diff --git a/app/Jobs/ApplicationDeploymentJob.php b/app/Jobs/ApplicationDeploymentJob.php
index e9203cbab4..6a3101e1cc 100644
--- a/app/Jobs/ApplicationDeploymentJob.php
+++ b/app/Jobs/ApplicationDeploymentJob.php
@@ -310,6 +310,7 @@ class ApplicationDeploymentJob implements ShouldBeEncrypted, ShouldQueue
         $this->application_deployment_queue->refresh();
         if ($this->application_deployment_queue->status === ApplicationDeploymentStatus::CANCELLED_BY_USER->value) {
             $this->application_deployment_queue->addLogEntry('Deployment was cancelled before starting.');
+            $this->handleCancelledDeployment();
 
             return;
         }
@@ -659,7 +660,7 @@ class ApplicationDeploymentJob implements ShouldBeEncrypted, ShouldQueue
     /**
      * An inline Dockerfile image has no commit tag. The main server builds it and pushes it with the
      * `latest` tag just before it queues the additional servers, so an additional server pulls that
-     * image instead of building its own. A failed pull fails the deployment.
+     * image instead of building its own. A missing image fails the deployment with a clear message.
      */
     private function pull_image_for_additional_server(): bool
     {
@@ -667,11 +668,28 @@ class ApplicationDeploymentJob implements ShouldBeEncrypted, ShouldQueue
             return false;
         }
 
+        $image = escapeshellarg($this->production_image_name);
         $this->application_deployment_queue->addLogEntry("Pulling image ({$this->production_image_name}) that the main server pushed. Build step skipped.");
-        $this->execute_remote_command([
-            'docker pull '.escapeshellarg($this->production_image_name),
-            'hidden' => true,
-        ]);
+        $this->execute_remote_command(
+            [
+                "docker pull {$image}",
+                'hidden' => true,
+                'ignore_errors' => true,
+            ],
+            [
+                "docker images -q {$image} 2>/dev/null",
+                'hidden' => true,
+                'save' => 'local_image_found',
+            ],
+        );
+
+        if (str($this->saved_outputs->get('local_image_found'))->isEmpty()) {
+            $message = "Image ({$this->production_image_name}) not found in the registry; the main server must push it first.";
+            if (! $this->server->canBuildApplications()) {
+                $message .= " The server ({$this->server->name}) is set to deployments only and cannot build it.";
+            }
+            throw new DeploymentException($message);
+        }
 
         return true;
     }
@@ -5614,12 +5632,24 @@ COPY ./nginx.conf /etc/nginx/conf.d/default.conf");
 
         if ($this->application_deployment_queue->status === ApplicationDeploymentStatus::CANCELLED_BY_USER->value) {
             $this->application_deployment_queue->addLogEntry('Deployment cancelled by user, stopping execution.');
+            $this->handleCancelledDeployment();
             throw new DeploymentException('Deployment cancelled by user', 69420);
         }
 
         return false;
     }
 
+    /**
+     * A cancelled deployment on an additional server can be the last one of a multi-server
+     * deployment, so it must also trigger the combined notification.
+     */
+    private function handleCancelledDeployment(): void
+    {
+        if (filled($this->application_deployment_queue->parent_deployment_uuid)) {
+            $this->sendMultiServerDeploymentNotification($this->application_deployment_queue->parent_deployment_uuid);
+        }
+    }
+
     /**
      * Update the deployment status in the database.
      */
diff --git a/bootstrap/helpers/applications.php b/bootstrap/helpers/applications.php
index 8dc623af41..8fe4c0d393 100644
--- a/bootstrap/helpers/applications.php
+++ b/bootstrap/helpers/applications.php
@@ -63,7 +63,7 @@ function queue_application_deployment(Application $application, string $deployme
             ];
         }
 
