diff --git a/mac/Tests/CodeBurnMenubarTests/AppStoreCopilotDisconnectTests.swift b/mac/Tests/CodeBurnMenubarTests/AppStoreCopilotDisconnectTests.swift
index 8a99d045..b6b0ce6f 100644
--- a/mac/Tests/CodeBurnMenubarTests/AppStoreCopilotDisconnectTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/AppStoreCopilotDisconnectTests.swift
@@ -145,11 +145,10 @@ struct AppStoreCopilotDisconnectTests {
 @MainActor
 private func withIsolatedCopilotStore(
     hasCredential: Bool,
+    testName: String = #function,
     _ body: @MainActor (AppStore, CopilotFetchStub, UserDefaults) async throws -> Void
 ) async throws {
-    let suiteName = "codeburn.copilot.disconnect.\(UUID().uuidString)"
-    let defaults = try #require(UserDefaults(suiteName: suiteName))
-    TestDefaults.forget(suiteName)
+    let (defaults, suiteName) = TestDefaults.make("codeburn.copilot.disconnect.\(testName)")
     defer { TestDefaults.forget(suiteName) }
 
     let fetch = CopilotFetchStub()
diff --git a/mac/Tests/CodeBurnMenubarTests/CapacityDockGlanceWindowTests.swift b/mac/Tests/CodeBurnMenubarTests/CapacityDockGlanceWindowTests.swift
index 0017b0a6..056908ac 100644
--- a/mac/Tests/CodeBurnMenubarTests/CapacityDockGlanceWindowTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/CapacityDockGlanceWindowTests.swift
@@ -171,9 +171,8 @@ struct CapacityDockGlanceWindowTests {
 
     // MARK: - Persistence
 
-    private func defaults() -> (UserDefaults, String) {
-        let suiteName = "CodeBurnMenubarTests.CapacityDockGlance.\(UUID().uuidString)"
-        return (UserDefaults(suiteName: suiteName)!, suiteName)
+    private func defaults(_ testName: String = #function) -> (UserDefaults, String) {
+        TestDefaults.make("CodeBurnMenubarTests.CapacityDockGlance.\(testName)")
     }
 
     @Test("the glance window defaults to the billing horizon for every provider")
diff --git a/mac/Tests/CodeBurnMenubarTests/CapacityDockInteractionStateTests.swift b/mac/Tests/CodeBurnMenubarTests/CapacityDockInteractionStateTests.swift
index aede6289..1eb07bc1 100644
--- a/mac/Tests/CodeBurnMenubarTests/CapacityDockInteractionStateTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/CapacityDockInteractionStateTests.swift
@@ -104,8 +104,7 @@ struct CapacityDockDragSettlingTests {
     @MainActor
     @Test("settling when no drag is in flight changes nothing")
     func settlingIdleDockIsANoOp() {
-        let name = "CodeBurnMenubarTests.DragSettle.\(UUID().uuidString)"
-        let defaults = UserDefaults(suiteName: name)!
+        let (defaults, name) = TestDefaults.make("CodeBurnMenubarTests.DragSettle.\(#function)")
         defer { TestDefaults.forget(name) }
 
         let controller = CapacityDockController(store: AppStore(), defaults: defaults)
diff --git a/mac/Tests/CodeBurnMenubarTests/CapacityDockPreferencesTests.swift b/mac/Tests/CodeBurnMenubarTests/CapacityDockPreferencesTests.swift
index 64636314..42f828d3 100644
--- a/mac/Tests/CodeBurnMenubarTests/CapacityDockPreferencesTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/CapacityDockPreferencesTests.swift
@@ -4,9 +4,8 @@ import Testing
 
 @Suite("Capacity Dock preferences")
 struct CapacityDockPreferencesTests {
-    private func defaults() -> (UserDefaults, String) {
-        let suiteName = "CodeBurnMenubarTests.CapacityDock.\(UUID().uuidString)"
-        return (UserDefaults(suiteName: suiteName)!, suiteName)
+    private func defaults(_ testName: String = #function) -> (UserDefaults, String) {
+        TestDefaults.make("CodeBurnMenubarTests.CapacityDock.\(testName)")
     }
 
