diff --git a/FirebaseAuth/Sources/Swift/Auth/Auth.swift b/FirebaseAuth/Sources/Swift/Auth/Auth.swift
index 8effd15c101..3ac7f43b9bc 100644
--- a/FirebaseAuth/Sources/Swift/Auth/Auth.swift
+++ b/FirebaseAuth/Sources/Swift/Auth/Auth.swift
@@ -295,9 +295,9 @@ extension Auth: AuthInterop {
       Task {
         do {
           let response = try await AuthBackend.call(with: request)
-          Auth.wrapMainAsync(callback: completion, withParam: response.signinMethods, error: nil)
+          Auth.wrapMainAsync(callback: completion, with: .success(response.signinMethods))
         } catch {
-          Auth.wrapMainAsync(callback: completion, withParam: nil, error: error)
+          Auth.wrapMainAsync(callback: completion, with: .failure(error))
         }
       }
     }
@@ -1004,9 +1004,9 @@ extension Auth: AuthInterop {
           let actionCodeInfo = ActionCodeInfo(withOperation: operation,
                                               email: email,
                                               newEmail: response.verifiedEmail)
-          Auth.wrapMainAsync(callback: completion, withParam: actionCodeInfo, error: nil)
+          Auth.wrapMainAsync(callback: completion, with: .success(actionCodeInfo))
         } catch {
-          Auth.wrapMainAsync(callback: completion, withParam: nil, error: error)
+          Auth.wrapMainAsync(callback: completion, with: .failure(error))
         }
       }
     }
@@ -2224,7 +2224,11 @@ extension Auth: AuthInterop {
     ((AuthDataResult?, Error?) -> Void)?) -> (AuthDataResult?, Error?) -> Void {
     let authDataCallback: (((AuthDataResult?, Error?) -> Void)?, AuthDataResult?, Error?) -> Void =
       { callback, result, error in
-        Auth.wrapMainAsync(callback: callback, withParam: result, error: error)
+        if let result {
+          Auth.wrapMainAsync(callback: callback, with: .success(result))
+        } else if let error {
+          Auth.wrapMainAsync(callback: callback, with: .failure(error))
+        }
       }
     return { authResult, error in
       if let error {
@@ -2261,11 +2265,14 @@ extension Auth: AuthInterop {
   }
 
   class func wrapMainAsync<T: Any>(callback: ((T?, Error?) -> Void)?,
-                                   withParam param: T?,
-                                   error: Error?) -> Void {
-    if let callback {
-      DispatchQueue.main.async {
-        callback(param, error)
+                                   with result: Result<T, Error>) -> Void {
+    guard let callback else { return }
+    DispatchQueue.main.async {
+      switch result {
+      case let .success(success):
+        callback(success, nil)
+      case let .failure(error):
+        callback(nil, error)
       }
     }
   }
diff --git a/FirebaseAuth/Sources/Swift/AuthProvider/PhoneAuthProvider.swift b/FirebaseAuth/Sources/Swift/AuthProvider/PhoneAuthProvider.swift
index 32d15499f74..a8651d12e01 100644
--- a/FirebaseAuth/Sources/Swift/AuthProvider/PhoneAuthProvider.swift
+++ b/FirebaseAuth/Sources/Swift/AuthProvider/PhoneAuthProvider.swift
@@ -86,9 +86,9 @@ import Foundation
               uiDelegate: uiDelegate,
               multiFactorSession: multiFactorSession
             )
-            Auth.wrapMainAsync(callback: completion, withParam: verificationID, error: nil)
+            Auth.wrapMainAsync(callback: completion, with: .success(verificationID))
           } catch {
-            Auth.wrapMainAsync(callback: completion, withParam: nil, error: error)
+            Auth.wrapMainAsync(callback: completion, with: .failure(error))
           }
         }
       }
@@ -184,7 +184,7 @@ import Foundation
     private func internalVerify(phoneNumber: String,
                                 uiDelegate: AuthUIDelegate?,
                                 multiFactorSession: MultiFactorSession? = nil) async throws
-      -> String? {
+      -> String {
       guard phoneNumber.count > 0 else {
         throw AuthErrorUtils.missingPhoneNumberError(message: nil)
       }
@@ -209,7 +209,7 @@ import Foundation
     private func verifyClAndSendVerificationCode(toPhoneNumber phoneNumber: String,
                                                  retryOnInvalidAppCredential: Bool,
                                                  uiDelegate: AuthUIDelegate?) async throws
-      -> String? {
+      -> String {
       let codeIdentity = try await verifyClient(withUIDelegate: uiDelegate)
       let request = SendVerificationCodeRequest(phoneNumber: phoneNumber,
                                                 codeIdentity: codeIdentity,
@@ -236,7 +236,7 @@ import Foundation
                                                  retryOnInvalidAppCredential: Bool,
                                                  multiFactorSession session: MultiFactorSession?,
                                                  uiDelegate: AuthUIDelegate?) async throws
-      -> String? {
+      -> String {
       if let settings = auth.settings,
          settings.isAppVerificationDisabledForTesting {
         let request = SendVerificationCodeRequest(
@@ -264,7 +264,10 @@ import Foundation
                                                   enrollmentInfo: startMFARequestInfo,
                                                   requestConfiguration: auth.requestConfiguration)
           let response = try await AuthBackend.call(with: request)
-          return response.phoneSessionInfo?.sessionInfo
+          guard let sessionInfo = response.phoneSessionInfo?.sessionInfo else {
+            throw AuthErrorUtils.error(code: .missingMultiFactorInfo)
+          }
+          return sessionInfo
         } else {
           let request = StartMFASignInRequest(MFAPendingCredential: session.mfaPendingCredential,
                                               MFAEnrollmentID: session.multiFactorInfo?.uid,
@@ -272,7 +275,10 @@ import Foundation
                                               requestConfiguration: auth.requestConfiguration)
 