-        return ApplicationDeploymentQueue::create([
+        $deployment = ApplicationDeploymentQueue::create([
             'application_id' => $application_id,
             'application_name' => $application->name,
             'server_id' => $server_id,
@@ -83,13 +83,24 @@ function queue_application_deployment(Application $application, string $deployme
             'only_this_server' => $only_this_server,
             'parent_deployment_uuid' => $parent_deployment_uuid,
         ]);
+
+        // Decide whether to start while the application and server rows are still locked,
+        // so concurrent requests cannot both see an idle queue and start two deployments.
+        $started = $no_questions_asked || next_queuable($server_id, $application_id, $commit, $pull_request_id);
+        if ($started) {
+            $deployment->update([
+                'status' => ApplicationDeploymentStatus::IN_PROGRESS->value,
+            ]);
+        }
+
+        return ['deployment' => $deployment, 'started' => $started];
     });
 
-    if (is_array($admission)) {
+    if (! isset($admission['deployment'])) {
         return $admission;
     }
 
-    $deployment = $admission;
+    $deployment = $admission['deployment'];
 
     if (auth()->check() && ! $is_webhook && ! $is_api && ! $rollback) {
         auditLog($restart_only ? 'ui.application.restarted' : 'ui.application.deployed', [
@@ -100,17 +111,7 @@ function queue_application_deployment(Application $application, string $deployme
         ]);
     }
 
-    if ($no_questions_asked) {
-        $deployment->update([
-            'status' => ApplicationDeploymentStatus::IN_PROGRESS->value,
-        ]);
-        ApplicationDeploymentJob::dispatch(
-            application_deployment_queue_id: $deployment->id,
-        );
-    } elseif (next_queuable($server_id, $application_id, $commit, $pull_request_id)) {
-        $deployment->update([
-            'status' => ApplicationDeploymentStatus::IN_PROGRESS->value,
-        ]);
+    if ($admission['started']) {
         ApplicationDeploymentJob::dispatch(
             application_deployment_queue_id: $deployment->id,
         );
@@ -141,19 +142,46 @@ function queue_next_deployment(Application $application)
         ->sortBy('created_at');
 
     foreach ($queued_deployments as $next_deployment) {
-        // Check if this queued deployment can actually run
-        if (next_queuable($next_deployment->server_id, $next_deployment->application_id, $next_deployment->commit, $next_deployment->pull_request_id)) {
-            $next_deployment->update([
-                'status' => ApplicationDeploymentStatus::IN_PROGRESS->value,
-            ]);
-
-            ApplicationDeploymentJob::dispatch(
-                application_deployment_queue_id: $next_deployment->id,
-            );
-        }
+        start_queued_deployment($next_deployment);
     }
 }
 
+/**
+ * Start a queued deployment if it can run now. The check and the status change run under
+ * the same application and server row locks as queue admission, so two workers cannot both
+ * start the same deployment or exceed the per-application and per-server limits.
+ */
+function start_queued_deployment(ApplicationDeploymentQueue $deployment): bool
+{
+    $started = DB::transaction(function () use ($deployment): bool {
+        Application::query()->whereKey($deployment->application_id)->lockForUpdate()->first();
+        Server::query()->whereKey($deployment->server_id)->lockForUpdate()->first();
+        $current = ApplicationDeploymentQueue::query()->whereKey($deployment->id)->lockForUpdate()->first();
+
+        if (! $current || $current->status !== ApplicationDeploymentStatus::QUEUED->value) {
+            return false;
+        }
+
+        if (! next_queuable($current->server_id, $current->application_id, $current->commit, $current->pull_request_id)) {
+            return false;
+        }
+
+        $current->update([
+            'status' => ApplicationDeploymentStatus::IN_PROGRESS->value,
+        ]);
+
+        return true;
+    });
+
+    if ($started) {
+        ApplicationDeploymentJob::dispatch(
+            application_deployment_queue_id: $deployment->id,
+        );
+    }
+
+    return $started;
+}
+
 function next_queuable(string $server_id, string $application_id, string $commit = 'HEAD', int $pull_request_id = 0): bool
 {
     // Check if there's already a deployment in progress for this application with the same pull_request_id
@@ -188,19 +216,8 @@ function next_after_cancel(?Server $server = null)
             ->get()
             ->sortBy('created_at');
 