     @Test("fresh installs keep the dock off with Codex as the only resting provider")
diff --git a/mac/Tests/CodeBurnMenubarTests/CapacityDockPresentationTests.swift b/mac/Tests/CodeBurnMenubarTests/CapacityDockPresentationTests.swift
index 4cc53de3..53234d76 100644
--- a/mac/Tests/CodeBurnMenubarTests/CapacityDockPresentationTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/CapacityDockPresentationTests.swift
@@ -25,8 +25,7 @@ struct CapacityDockPresentationTests {
     @MainActor
     @Test("rail body length follows presentation progress instead of snapping to interaction state")
     func presentationLengthInterpolates() {
-        let suite = "CodeBurnMenubarTests.CapacityDock.Presentation.\(UUID().uuidString)"
-        let defaults = UserDefaults(suiteName: suite)!
+        let (defaults, suite) = TestDefaults.make("CodeBurnMenubarTests.CapacityDock.Presentation")
         defer { TestDefaults.forget(suite) }
         CapacityDockPreferences.setSelectedProviders([.codex, .claude, .gemini], defaults: defaults)
         let model = CapacityDockViewModel(preferences: CapacityDockPreferences.load(defaults: defaults))
@@ -43,8 +42,7 @@ struct CapacityDockPresentationTests {
     @MainActor
     @Test("Resting provider stays at the reveal anchor")
     func restingProviderFollowsExpansionAnchor() {
-        let suite = "CodeBurnMenubarTests.CapacityDock.AnchorOrder.\(UUID().uuidString)"
-        let defaults = UserDefaults(suiteName: suite)!
+        let (defaults, suite) = TestDefaults.make("CodeBurnMenubarTests.CapacityDock.AnchorOrder")
         defer { TestDefaults.forget(suite) }
         CapacityDockPreferences.setSelectedProviders([.codex, .claude, .gemini], defaults: defaults)
         let model = CapacityDockViewModel(preferences: CapacityDockPreferences.load(defaults: defaults))
@@ -59,8 +57,7 @@ struct CapacityDockPresentationTests {
     @MainActor
     @Test("Attachment morphs inside the body without changing the panel size")
     func attachmentKeepsPanelSizeStable() {
-        let suite = "CodeBurnMenubarTests.CapacityDock.EdgeSpread.\(UUID().uuidString)"
-        let defaults = UserDefaults(suiteName: suite)!
+        let (defaults, suite) = TestDefaults.make("CodeBurnMenubarTests.CapacityDock.EdgeSpread")
         defer { TestDefaults.forget(suite) }
         CapacityDockPreferences.setScale(1.2, defaults: defaults)
         let model = CapacityDockViewModel(preferences: CapacityDockPreferences.load(defaults: defaults))
@@ -79,8 +76,7 @@ struct CapacityDockPresentationTests {
     @MainActor
     @Test("Supported settled scales and edges compose integral panel dimensions")
     func supportedSettledGeometryIsIntegral() {
-        let suite = "CodeBurnMenubarTests.CapacityDock.IntegralGeometry.\(UUID().uuidString)"
-        let defaults = UserDefaults(suiteName: suite)!
+        let (defaults, suite) = TestDefaults.make("CodeBurnMenubarTests.CapacityDock.IntegralGeometry")
         defer { TestDefaults.forget(suite) }
         CapacityDockPreferences.setSelectedProviders([.codex, .claude, .gemini], defaults: defaults)
 
@@ -228,8 +224,7 @@ struct CapacityDockPresentationTests {
     @MainActor
     @Test("Selected provider identity remains stable throughout reveal and retraction")
     func selectedProviderIdentityStaysStable() {
-        let suite = "CodeBurnMenubarTests.CapacityDock.Identity.\(UUID().uuidString)"
-        let defaults = UserDefaults(suiteName: suite)!
+        let (defaults, suite) = TestDefaults.make("CodeBurnMenubarTests.CapacityDock.Identity")
         defer { TestDefaults.forget(suite) }
         CapacityDockPreferences.setSelectedProviders([.codex, .claude, .gemini], defaults: defaults)
         CapacityDockPreferences.setPreferredProvider(.codex, defaults: defaults)
@@ -252,8 +247,7 @@ struct CapacityDockPresentationTests {
     @MainActor
     @Test("Resting provider stays visible when the rail expands toward its start edge")
     func restingProviderNeverFlashesToAnotherIcon() {
-        let suite = "CodeBurnMenubarTests.CapacityDock.NoIconFlash.\(UUID().uuidString)"
-        let defaults = UserDefaults(suiteName: suite)!
+        let (defaults, suite) = TestDefaults.make("CodeBurnMenubarTests.CapacityDock.NoIconFlash")
         defer { TestDefaults.forget(suite) }
         CapacityDockPreferences.setSelectedProviders([.codex, .claude, .gemini], defaults: defaults)
         CapacityDockPreferences.setPreferredProvider(.codex, defaults: defaults)
diff --git a/mac/Tests/CodeBurnMenubarTests/CapacityDockProviderQuotaServiceTests.swift b/mac/Tests/CodeBurnMenubarTests/CapacityDockProviderQuotaServiceTests.swift
index 7c5e013a..c954e7fb 100644
--- a/mac/Tests/CodeBurnMenubarTests/CapacityDockProviderQuotaServiceTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/CapacityDockProviderQuotaServiceTests.swift
@@ -436,8 +436,7 @@ struct CapacityDockProviderQuotaServiceTests {
 
