokens()));
+                }
+                this.maxTokens = maxTokens;
+                return this;
             }
-            this.maxTokens = maxTokens;
-            return this;
         }
 
         private ChatEntityBuilder stream()
diff --git a/src/test/java/org/devlive/sdk/openai/OpenAiClientTest.java b/src/test/java/org/devlive/sdk/openai/OpenAiClientTest.java
index cd19897..5266e08 100644
--- a/src/test/java/org/devlive/sdk/openai/OpenAiClientTest.java
+++ b/src/test/java/org/devlive/sdk/openai/OpenAiClientTest.java
@@ -15,6 +15,7 @@
 import org.devlive.sdk.common.exception.RequestException;
 import org.devlive.sdk.openai.model.CompletionModel;
 import org.devlive.sdk.openai.model.EditModel;
+import org.devlive.sdk.openai.response.ChatResponse;
 import org.junit.Assert;
 import org.junit.Before;
 import org.junit.Test;
@@ -94,6 +95,30 @@ public void testCreateCompletion()
         Assert.assertTrue(client.createCompletion(configure).getChoices().size() > 0);
     }
 
+    @Test
+    public void testCustomizedModel()
+    {
+        client = OpenAiClient.builder()
+                .apiHost(System.getProperty("proxy.host"))
+                .apiKey(System.getProperty("openai.token"))
+                .model("text-davinci-003")
+                .build();
+
+        List<MessageEntity> messages = Lists.newArrayList();
+        messages.add(MessageEntity.builder()
+                .content("Hello, please show me a jok!")
+                .build());
+
+        ChatEntity configure = ChatEntity.builder()
+                .messages(messages)
+                .model("text-davinci-003")
+                .build();
+        ChatResponse chatCompletion = client.createChatCompletion(configure);
+        String content = chatCompletion.getChoices().get(0).getMessage().getContent();
+        // System.out.println(content);
+        Assert.assertNotNull(content);
+    }
+
     @Test
     public void testCreateChatCompletion()
     {