diff --git a/Sources/VRCKit/APIClient.swift b/Sources/VRCKit/APIClient.swift
index f272b65..5f5be28 100644
--- a/Sources/VRCKit/APIClient.swift
+++ b/Sources/VRCKit/APIClient.swift
@@ -43,7 +43,7 @@ public final actor APIClient {
     /// - Throws: `VRCKitError.unexpectedError` if the username and password cannot be converted to UTF-8 data.
     private func encodeAuthorization(_ credential: Credential) throws -> String {
         guard let payload = credential.authString.data(using: .utf8) else {
-            throw VRCKitError.unexpected
+            throw VRCKitError.credentialNotSet
         }
         return "Basic \(payload.base64EncodedString())"
     }
@@ -108,7 +108,7 @@ public final actor APIClient {
                     return
                 }
                 guard let data = data, let reponse = urlResponse as? HTTPURLResponse else {
-                    continuation.resume(throwing: VRCKitError.invalidResponse)
+                    continuation.resume(throwing: VRCKitError.invalidResponse(String(describing: data)))
                     return
                 }
                 continuation.resume(returning: (data, reponse))
@@ -121,7 +121,7 @@ public final actor APIClient {
     private func requestWithFoundation(_ request: URLRequest) async throws -> HTTPResponse {
         let (data, response) = try await URLSession.shared.data(for: request)
         guard let response = response as? HTTPURLResponse else {
-            throw VRCKitError.invalidResponse
+            throw VRCKitError.invalidResponse(String(describing: data))
         }
         return (data, response)
     }
diff --git a/Sources/VRCKit/Errors.swift b/Sources/VRCKit/Errors.swift
index 66894ec..cfcc4ca 100644
--- a/Sources/VRCKit/Errors.swift
+++ b/Sources/VRCKit/Errors.swift
@@ -21,8 +21,11 @@ public enum VRCKitError: Error, LocalizedError, Equatable {
     /// Represents an error indicating that the client has been deallocated.
     case clientDeallocated
 
+    /// Represents an error indicating that credential not set.
+    case credentialNotSet
+
     /// Represents an error indicating an invalid response was received.
-    case invalidResponse
+    case invalidResponse(_ details: String)
 
     /// Represents an error indicating an invalid request with additional details.
     case invalidRequest(_ details: String)
@@ -30,9 +33,6 @@ public enum VRCKitError: Error, LocalizedError, Equatable {
     /// Represents an error indicating an authentication failure.
     case unauthorized
 
-    /// Represents an unexpected error.
-    case unexpected
-
     /// Represents an url error.
     case urlError
 
@@ -42,10 +42,10 @@ public enum VRCKitError: Error, LocalizedError, Equatable {
         case .apiError: "API Error"
         case .badGateway: "Bad Gateway"
         case .clientDeallocated: "Client Deallocated"
+        case .credentialNotSet: "Credential Error"
         case .invalidResponse: "Invalid Response"
         case .invalidRequest: "Invalid Request"
         case .unauthorized: "Unauthorized"
-        case .unexpected: "Unexpected"
         case .urlError: "URL Error"
         }
     }
@@ -53,10 +53,10 @@ public enum VRCKitError: Error, LocalizedError, Equatable {
     /// Provides a localized failure reason for the error.
     public var failureReason: String? {
         switch self {
-        case .apiError(let details), .invalidRequest(let details):
-            details
-        default:
-            errorDescription
+        case .apiError(let details): details
+        case .invalidRequest(let details): details
+        case .invalidResponse(let details): details
+        default: errorDescription
         }
     }
 }
diff --git a/Sources/VRCKit/Services/AuthenticationService.swift b/Sources/VRCKit/Services/AuthenticationService.swift
index 086142c..5662185 100644
--- a/Sources/VRCKit/Services/AuthenticationService.swift
+++ b/Sources/VRCKit/Services/AuthenticationService.swift
@@ -30,7 +30,7 @@ public final actor AuthenticationService: APIService, AuthenticationServiceProto
         } catch _ as DecodingError {
             let result: RequiresTwoFactorAuthResponse = try Serializer.shared.decode(response.data)
             guard let requires = result.requires else {
-                throw VRCKitError.unexpected
+                throw VRCKitError.invalidResponse("\(result.requires?.rawValue)")
             }
             return .right(requires)
         }
diff --git a/Sources/VRCKit/Utils/Serializer.swift b/Sources/VRCKit/Utils/Serializer.swift
index 451ac6c..174d192 100644
--- a/Sources/VRCKit/Utils/Serializer.swift
+++ b/Sources/VRCKit/Utils/Serializer.swift
@@ -35,7 +35,7 @@ final class Serializer: Sendable {
             do {
                 let errorResponse = try decoder.decode(ErrorResponse.self, from: data)
                 if errorResponse.error.statusCode == 401 {
-                    throw VRCKitError.unauthorized
+                    throw VRCKitError.credentialNotSet
                 } else {
                     throw VRCKitError.apiError(errorResponse.error.message)
                 }