diff --git a/cmux.xcodeproj/project.pbxproj b/cmux.xcodeproj/project.pbxproj
index 0e832f4f7c36..95c4d9f7e5d9 100644
--- a/cmux.xcodeproj/project.pbxproj
+++ b/cmux.xcodeproj/project.pbxproj
@@ -17705,6 +17705,7 @@
 				CODE_SIGN_STYLE = Automatic;
 				CURRENT_PROJECT_VERSION = 106;
 				GENERATE_INFOPLIST_FILE = YES;
+				INFOPLIST_KEY_NSPrincipalClass = CmuxTestsPrincipal;
 				MACOSX_DEPLOYMENT_TARGET = 14.0;
 				MARKETING_VERSION = 0.64.25;
 				ONLY_ACTIVE_ARCH = YES;
@@ -17725,6 +17726,7 @@
 				CODE_SIGN_STYLE = Automatic;
 				CURRENT_PROJECT_VERSION = 106;
 				GENERATE_INFOPLIST_FILE = YES;
+				INFOPLIST_KEY_NSPrincipalClass = CmuxTestsPrincipal;
 				MACOSX_DEPLOYMENT_TARGET = 14.0;
 				MARKETING_VERSION = 0.64.25;
 				ONLY_ACTIVE_ARCH = NO;
diff --git a/cmuxTests/AppDelegateDisplayConfigRestoreTests.swift b/cmuxTests/AppDelegateDisplayConfigRestoreTests.swift
index 4a0249200440..59de86f6c07c 100644
--- a/cmuxTests/AppDelegateDisplayConfigRestoreTests.swift
+++ b/cmuxTests/AppDelegateDisplayConfigRestoreTests.swift
@@ -8,7 +8,7 @@ import Testing
 @testable import cmux
 #endif
 /// Round-trip coverage for per-monitor window-geometry memory (issue #2135).
-@Suite(.serialized)
+@Suite(.serialized, .exclusiveAppContext)
 @MainActor
 struct AppDelegateDisplayConfigRestoreTests {
     // MARK: fixtures
diff --git a/cmuxTests/AppDelegateMainWindowTestingSupport.swift b/cmuxTests/AppDelegateMainWindowTestingSupport.swift
index c8f460fc214d..77d0a381344d 100644
--- a/cmuxTests/AppDelegateMainWindowTestingSupport.swift
+++ b/cmuxTests/AppDelegateMainWindowTestingSupport.swift
@@ -1,5 +1,8 @@
 import AppKit
+import CmuxTerminal
 import Foundation
+import Testing
+import XCTest
 