     @Test("removeProvider drops only the target and keeps manual selection unlatched")
     func removeProviderPersistence() throws {
-        let suiteName = "CodeBurnMenubarTests.CapacityDockRemove.\(UUID().uuidString)"
-        let defaults = try #require(UserDefaults(suiteName: suiteName))
+        let (defaults, suiteName) = TestDefaults.make("CodeBurnMenubarTests.CapacityDockRemove")
         defer { TestDefaults.forget(suiteName) }
         let cursor = try #require(CapacityDockProvider(rawValue: "cursor"))
         defaults.set(["codex", "cursor"], forKey: CapacityDockPreferences.selectedProvidersKey)
diff --git a/mac/Tests/CodeBurnMenubarTests/CodexBankedResetTests.swift b/mac/Tests/CodeBurnMenubarTests/CodexBankedResetTests.swift
index f14341f8..53df7490 100644
--- a/mac/Tests/CodeBurnMenubarTests/CodexBankedResetTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/CodexBankedResetTests.swift
@@ -351,6 +351,7 @@ struct CodexBankedResetAnnouncerTests {
 
 @MainActor
 private func withAnnouncer(
+    testName: String = #function,
     _ body: @MainActor (
         CodexBankedResetAnnouncer,
         RecordingBankedResetNotifier,
@@ -358,9 +359,7 @@ private func withAnnouncer(
         MemoryBankedResetStore
     ) async throws -> Void
 ) async throws {
-    let suiteName = "codeburn.codex.bankedResets.\(UUID().uuidString)"
-    let defaults = try #require(UserDefaults(suiteName: suiteName))
-    TestDefaults.forget(suiteName)
+    let (defaults, suiteName) = TestDefaults.make("codeburn.codex.bankedResets.\(testName)")
     defer { TestDefaults.forget(suiteName) }
 
     let notifier = RecordingBankedResetNotifier()
diff --git a/mac/Tests/CodeBurnMenubarTests/CopilotQuotaPresentationTests.swift b/mac/Tests/CodeBurnMenubarTests/CopilotQuotaPresentationTests.swift
index ca8dee48..a25dcc53 100644
--- a/mac/Tests/CodeBurnMenubarTests/CopilotQuotaPresentationTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/CopilotQuotaPresentationTests.swift
@@ -50,9 +50,7 @@ struct CopilotQuotaPresentationTests {
 
     @Test("explicit disconnect copy differs from first-use; clearing the flag restores first-use")
     func explicitDisconnectDiffersFromFirstUseAndClears() throws {
-        let suiteName = "codeburn.copilot.presentation.\(UUID().uuidString)"
-        let defaults = try #require(UserDefaults(suiteName: suiteName))
-        TestDefaults.forget(suiteName)
+        let (defaults, suiteName) = TestDefaults.make("codeburn.copilot.presentation.\(#function)")
         defer { TestDefaults.forget(suiteName) }
 
         #expect(!CopilotExplicitDisconnect.isSet(defaults: defaults))
diff --git a/mac/Tests/CodeBurnMenubarTests/CredentialKeychainCacheRedTests.swift b/mac/Tests/CodeBurnMenubarTests/CredentialKeychainCacheRedTests.swift
index dc8c9a51..1de226fc 100644
--- a/mac/Tests/CodeBurnMenubarTests/CredentialKeychainCacheRedTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/CredentialKeychainCacheRedTests.swift
@@ -12,6 +12,7 @@ struct CredentialKeychainCacheRedTests {
     private let accountSentinel = "cb-red-account-sentinel"
 
     private func withIsolatedSeams(
+        testName: String = #function,
         _ body: (URL, InMemoryKeychainCredentialCache) throws -> Void
     ) throws {
         CredentialStoreTestIsolation.lock.lock()
@@ -21,9 +22,7 @@ struct CredentialKeychainCacheRedTests {
             .appendingPathComponent("codeburn-0b-red-\(UUID().uuidString)", isDirectory: true)
         let support = root.appendingPathComponent("Application Support", isDirectory: true)
         try FileManager.default.createDirectory(at: support, withIntermediateDirectories: true)
-        let suiteName = "codeburn.0b.red.\(UUID().uuidString)"
-        let defaults = UserDefaults(suiteName: suiteName)!
-        TestDefaults.forget(suiteName)
+        let (defaults, suiteName) = TestDefaults.make("codeburn.0b.red.\(testName)")
 
