diff --git a/Package.swift b/Package.swift
index 491da38..8a1b338 100644
--- a/Package.swift
+++ b/Package.swift
@@ -39,7 +39,6 @@ let package = Package(
 
 if isDevelop {
     package.dependencies.append(contentsOf: [
-        .package(url: "https://github.com/apple/swift-testing", exact: "0.3.0"),
         .package(url: "https://github.com/Alamofire/Alamofire.git", from: "5.10.0"),
     ])
     if isObjcAvailable {
@@ -53,7 +52,6 @@ if isDevelop {
         name: "StubNetworkKitTests",
         dependencies: [
             "StubNetworkKit",
-            .product(name: "Testing", package: "swift-testing"),
             "Alamofire",
         ],
         resources: [.copy("_Fixtures")]
@@ -63,6 +61,14 @@ if isDevelop {
             "APIKit",
         ])
     }
+    #if compiler(<6.0)
+        package.dependencies.append(contentsOf: [
+            .package(url: "https://github.com/apple/swift-testing", exact: "0.3.0"),
+        ])
+        testTarget.dependencies.append(contentsOf: [
+            .product(name: "Testing", package: "swift-testing"),
+        ])
+    #endif
     package.targets.append(testTarget)
 
     if isObjcAvailable {
diff --git a/Sources/StubNetworkKit/StubURLProtocol.swift b/Sources/StubNetworkKit/StubURLProtocol.swift
index 27337fa..28068da 100644
--- a/Sources/StubNetworkKit/StubURLProtocol.swift
+++ b/Sources/StubNetworkKit/StubURLProtocol.swift
@@ -11,6 +11,8 @@ final class StubURLProtocol: URLProtocol {
     private(set) static var stubs: [Stub] = []
     #endif
 
+    private static let lock = NSLock()
+
     override static func canInit(with request: URLRequest) -> Bool {
         true
     }
@@ -58,10 +60,14 @@ final class StubURLProtocol: URLProtocol {
 
 extension StubURLProtocol {
     static func register(_ stub: Stub) {
+        lock.lock()
+        defer { lock.unlock() }
         stubs.append(stub)
     }
 
     static func reset() {
+        lock.lock()
+        defer { lock.unlock() }
         stubs = []
     }
 }
diff --git a/TestPlan.xctestplan b/TestPlan.xctestplan
index b39a742..1b0ae2f 100644
--- a/TestPlan.xctestplan
+++ b/TestPlan.xctestplan
@@ -11,7 +11,6 @@
   "defaultOptions" : {
     "maximumTestRepetitions" : 3,
     "nsZombieEnabled" : true,
-    "repeatInNewRunnerProcess" : true,
     "testRepetitionMode" : "retryOnFailure",
     "testTimeoutsEnabled" : true
   },
diff --git a/Tests/StubNetworkKitTests/Examples/StubNetworkKitTests_SwiftTesting.swift b/Tests/StubNetworkKitTests/Examples/StubNetworkKitTests_SwiftTesting.swift
index e0f40f1..5fb7952 100644
--- a/Tests/StubNetworkKitTests/Examples/StubNetworkKitTests_SwiftTesting.swift
+++ b/Tests/StubNetworkKitTests/Examples/StubNetworkKitTests_SwiftTesting.swift
@@ -5,7 +5,11 @@ import FoundationNetworking
 import Testing
 import StubNetworkKit
 
+#if compiler(<6.0)
 @Suite
+#else
+@Suite(.serialized)
+#endif
 final class StubNetworkKitTests_SwiftTesting {
     init() {
         StubNetworking.option(printDebugLog: true,