diff --git a/Package.swift b/Package.swift
index e1ebd75..c80fee6 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 0d56ebb..b232877 100644
--- a/Sources/StubNetworkKit/StubURLProtocol.swift
+++ b/Sources/StubNetworkKit/StubURLProtocol.swift
@@ -5,7 +5,7 @@ import FoundationNetworking
 
 // based on https://github.com/417-72KI/MultipartFormDataParser/blob/main/Tests/MultipartFormDataParserTests/StubURLProtocol.swift
 final class StubURLProtocol: URLProtocol {
-    nonisolated(unsafe) private(set) static var stubs: [Stub] = []
+    private static let stubs = Lock([Stub]())
 
     override static func canInit(with request: URLRequest) -> Bool {
         true
@@ -54,17 +54,22 @@ final class StubURLProtocol: URLProtocol {
 
 extension StubURLProtocol {
     static func register(_ stub: Stub) {
-        stubs.append(stub)
+        stubs.withLock { stubs in
+            stubs.append(stub)
+        }
     }
 
     static func reset() {
-        stubs = []
+        stubs.withLock {
+            $0 = []
+        }
     }
 }
 
 private extension StubURLProtocol {
     func stub(with request: URLRequest) -> Stub? {
-        Self.stubs
-            .last(where: { $0.matcher(request) })
+        Self.stubs.withLock {
+            $0.last(where: { $0.matcher(request) })
+        }
     }
 }
diff --git a/Sources/StubNetworkKit/Util/Lock.swift b/Sources/StubNetworkKit/Util/Lock.swift
new file mode 100644
index 0000000..78165a8
--- /dev/null
+++ b/Sources/StubNetworkKit/Util/Lock.swift
@@ -0,0 +1,39 @@
+import Foundation
+
+#if canImport(os)
+import os
+
+final class Lock<State: Sendable>: Sendable {
+    let lock: OSAllocatedUnfairLock<State>
+
+    init(_ initialState: State) {
+        lock = .init(initialState: initialState)
+    }
+}
+
+extension Lock {
+    @inlinable
+    func withLock<R: Sendable>(_ body: @Sendable (inout State) throws -> R) rethrows -> R {
+        try lock.withLock(body)
+    }
+}
+#elseif canImport(Synchronization)
+import Synchronization
+
+@available(macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0, *)
+final class Lock<Value: Sendable>: Sendable {
+    let lock: Mutex<Value>
+
+    init(_ initialValue: Value) {
+        lock = .init(initialValue)
+    }
+}
+
+@available(macOS 15.0, iOS 18.0, watchOS 11.0, tvOS 18.0, visionOS 2.0, *)
+extension Lock {
+    @inlinable
+    func withLock<Result: ~Copyable, E: Error>(_ body: (inout sending Value) throws(E) -> sending Result) throws(E) -> sending Result {
+        try lock.withLock(body)
+    }
+}
+#endif
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,