         let fakeKeychain = InMemoryKeychainCredentialCache()
         ClaudeCredentialStore.resetTestSeams()
diff --git a/mac/Tests/CodeBurnMenubarTests/CredentialKeychainContinuityTests.swift b/mac/Tests/CodeBurnMenubarTests/CredentialKeychainContinuityTests.swift
index 278f8af3..33574df8 100644
--- a/mac/Tests/CodeBurnMenubarTests/CredentialKeychainContinuityTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/CredentialKeychainContinuityTests.swift
@@ -29,6 +29,7 @@ struct CredentialKeychainContinuityTests {
     }
 
     private func withHarness(
+        testName: String = #function,
         _ body: (Harness) throws -> Void
     ) throws {
         CredentialStoreTestIsolation.lock.lock()
@@ -38,9 +39,7 @@ struct CredentialKeychainContinuityTests {
             .appendingPathComponent("codeburn-0b-cont-\(UUID().uuidString)", isDirectory: true)
         let support = root.appendingPathComponent("Application Support", isDirectory: true)
         try FileManager.default.createDirectory(at: support, withIntermediateDirectories: true)
-        let suiteName = "codeburn.0b.cont.\(UUID().uuidString)"
-        let defaults = UserDefaults(suiteName: suiteName)!
-        TestDefaults.forget(suiteName)
+        let (defaults, suiteName) = TestDefaults.make("codeburn.0b.cont.\(testName)")
         let fake = InMemoryKeychainCredentialCache()
 
         ClaudeCredentialStore.resetTestSeams()
@@ -166,10 +165,8 @@ struct CredentialKeychainContinuityTests {
 
     @Test("concurrent provider-presence updates do not lose identifiers")
     func providerPresenceUpdatesAreAtomic() async {
-        let suiteName = "codeburn.capacity-dock-presence.\(UUID().uuidString)"
-        let defaults = UserDefaults(suiteName: suiteName)!
+        let (defaults, suiteName) = TestDefaults.make("codeburn.capacity-dock-presence.\(#function)")
         let defaultsBox = SendableUserDefaults(defaults)
-        TestDefaults.forget(suiteName)
         defer { TestDefaults.forget(suiteName) }
         let providerIDs = Array(CapacityDockPreferences.supportedProviders.prefix(24).map(\.id))
 
diff --git a/mac/Tests/CodeBurnMenubarTests/DefaultsKeyObserverTests.swift b/mac/Tests/CodeBurnMenubarTests/DefaultsKeyObserverTests.swift
index 1d754249..2d17c6d1 100644
--- a/mac/Tests/CodeBurnMenubarTests/DefaultsKeyObserverTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/DefaultsKeyObserverTests.swift
@@ -7,9 +7,8 @@ import Testing
 /// external write (verified out of process), KVO does, so the observer is what is tested.
 @Suite("Defaults key observer")
 struct DefaultsKeyObserverTests {
-    private func suite() -> (UserDefaults, String) {
-        let name = "CodeBurnMenubarTests.DefaultsKeyObserver.\(UUID().uuidString)"
-        return (UserDefaults(suiteName: name)!, name)
+    private func suite(_ testName: String = #function) -> (UserDefaults, String) {
+        TestDefaults.make("CodeBurnMenubarTests.DefaultsKeyObserver.\(testName)")
     }
 
     @Test("a change to the watched key calls back")
diff --git a/mac/Tests/CodeBurnMenubarTests/EarlyQuotaResetTests.swift b/mac/Tests/CodeBurnMenubarTests/EarlyQuotaResetTests.swift
index 6a03dab9..063d8213 100644
--- a/mac/Tests/CodeBurnMenubarTests/EarlyQuotaResetTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/EarlyQuotaResetTests.swift
@@ -608,11 +608,10 @@ private func seedBaseline(_ monitor: EarlyQuotaResetMonitor) async {
 
 @MainActor
 private func withIsolatedMonitor(
+    testName: String = #function,
     _ body: @MainActor (EarlyQuotaResetMonitor, RecordingEarlyResetNotifier, UserDefaults) async throws -> Void
 ) async throws {
-    let suiteName = "codeburn.quota.earlyReset.\(UUID().uuidString)"
-    let defaults = try #require(UserDefaults(suiteName: suiteName))
-    TestDefaults.forget(suiteName)
+    let (defaults, suiteName) = TestDefaults.make("codeburn.quota.earlyReset.monitor.\(testName)")
     defer { TestDefaults.forget(suiteName) }
 
     let notifier = RecordingEarlyResetNotifier()
@@ -952,11 +951,10 @@ struct EarlyQuotaResetCodexWiringTests {
     }
 
