diff --git a/AppUpdater.swift b/AppUpdater.swift
index 286fd04..a3df8b9 100644
--- a/AppUpdater.swift
+++ b/AppUpdater.swift
@@ -361,7 +361,7 @@ public enum AppUpdaterError: LocalizedError, Equatable {
         case .processFailed(let executable, let status, let stderr):
             "\(executable.path) failed with status \(status): \(stderr)"
         case .relaunchFailed:
-            "The updated app did not launch a new application instance."
+            "The updated app could not be relaunched safely."
         case .resourceLimitExceeded(let resource):
             "The update exceeded the configured \(resource) limit."
         case .rollbackFailed:
@@ -1066,6 +1066,7 @@ enum ApplicationLauncher {
         else {
             throw AppUpdaterError.invalidDownloadedBundle
         }
+        var probe: Process?
         try await launchNewInstance(
             spawn: {
                 let process = Process()
@@ -1074,6 +1075,7 @@ enum ApplicationLauncher {
                 process.standardOutput = FileHandle.nullDevice
                 process.standardError = FileHandle.nullDevice
                 try process.run()
+                probe = process
                 return process.processIdentifier
             },
             isReady: { processIdentifier in
@@ -1085,10 +1087,13 @@ enum ApplicationLauncher {
                     application.bundleIdentifier == identifier,
                     application.executableURL?.resolvingSymlinksInPath() == executableURL
                 else { return false }
-                application.activate()
                 return true
             },
-            armFallback: { _ in
+            terminateProbe: {
+                if probe?.isRunning == true { probe?.terminate() }
+            },
+            isProbeTerminated: { probe?.isRunning == false },
+            armFallback: {
                 try armFallback(
                     oldProcessIdentifier: getpid(),
                     applicationURL: url
@@ -1130,14 +1135,23 @@ enum ApplicationLauncher {
         attempts: Int = 200,
         spawn: () throws -> pid_t,
         isReady: (pid_t) -> Bool,
-        armFallback: (pid_t) throws -> Void = { _ in },
+        terminateProbe: () -> Void,
+        isProbeTerminated: () -> Bool,
+        armFallback: () throws -> Void = {},
         pause: () async throws -> Void
     ) async throws {
         let processIdentifier = try spawn()
         for _ in 0..<attempts {
             if isReady(processIdentifier) {
-                try armFallback(processIdentifier)
-                return
+                terminateProbe()
+                for _ in 0..<attempts {
+                    if isProbeTerminated() {
+                        try armFallback()
+                        return
+                    }
+                    try await pause()
+                }
+                throw AppUpdaterError.relaunchFailed
             }
             try await pause()
         }
diff --git a/README.md b/README.md
index f41fd6f..2e756ee 100644
--- a/README.md
+++ b/README.md
@@ -82,9 +82,10 @@ decide not to continue.
 Call `installAndRelaunch()` only after the host has saved its state, stopped
 background work, and ceased loading bundle code or resources. The running
 instance moves itself to a backup, copies the validated candidate into its old
-path, validates the installed copy, and launches a new instance. It restores
-the backup if copying, final validation, or launch fails. The old instance exits
-after the new instance launches.
+path, and validates that the installed copy launches. It stops the temporary
+validation instance before committing the transaction and restores the backup
+if copying, final validation, launch, or probe termination fails. On success,
+the old instance exits and an armed fallback launches the installed app.
 
 > [!IMPORTANT]
 > Quiescing must not cause the old instance to exit. In particular, hosts that
diff --git a/Tests/AppUpdaterTests/AppUpdaterTests.swift b/Tests/AppUpdaterTests/AppUpdaterTests.swift
index 7e5a4f9..1793124 100644
--- a/Tests/AppUpdaterTests/AppUpdaterTests.swift
+++ b/Tests/AppUpdaterTests/AppUpdaterTests.swift
@@ -916,10 +916,12 @@ final class AppUpdaterTests: XCTestCase {
     }
 
     @MainActor
-    func testRelaunchWaitsForSpawnedProcess() async throws {
-        var samples = [false, false, true]
+    func testRelaunchTerminatesProbeBeforeArmingFallback() async throws {
+        var readiness = [false, false, true]
+        var termination = [false, true]
+        var events: [String] = []
         var launches = 0
-        var protectedProcessIdentifier: pid_t?
+        var armedFallback = false
         try await ApplicationLauncher.launchNewInstance(
             attempts: 3,
             spawn: {
@@ -928,13 +930,24 @@ final class AppUpdaterTests: XCTestCase {
             },
             isReady: { processIdentifier in
                 XCTAssertEqual(processIdentifier, 42)
-                return samples.removeFirst()
+                return readiness.removeFirst()
+            },
+            terminateProbe: { events.append("terminate probe") },
+            isProbeTerminated: {
+                events.append("check probe")
+                return termination.removeFirst()
+            },
+            armFallback: {
+                events.append("arm fallback")
+                armedFallback = true
             },
-            armFallback: { protectedProcessIdentifier = $0 },
             pause: {}
         )
         XCTAssertEqual(launches, 1)
-        XCTAssertEqual(protectedProcessIdentifier, 42)
+        XCTAssertTrue(armedFallback)
+        XCTAssertEqual(events, [
+            "terminate probe", "check probe", "check probe", "arm fallback",
+        ])
     }
 
     @MainActor
@@ -944,6 +957,8 @@ final class AppUpdaterTests: XCTestCase {
                 attempts: 2,
                 spawn: { 42 },
                 isReady: { _ in false },
+                terminateProbe: {},
+                isProbeTerminated: { false },
                 pause: {}
             )
             XCTFail("launch should fail until the spawned process is ready")
@@ -952,6 +967,26 @@ final class AppUpdaterTests: XCTestCase {
         }
     }
 
+    @MainActor
+    func testRelaunchRejectsUnstoppableProbe() async {
+        var armedFallback = false
+        do {
+            try await ApplicationLauncher.launchNewInstance(
+                attempts: 1,
+                spawn: { 42 },
+                isReady: { _ in true },
+                terminateProbe: {},
+                isProbeTerminated: { false },
+                armFallback: { armedFallback = true },
+                pause: {}
+            )
+            XCTFail("launch should fail while the validation process is running")
+        } catch {
+            XCTAssertEqual(error as? AppUpdaterError, .relaunchFailed)
+            XCTAssertFalse(armedFallback)
+        }
+    }
+
     @MainActor
     func testInstalledValidationFailureRollsBack() async throws {
         var validations = 0