 #if canImport(cmux_DEV)
 @testable import cmux_DEV
@@ -290,3 +293,76 @@ struct PortalRenderingAuthorityUnavailable: Error, CustomStringConvertible {
 final class KeyStatusTestWindow: NSWindow {
     override var isKeyWindow: Bool { true }
 }
+
+/// The cmuxTests bundle's NSPrincipalClass. XCTest creates it when the bundle
+/// loads, before the first test, and it restores `AppDelegate.shared` after
+/// every XCTest case.
+///
+/// `AppDelegate.init` installs the new delegate as `shared`, and hundreds of
+/// tests build a throwaway delegate without restoring the host's. Whichever
+/// test ran next in the same host inherited the leftover, and which tests
+/// share a host depends on the timing-based shard layout, so the resulting
+/// failures moved from run to run. `AppDelegate.init` also points the surface
+/// registry's weak route retirer at itself, so that is put back too. Swift
+/// Testing tests are not observed here; a Swift Testing suite that constructs
+/// `AppDelegate()` or reads `shared` across a suspension point takes
+/// `.exclusiveAppContext`, which serializes it with the other app-context tests
+/// and restores `shared` the same way.
+@objc(CmuxTestsPrincipal)
+final class CmuxTestsPrincipal: NSObject, XCTestObservation {
+    private var sharedAtStart: AppDelegate?
+
+    override init() {
+        super.init()
+        XCTestObservationCenter.shared.addTestObserver(self)
+    }
+
+    func testCaseWillStart(_ testCase: XCTestCase) {
+        sharedAtStart = AppDelegate.shared
+    }
+
+    func testCaseDidFinish(_ testCase: XCTestCase) {
+        if AppDelegate.shared !== sharedAtStart {
+            AppDelegate.shared = sharedAtStart
+            if let sharedAtStart {
+                GhosttyApp.terminalSurfaceRegistry.attachRouteRetirer(sharedAtStart)
+            }
+        }
+        sharedAtStart = nil
+    }
+}
+
+/// Swift Testing counterpart of `CmuxTestsPrincipal`: runs each test in the
+/// suite inside `AppContextSerialGate`, so suites in parallel cannot swap
+/// `AppDelegate.shared` under each other at a suspension point, and then puts
+/// `shared` and the surface registry's route retirer back.
+struct ExclusiveAppContextTrait: SuiteTrait, TestTrait, TestScoping {
+    var isRecursive: Bool { true }
+
+    func scopeProvider(for test: Test, testCase: Test.Case?) -> Self? {
+        testCase == nil ? nil : self
+    }
+
+    func provideScope(
+        for test: Test,
+        testCase: Test.Case?,
+        performing function: @Sendable () async throws -> Void
+    ) async throws {
+        try await AppContextSerialGate.withExclusiveAppContext {
+            let sharedAtStart = AppDelegate.shared
+            defer {
+                if AppDelegate.shared !== sharedAtStart {
+                    AppDelegate.shared = sharedAtStart
+                    if let sharedAtStart {
+                        GhosttyApp.terminalSurfaceRegistry.attachRouteRetirer(sharedAtStart)
+                    }
+                }
+            }
+            try await function()
+        }
+    }
+}
+
+extension Trait where Self == ExclusiveAppContextTrait {
+    static var exclusiveAppContext: Self { Self() }
+}
diff --git a/cmuxTests/NewCloudWorkspaceShortcutTests.swift b/cmuxTests/NewCloudWorkspaceShortcutTests.swift
index b1331828b9b0..a99aea1af232 100644
--- a/cmuxTests/NewCloudWorkspaceShortcutTests.swift
+++ b/cmuxTests/NewCloudWorkspaceShortcutTests.swift
@@ -14,7 +14,7 @@ import Testing
 /// rows with their live shortcut hints, and the shared action every
 /// entrypoint routes through.
 @MainActor
-@Suite(.serialized)
+@Suite(.serialized, .exclusiveAppContext)
 final class NewCloudWorkspaceShortcutTests {
     private final class RecordingSheetPresenter: NewMachineSheetPresenting {
         private(set) var presentCount = 0
diff --git a/cmuxTests/WindowAndDragTests.swift b/cmuxTests/WindowAndDragTests.swift
index cc9a2d4ec8bc..59d2d8f6f1fe 100644
--- a/cmuxTests/WindowAndDragTests.swift
+++ b/cmuxTests/WindowAndDragTests.swift
@@ -587,6 +587,32 @@ final class AppDelegateWindowContextRoutingTests: XCTestCase {
 }
 
 
+/// `AppDelegate.init` installs the new delegate as `AppDelegate.shared`, and
+/// many tests build a throwaway one without putting the host's back. The next
+/// test in the same host then ran against the leftover: detached-inspector
+/// Cmd-W tests failed on main whenever the shard layout placed them after
+/// AppDelegateWindowContextRoutingTests. XCTest runs these two in name order.
+@MainActor
+final class AppDelegateSharedIsolationTests: XCTestCase {
+    private static var sharedBeforeLeak: AppDelegate??
+
+    func test1ConstructingAnAppDelegateReplacesShared() {
+        Self.sharedBeforeLeak = .some(AppDelegate.shared)
+        let leaked = AppDelegate()
+        XCTAssertTrue(AppDelegate.shared === leaked)
+    }
+
+    func test2NextTestStartsWithTheHostSharedDelegate() throws {
+        guard let expected = Self.sharedBeforeLeak else {
+            throw XCTSkip("Runs after test1ConstructingAnAppDelegateReplacesShared in the same host")
+        }
+        XCTAssertTrue(
+            AppDelegate.shared === expected,
+            "A delegate a previous test constructed must not stay installed as AppDelegate.shared"
+        )
+    }
+}
+
 @MainActor
 final class AppDelegateLaunchServicesRegistrationTests: XCTestCase {
     func testDefaultTerminalRegistrationKeepsAllAdvertisedTargets() {
diff --git a/cmuxTests/WorkspaceGroupCycleShortcutTests.swift b/cmuxTests/WorkspaceGroupCycleShortcutTests.swift
index 4f1cebc08f44..107b40839c9f 100644
--- a/cmuxTests/WorkspaceGroupCycleShortcutTests.swift
+++ b/cmuxTests/WorkspaceGroupCycleShortcutTests.swift
@@ -11,7 +11,7 @@ import Testing
 
 #if DEBUG
 @MainActor
-@Suite("Workspace group cycle shortcuts", .serialized)
+@Suite("Workspace group cycle shortcuts", .serialized, .exclusiveAppContext)
 struct WorkspaceGroupCycleShortcutTests {
     @Test func actionsAreVisibleAndUnboundByDefault() throws {
         let actions: [KeyboardShortcutSettings.Action] = [