     private func withCodexStore(
+        testName: String = #function,
         _ body: @MainActor (AppStore, RecordingEarlyResetNotifier) async throws -> Void
     ) async throws {
-        let suiteName = "codeburn.quota.earlyReset.\(UUID().uuidString)"
-        let defaults = try #require(UserDefaults(suiteName: suiteName))
-        TestDefaults.forget(suiteName)
+        let (defaults, suiteName) = TestDefaults.make("codeburn.quota.earlyReset.codexWiring.\(testName)")
         defer { TestDefaults.forget(suiteName) }
 
         let notifier = RecordingEarlyResetNotifier()
diff --git a/mac/Tests/CodeBurnMenubarTests/LanguagePreferenceTests.swift b/mac/Tests/CodeBurnMenubarTests/LanguagePreferenceTests.swift
index 57346d9b..0e7bf781 100644
--- a/mac/Tests/CodeBurnMenubarTests/LanguagePreferenceTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/LanguagePreferenceTests.swift
@@ -62,7 +62,7 @@ struct LanguagePreferenceTests {
     func applyWritesAppleLanguages() {
         let domain = domain("apply")
         defer { TestDefaults.forget(domain) }
-        let scratch = UserDefaults(suiteName: domain)!
+        let scratch = TestDefaults.open(domain)
         LanguagePreference.apply(.chineseSimplified, defaults: scratch)
         #expect(read(domain) == .chineseSimplified)
         LanguagePreference.apply(.system, defaults: scratch)
diff --git a/mac/Tests/CodeBurnMenubarTests/MenubarPeriodSettingsTests.swift b/mac/Tests/CodeBurnMenubarTests/MenubarPeriodSettingsTests.swift
index 451240a5..ab210306 100644
--- a/mac/Tests/CodeBurnMenubarTests/MenubarPeriodSettingsTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/MenubarPeriodSettingsTests.swift
@@ -42,8 +42,7 @@ struct MenubarPeriodSettingsTests {
 
     @Test("periods persist canonical defaults values")
     func periodsPersistCanonicalDefaultsValues() {
-        let suiteName = "CodeBurnMenubarTests.\(UUID().uuidString)"
-        let defaults = UserDefaults(suiteName: suiteName)!
+        let (defaults, suiteName) = TestDefaults.make("CodeBurnMenubarTests.\(#function)")
         defer { TestDefaults.forget(suiteName) }
 
         Period.sevenDays.persistAsMenubarDefault(defaults: defaults)
@@ -61,8 +60,7 @@ struct MenubarPeriodSettingsTests {
 
     @Test("menubar scope persistence defaults to local and round-trips")
     func menubarScopePersistenceDefaultsToLocalAndRoundTrips() {
-        let suiteName = "CodeBurnMenubarTests.\(UUID().uuidString)"
-        let defaults = UserDefaults(suiteName: suiteName)!
+        let (defaults, suiteName) = TestDefaults.make("CodeBurnMenubarTests.\(#function)")
         defer { TestDefaults.forget(suiteName) }
 
         #expect(MenubarScope.savedMenubarScope(defaults: defaults) == .local)
diff --git a/mac/Tests/CodeBurnMenubarTests/MenubarSecondRowTests.swift b/mac/Tests/CodeBurnMenubarTests/MenubarSecondRowTests.swift
index 003db8c6..75e8d800 100644
--- a/mac/Tests/CodeBurnMenubarTests/MenubarSecondRowTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/MenubarSecondRowTests.swift
@@ -467,8 +467,7 @@ struct MenubarSecondRowTests {
 
     @Test("settings default to off and round-trip through UserDefaults")
     func preferencesRoundTrip() {
-        let suiteName = "CodeBurnMenubarTests.\(UUID().uuidString)"
-        let defaults = UserDefaults(suiteName: suiteName)!
+        let (defaults, suiteName) = TestDefaults.make("CodeBurnMenubarTests.\(#function)")
         defer { TestDefaults.forget(suiteName) }
 
         #expect(MenubarRowPreferences.load(defaults: defaults) == MenubarRowSettings.default)
diff --git a/mac/Tests/CodeBurnMenubarTests/ProviderSettingsEditorStateTests.swift b/mac/Tests/CodeBurnMenubarTests/ProviderSettingsEditorStateTests.swift
index f56c3123..ccb5e67d 100644
--- a/mac/Tests/CodeBurnMenubarTests/ProviderSettingsEditorStateTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/ProviderSettingsEditorStateTests.swift
@@ -115,8 +115,7 @@ struct ProviderSettingsEditorStateTests {
 
