diff --git a/FirebaseAuth/Sources/Swift/Backend/IdentityToolkitRequest.swift b/FirebaseAuth/Sources/Swift/Backend/IdentityToolkitRequest.swift
index 3ee424459a5..3a5d5a3b18a 100644
--- a/FirebaseAuth/Sources/Swift/Backend/IdentityToolkitRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/IdentityToolkitRequest.swift
@@ -19,7 +19,15 @@ private let kHttpProtocol = "http:"
 
 private let kEmulatorHostAndPrefixFormat = "%@/%@"
 
-private var gAPIHost = "www.googleapis.com"
+#if compiler(>=6)
+  /// Host for server API calls. This should be changed via
+  /// `IdentityToolkitRequest.setHost(_ host:)` for testing purposes only.
+  private nonisolated(unsafe) var gAPIHost = "www.googleapis.com"
+#else
+  /// Host for server API calls. This should be changed via
+  /// `IdentityToolkitRequest.setHost(_ host:)` for testing purposes only.
+  private var gAPIHost = "www.googleapis.com"
+#endif // compiler(>=6)
 
 private let kFirebaseAuthAPIHost = "www.googleapis.com"
 private let kIdentityPlatformAPIHost = "identitytoolkit.googleapis.com"
diff --git a/FirebaseAuth/Sources/Swift/Backend/RPC/SecureTokenRequest.swift b/FirebaseAuth/Sources/Swift/Backend/RPC/SecureTokenRequest.swift
index c07d60c7a60..9dfb1af0e3c 100644
--- a/FirebaseAuth/Sources/Swift/Backend/RPC/SecureTokenRequest.swift
+++ b/FirebaseAuth/Sources/Swift/Backend/RPC/SecureTokenRequest.swift
@@ -54,8 +54,15 @@ private let kRefreshTokenKey = "refreshToken"
 /// The key for the "code" parameter in the request.
 private let kCodeKey = "code"
 
-/// Host for server API calls.
-private var gAPIHost = "securetoken.googleapis.com"
+#if compiler(>=6)
+  /// Host for server API calls. This should be changed via
+  /// `SecureTokenRequest.setHost(_ host:)` for testing purposes only.
+  private nonisolated(unsafe) var gAPIHost = "securetoken.googleapis.com"
+#else
+  /// Host for server API calls. This should be changed via
+  /// `SecureTokenRequest.setHost(_ host:)` for testing purposes only.
+  private var gAPIHost = "securetoken.googleapis.com"
+#endif // compiler(>=6)
 
 /// Represents the parameters for the token endpoint.
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)