           let response = try await AuthBackend.call(with: request)
-          return response.responseInfo?.sessionInfo
+          guard let sessionInfo = response.responseInfo?.sessionInfo else {
+            throw AuthErrorUtils.error(code: .multiFactorInfoNotFound)
+          }
+          return sessionInfo
         }
       } catch {
         return try await handleVerifyErrorWithRetry(
@@ -289,7 +295,7 @@ import Foundation
                                             phoneNumber: String,
                                             retryOnInvalidAppCredential: Bool,
                                             multiFactorSession session: MultiFactorSession?,
-                                            uiDelegate: AuthUIDelegate?) async throws -> String? {
+                                            uiDelegate: AuthUIDelegate?) async throws -> String {
       if (error as NSError).code == AuthErrorCode.invalidAppCredential.rawValue {
         if retryOnInvalidAppCredential {
           auth.appCredentialManager.clearCredential()
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/SignIn/StartMFASignInResponse.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/SignIn/StartMFASignInResponse.swift
index cc597172010..deba2ba1557 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/SignIn/StartMFASignInResponse.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/MultiFactor/SignIn/StartMFASignInResponse.swift
@@ -16,7 +16,8 @@ import Foundation
 
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
 struct StartMFASignInResponse: AuthRPCResponse {
-  var responseInfo: AuthProtoStartMFAPhoneResponseInfo?
+//  var responseInfo: AuthProtoStartMFAPhoneResponseInfo?
+  private(set) var responseInfo: AuthProtoStartMFAPhoneResponseInfo?
 
   mutating func setFields(dictionary: [String: AnyHashable]) throws {
     if let data = dictionary["phoneResponseInfo"] as? [String: AnyHashable] {
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/SendVerificationTokenResponse.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/SendVerificationTokenResponse.swift
index f9e6872cfcb..8f844c25f8c 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/SendVerificationTokenResponse.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/SendVerificationTokenResponse.swift
@@ -15,9 +15,13 @@
 import Foundation
 
 struct SendVerificationCodeResponse: AuthRPCResponse {
-  var verificationID: String?
+  // Default value will be overridden when `setField(dictionary:)` is called.
+  private(set) var verificationID: String = ""
 
   mutating func setFields(dictionary: [String: AnyHashable]) throws {
-    verificationID = dictionary["sessionInfo"] as? String
+    guard let verificationID = dictionary["sessionInfo"] as? String else {
+      throw AuthErrorUtils.unexpectedResponse(deserializedResponse: dictionary)
+    }
+    self.verificationID = verificationID
   }
 }
diff --git a/FirebaseAuth/Tests/Unit/RPCBaseTests.swift b/FirebaseAuth/Tests/Unit/RPCBaseTests.swift
index 009289a1b44..06ef5cf7151 100644
--- a/FirebaseAuth/Tests/Unit/RPCBaseTests.swift
+++ b/FirebaseAuth/Tests/Unit/RPCBaseTests.swift
@@ -97,7 +97,7 @@ class RPCBaseTests: XCTestCase {
         XCTFail("decodedRequest is not a dictionary")
       }
       // Dummy response to unblock await.
-      let _ = try self.rpcIssuer?.respond(withJSON: [:])
+      let _ = try self.rpcIssuer?.respond(withJSON: ["sessionInfo": "verification_id_123"])
     }
     let _ = try await AuthBackend.call(with: request)
   }