     @Test("presence writes allow a synchronous UserDefaults callback to read presence")
     func presenceWritesDoNotDeadlockSwiftUICallbacks() async throws {
-        let suiteName = "CodeBurnMenubarTests.PresenceReentry.\(UUID().uuidString)"
-        let defaults = try #require(UserDefaults(suiteName: suiteName))
+        let (defaults, suiteName) = TestDefaults.make("CodeBurnMenubarTests.PresenceReentry")
         let defaultsBox = SendableUserDefaults(defaults)
         let callbackFinished = LockedCompletionFlag()
         let writeFinished = LockedCompletionFlag()
diff --git a/mac/Tests/CodeBurnMenubarTests/QuotaCrossingTests.swift b/mac/Tests/CodeBurnMenubarTests/QuotaCrossingTests.swift
index 2ef4d8f6..4f960a4a 100644
--- a/mac/Tests/CodeBurnMenubarTests/QuotaCrossingTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/QuotaCrossingTests.swift
@@ -150,11 +150,10 @@ struct QuotaCrossingMonitorTests {
 
 @MainActor
 private func withMonitor(
+    testName: String = #function,
     _ body: @MainActor (QuotaCrossingMonitor, RecordingCrossingNotifier, UserDefaults) async throws -> Void
 ) async throws {
-    let suiteName = "codeburn.quota.crossing.\(UUID().uuidString)"
-    let defaults = try #require(UserDefaults(suiteName: suiteName))
-    TestDefaults.forget(suiteName)
+    let (defaults, suiteName) = TestDefaults.make("codeburn.quota.crossing.\(testName)")
     defer { TestDefaults.forget(suiteName) }
 
     let notifier = RecordingCrossingNotifier()
diff --git a/mac/Tests/CodeBurnMenubarTests/TelemetryTests.swift b/mac/Tests/CodeBurnMenubarTests/TelemetryTests.swift
index deb873e9..9ef78bf5 100644
--- a/mac/Tests/CodeBurnMenubarTests/TelemetryTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/TelemetryTests.swift
@@ -64,13 +64,16 @@ struct TelemetryTests {
 
     /// A defaults suite and a directory that exist only for one test.
     final class Scratch {
-        let suiteName = "CodeBurnMenubarTests.Telemetry.\(UUID().uuidString)"
+        let suiteName: String
         let directory = FileManager.default.temporaryDirectory
             .appendingPathComponent("codeburn-telemetry-tests-\(UUID().uuidString)")
         let defaults: UserDefaults
 
-        init() {
-            defaults = UserDefaults(suiteName: suiteName)!
+        /// `label` distinguishes several `Scratch`es created by one test
+        /// (a loop, or a few named cases); it must be unique per call site.
+        init(_ label: String = "", testName: String = #function) {
+            let name = label.isEmpty ? testName : "\(testName).\(label)"
+            (defaults, suiteName) = TestDefaults.make("CodeBurnMenubarTests.Telemetry.\(name)")
             try? FileManager.default.createDirectory(
                 at: directory, withIntermediateDirectories: true)
         }
@@ -206,17 +209,17 @@ struct TelemetryTests {
 
     @Test("standalone defaults on outside the default-off region, off inside it and off when unknown")
     func standaloneRegionDefaults() {
-        let us = Scratch()
+        let us = Scratch("us")
         #expect(Self.client(us, region: "US").status().enabled)
 
-        let de = Scratch()
+        let de = Scratch("de")
         #expect(Self.client(de, region: "DE").status().enabled == false)
 
-        let unknown = Scratch()
+        let unknown = Scratch("unknown")
         #expect(Self.client(unknown, region: nil).status().enabled == false)
 
         // A UN M.49 region is nobody's country code, so it reads as unknown.
-        let m49 = Scratch()
+        let m49 = Scratch("m49")
         #expect(Self.client(m49, region: "419").status().enabled == false)
     }
 
@@ -296,7 +299,7 @@ struct TelemetryTests {
     func toggleStateForEveryCombination() {
         for desktopEnabled in [true, false] {
             for vetoed in [true, false] {
-                let scratch = Scratch()
+                let scratch = Scratch("desktop\(desktopEnabled)-veto\(vetoed)")
                 scratch.defaults.set(vetoed, forKey: Telemetry.localOptOutKey)
                 let state = scratch.writeDesktopState(
                     """
@@ -312,7 +315,7 @@ struct TelemetryTests {
             }
         }
 
-        let scratch = Scratch()
+        let scratch = Scratch("standalone")
         let standalone = Self.client(scratch, region: "US")
         #expect(standalone.status().source == .app)
         #expect(standalone.status().isLocked == false, "standalone the toggle is never a readout")
@@ -376,12 +379,12 @@ struct TelemetryTests {
     func standaloneExplicitDecisionSwitch() {
         // Both settings of Telemetry.standaloneRequiresExplicitDecision, so the
         // product call is a one-line change with tests already standing.
-        let regionDecides = Scratch()
+        let regionDecides = Scratch("regionDecides")
         let deciding = Self.client(regionDecides, region: "US", requiresExplicitDecision: false)
         deciding.track("app_open")
         #expect(deciding.queuedEvents.count == 1, "the region default is itself the answer")
 
