diff --git a/src/main/kotlin/com/lauvsong/refactorgpt/dto/request/ChatGptRequest.kt b/src/main/kotlin/com/lauvsong/refactorgpt/dto/request/ChatGptRequest.kt
index 2f46e6f..1ecbdf1 100644
--- a/src/main/kotlin/com/lauvsong/refactorgpt/dto/request/ChatGptRequest.kt
+++ b/src/main/kotlin/com/lauvsong/refactorgpt/dto/request/ChatGptRequest.kt
@@ -1,9 +1,9 @@
 package com.lauvsong.refactorgpt.dto.request
 
 data class ChatGptRequest(
-    val model: String = "text-davinci-003",
+    val model: String = "gpt-3.5-turbo-instruct",
     val prompt: String,
-    val maxTokens: Int = 500,
+    val maxTokens: Int = 1600,
     val temperature: Int = 0
 ) {
 
@@ -13,15 +13,14 @@ data class ChatGptRequest(
 
         private fun makePrompt(fileExtension: String, code: String): String =
             """
-                You role is perfect code refactoring prompt.
+                You role is code refactoring output.
                 Refactor the following code for better readability and maintainability.
-                Don't say ANY explain. Just response the code strictly.
+                Return only the refactored code and don't explain anything.
                 This code's file extension: $fileExtension
                 Here is the code:
                 ```
                 $code
                 ```
-
                 Respond start with the line 'Code:'
             """.trimIndent()
     }
diff --git a/src/main/kotlin/com/lauvsong/refactorgpt/dto/response/ChatGptResponse.kt b/src/main/kotlin/com/lauvsong/refactorgpt/dto/response/ChatGptResponse.kt
index 2b0af79..854619e 100644
--- a/src/main/kotlin/com/lauvsong/refactorgpt/dto/response/ChatGptResponse.kt
+++ b/src/main/kotlin/com/lauvsong/refactorgpt/dto/response/ChatGptResponse.kt
@@ -18,4 +18,8 @@ data class ChatGptResponse(
             code = choices.first().text.substringAfter("Code:").trim('`', '\n', ' ')
         )
     }
+
+    fun getFinishReason(): String {
+        return choices.first().finishReason
+    }
 }
diff --git a/src/main/kotlin/com/lauvsong/refactorgpt/exception/ChatGptAuthenticationException.kt b/src/main/kotlin/com/lauvsong/refactorgpt/exception/ChatGptAuthenticationException.kt
index b7c661c..7d00a74 100644
--- a/src/main/kotlin/com/lauvsong/refactorgpt/exception/ChatGptAuthenticationException.kt
+++ b/src/main/kotlin/com/lauvsong/refactorgpt/exception/ChatGptAuthenticationException.kt
@@ -2,6 +2,10 @@ package com.lauvsong.refactorgpt.exception
 
 class ChatGptAuthenticationException(
     message: String? = """
-        Authentication failed. Check API key settings.
+        Authentication failed. Check below things.
+        1. API key settings configured in Settings.
+        2. API key settings configured in OpenAI.
+        3. Ensure a credit card is registered with OpenAI.
+        If these steps do not resolve the issue, please get a new API key and try again.
         """
 ) : Exception(message)
diff --git a/src/main/kotlin/com/lauvsong/refactorgpt/service/ChatGptService.kt b/src/main/kotlin/com/lauvsong/refactorgpt/service/ChatGptService.kt
index a1ac363..a41545a 100644
--- a/src/main/kotlin/com/lauvsong/refactorgpt/service/ChatGptService.kt
+++ b/src/main/kotlin/com/lauvsong/refactorgpt/service/ChatGptService.kt
@@ -21,14 +21,38 @@ class ChatGptService(
             onSuccess = { response -> onRefactorSuccess(response) },
             onFailure = { exception ->
                 if (exception is SocketTimeoutException) {
-                    throw ChatGptFetchFailureException("timeout error.\nPlease check your network or set longer timeout in settings.")
+                    throw ChatGptFetchFailureException(
+                        """
+                        timeout error.
+                        Please check your network or set longer timeout in settings.
+                        """
+                    )
                 }
                 throw ChatGptFetchFailureException(exception.message)
             }
         )
 
-    private fun onRefactorSuccess(response: Response<ChatGptResponse>): Refactored =
-        takeUnless { response.code() == 401 }
-            ?.let { response.body()?.toRefactored() }
-            ?: throw ChatGptAuthenticationException()
+    private fun onRefactorSuccess(response: Response<ChatGptResponse>): Refactored {
+        if (response.code() == 401) {
+            throw ChatGptAuthenticationException()
+        }
+
+        val body = response.body()
+            ?: throw ChatGptFetchFailureException("OpenAI's response body is null.")
+
+        if (response.isSuccessful.not()) {
+            throw ChatGptFetchFailureException("${response.errorBody()?.string()}")
+        }
+
+        if (body.getFinishReason() == "length") {
+            throw ChatGptFetchFailureException(
+                """
+                The response exceeds the maximum token limit.
+                Please try again with a shorter code.
+                """
+            )
+        }
+
+        return body.toRefactored()
+    }
 }