-        let mustBeAsked = Scratch()
+        let mustBeAsked = Scratch("mustBeAsked")
         let waiting = Self.client(mustBeAsked, region: "US", requiresExplicitDecision: true)
         waiting.track("app_open")
         #expect(waiting.queuedEvents.isEmpty, "nothing before the question has been answered")
diff --git a/mac/Tests/CodeBurnMenubarTests/TerminalLauncherTests.swift b/mac/Tests/CodeBurnMenubarTests/TerminalLauncherTests.swift
index 5b4c5946..cf458cca 100644
--- a/mac/Tests/CodeBurnMenubarTests/TerminalLauncherTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/TerminalLauncherTests.swift
@@ -265,8 +265,7 @@ struct TerminalLauncherTests {
 
     @Test("preference defaults to Terminal.app when unset, preserving pre-#877 behaviour")
     func defaultsToTerminalWhenUnset() {
-        let suiteName = "CodeBurnMenubarTests.\(UUID().uuidString)"
-        let defaults = UserDefaults(suiteName: suiteName)!
+        let (defaults, suiteName) = TestDefaults.make("CodeBurnMenubarTests.\(#function)")
         defer { TestDefaults.forget(suiteName) }
 
         #expect(PreferredTerminal.saved(defaults: defaults) == .terminal)
@@ -275,8 +274,7 @@ struct TerminalLauncherTests {
 
     @Test("preference round-trips and unknown values collapse to the default")
     func preferenceRoundTripsAndRejectsGarbage() {
-        let suiteName = "CodeBurnMenubarTests.\(UUID().uuidString)"
-        let defaults = UserDefaults(suiteName: suiteName)!
+        let (defaults, suiteName) = TestDefaults.make("CodeBurnMenubarTests.\(#function)")
         defer { TestDefaults.forget(suiteName) }
 
         PreferredTerminal.iTerm2.persist(defaults: defaults)
diff --git a/mac/Tests/CodeBurnMenubarTests/TestDefaults.swift b/mac/Tests/CodeBurnMenubarTests/TestDefaults.swift
index 244690f6..78723c22 100644
--- a/mac/Tests/CodeBurnMenubarTests/TestDefaults.swift
+++ b/mac/Tests/CodeBurnMenubarTests/TestDefaults.swift
@@ -11,6 +11,26 @@ import Foundation
 /// right after creating it -- so a full `swift test` run does not grow the
 /// file count under Preferences.
 enum TestDefaults {
+    /// A `UserDefaults` suite named `name`, paired with that name for
+    /// `forget(_:)`. `name` must be fixed, not a uuid: cfprefsd can still
+    /// write a cleared domain's plist back to disk after `forget(_:)`
+    /// deletes it, and a fixed name lands that write on the same file every
+    /// run instead of a new one. A shared helper, a loop, or a parameterized
+    /// test must fold something distinguishing -- the caller's `#function`,
+    /// an index, an argument -- into `name` so no two tests share one.
+    static func make(_ name: String) -> (UserDefaults, String) {
+        (open(name), name)
+    }
+
+    /// Opens a suite/domain name as `UserDefaults`, for call sites that
+    /// already have their own name instead of calling `make(_:)`. Forgets it
+    /// first, so a fixed name starts clean even after a run that got killed
+    /// before its own `forget(_:)` ran.
+    static func open(_ suiteName: String) -> UserDefaults {
+        forget(suiteName)
+        return UserDefaults(suiteName: suiteName)!
+    }
+
     static func forget(_ suiteName: String) {
         // A `UserDefaults` instance scoped to the suite -- not `.standard` --
         // so `synchronize()` below flushes *this* domain's own dirty state.
diff --git a/mac/Tests/CodeBurnMenubarTests/UpdateNotificationTests.swift b/mac/Tests/CodeBurnMenubarTests/UpdateNotificationTests.swift
index 476285ab..14b06a92 100644
--- a/mac/Tests/CodeBurnMenubarTests/UpdateNotificationTests.swift
+++ b/mac/Tests/CodeBurnMenubarTests/UpdateNotificationTests.swift
@@ -98,11 +98,10 @@ struct UpdateNotificationTests {
 
 @MainActor
 private func withIsolatedChecker(
+    testName: String = #function,
     _ body: @MainActor (UpdateChecker, RecordingUpdateNotifier, UserDefaults) async throws -> Void
 ) async throws {
-    let suiteName = "codeburn.update.notifications.\(UUID().uuidString)"
-    let defaults = try #require(UserDefaults(suiteName: suiteName))
-    TestDefaults.forget(suiteName)
+    let (defaults, suiteName) = TestDefaults.make("codeburn.update.notifications.\(testName)")
     defer { TestDefaults.forget(suiteName) }
 
     let notifier = RecordingUpdateNotifier()
diff --git a/mac/Tests/CodeBurnMenubarTests/UserDefaultsSuiteLeakGuardTests.swift b/mac/Tests/CodeBurnMenubarTests/UserDefaultsSuiteLeakGuardTests.swift
new file mode 100644
index 00000000..f5161c93
--- /dev/null
+++ b/mac/Tests/CodeBurnMenubarTests/UserDefaultsSuiteLeakGuardTests.swift
@@ -0,0 +1,83 @@
+import Foundation
+import Testing
+
+/// Guards the fix for the test-only `UserDefaults(suiteName:` leak: every
+/// scratch suite must go through `TestDefaults.make`/`open`, whose only
+/// direct call sites live in `TestDefaults.swift`, so its plist actually gets
+/// cleaned up via `TestDefaults.forget`.
+@Suite("UserDefaults suite leak guard")
+struct UserDefaultsSuiteLeakGuardTests {
+    private static var testsDirectory: URL {
+        URL(fileURLWithPath: #filePath)
+            .deletingLastPathComponent()  // CodeBurnMenubarTests
+            .deletingLastPathComponent()  // Tests
+    }
+
+    @Test("the guard can actually see the files it is meant to check")
+    func testsAreReachable() throws {
+        let files = try swiftFiles(in: Self.testsDirectory)
+        #expect(
+            files.count > 10,
+            "found \(files.count) Swift files under \(Self.testsDirectory.path); if the tree moved, this guard checks nothing"
+        )
+    }
+
+    @Test("no test file creates a UserDefaults suite outside TestDefaults.swift")
+    func noDirectSuiteCreation() throws {
+        let needle = "UserDefaults(" + "suiteName"
+        let exempt: Set<String> = ["TestDefaults.swift", "UserDefaultsSuiteLeakGuardTests.swift"]
+        var offenders: [String] = []
+        for file in try swiftFiles(in: Self.testsDirectory) where !exempt.contains(file.lastPathComponent) {
+            let contents = try String(contentsOf: file, encoding: .utf8)
+            for (index, line) in contents.components(separatedBy: "\n").enumerated() where line.contains(needle) {
+                offenders.append("\(file.lastPathComponent):\(index + 1)")
+            }
+        }
+        #expect(
+            offenders.isEmpty,
+            """
+            \(offenders.count) test call site(s) create a UserDefaults suite directly, \
+            bypassing TestDefaults.forget and leaking its plist: \(offenders). \
+            Use TestDefaults.make(_:) or TestDefaults.open(_:) instead.
+            """
+        )
+    }
+
+    @Test("no two TestDefaults.make/open call sites use the same static suite name")
+    func noDuplicateSuiteNames() throws {
+        let pattern = try NSRegularExpression(pattern: #"TestDefaults\.(?:make|open)\(\s*"([^"]*)""#)
+        var locations: [String: [String]] = [:]
+        for file in try swiftFiles(in: Self.testsDirectory) where file.lastPathComponent != "TestDefaults.swift" {
+            let contents = try String(contentsOf: file, encoding: .utf8)
+            for (index, line) in contents.components(separatedBy: "\n").enumerated() {
+                let range = NSRange(line.startIndex..., in: line)
+                guard let match = pattern.firstMatch(in: line, range: range),
+                      let nameRange = Range(match.range(at: 1), in: line) else { continue }
+                let name = String(line[nameRange])
+                // A name built from the caller's own `#function` is unique per call
+                // site by construction (a different function, a different name);
+                // only a name with no such per-caller piece can silently collide.
+                guard !name.contains("#function") else { continue }
+                locations[name, default: []].append("\(file.lastPathComponent):\(index + 1)")
+            }
+        }
+        let duplicates = locations.filter { $0.value.count > 1 }
+        #expect(
+            duplicates.isEmpty,
+            """
+            \(duplicates.count) suite name(s) are reused across call sites, so two tests \
+            can write and forget the same plist concurrently: \
+            \(duplicates.map { "\"\($0.key)\" at \($0.value)" }.joined(separator: "; ")). \
+            Fold something distinguishing into the name -- the caller's #function, a loop \
+            index, a parameterized test's argument.
+            """
+        )
+    }
+
+    private func swiftFiles(in directory: URL) throws -> [URL] {
+        guard let enumerator = FileManager.default.enumerator(
+            at: directory, includingPropertiesForKeys: nil
+        ) else { return [] }
+        return enumerator.compactMap { $0 as? URL }.filter { $0.pathExtension == "swift" }
+    }
+}