-        if ($next_found->count() > 0) {
-            foreach ($next_found as $next) {
-                // Use next_queuable to properly check if this deployment can run
-                if (next_queuable($next->server_id, $next->application_id, $next->commit, $next->pull_request_id)) {
-                    $next->update([
-                        'status' => ApplicationDeploymentStatus::IN_PROGRESS->value,
-                    ]);
-
-                    ApplicationDeploymentJob::dispatch(
-                        application_deployment_queue_id: $next->id,
-                    );
-                }
-            }
+        foreach ($next_found as $next) {
+            start_queued_deployment($next);
         }
     }
 }
diff --git a/tests/Feature/MultiServerApplicationTest.php b/tests/Feature/MultiServerApplicationTest.php
index 13cac656f7..260c5747d7 100644
--- a/tests/Feature/MultiServerApplicationTest.php
+++ b/tests/Feature/MultiServerApplicationTest.php
@@ -273,6 +273,46 @@ describe('combined deployment notification', function () {
         Notification::assertNotSentTo($this->team, DeploymentFailed::class);
     });
 
+    test('sends the summary when the last additional server deployment is cancelled while running', function () {
+        $parentUuid = new_public_id();
+        Cache::put("multi-server-deployment-queued:{$parentUuid}", true);
+        createMultiServerDeployment($this->application, $this->extraServer, ApplicationDeploymentStatus::FINISHED->value, $parentUuid);
+        $cancelled = createMultiServerDeployment($this->application, $this->extraServer, ApplicationDeploymentStatus::CANCELLED_BY_USER->value, $parentUuid);
+        $job = makeMultiServerDeploymentJob(['application' => $this->application, 'application_deployment_queue' => $cancelled]);
+
+        expect(fn () => callMultiServerDeploymentJob($job, 'failDeployment'))
+            ->toThrow(DeploymentException::class, 'Deployment cancelled by user');
+
+        Notification::assertSentToTimes($this->team, DeploymentFailed::class, 1);
+        Notification::assertSentTo($this->team, DeploymentFailed::class, fn (DeploymentFailed $notification) => $notification->deployment_uuid === $cancelled->deployment_uuid);
+        Notification::assertNotSentTo($this->team, DeploymentSuccess::class);
+    });
+
+    test('sends the summary when the last additional server deployment is cancelled before it starts', function () {
+        $parentUuid = new_public_id();
+        Cache::put("multi-server-deployment-queued:{$parentUuid}", true);
+        createMultiServerDeployment($this->application, $this->extraServer, ApplicationDeploymentStatus::FINISHED->value, $parentUuid);
+        $cancelled = createMultiServerDeployment($this->application, $this->extraServer, ApplicationDeploymentStatus::CANCELLED_BY_USER->value, $parentUuid);
+        [$job] = makeMultiServerDeploymentJob(['application' => $this->application, 'application_deployment_queue' => $cancelled]);
+
+        $job->handle();
+
+        Notification::assertSentToTimes($this->team, DeploymentFailed::class, 1);
+        Notification::assertSentTo($this->team, DeploymentFailed::class, fn (DeploymentFailed $notification) => $notification->deployment_uuid === $cancelled->deployment_uuid);
+    });
+
+    test('does not notify when a cancelled additional server deployment is not the last one', function () {
+        $parentUuid = new_public_id();
+        Cache::put("multi-server-deployment-queued:{$parentUuid}", true);
+        createMultiServerDeployment($this->application, $this->extraServer, ApplicationDeploymentStatus::IN_PROGRESS->value, $parentUuid);
+        $cancelled = createMultiServerDeployment($this->application, $this->extraServer, ApplicationDeploymentStatus::CANCELLED_BY_USER->value, $parentUuid);
+        [$job] = makeMultiServerDeploymentJob(['application' => $this->application, 'application_deployment_queue' => $cancelled]);
+
+        $job->handle();
+
+        Notification::assertNothingSent();
+    });
+
     test('sends a failed notification that links to the failed server deployment', function () {
         $parentUuid = new_public_id();
         Cache::put("multi-server-deployment-queued:{$parentUuid}", true);
@@ -297,15 +337,21 @@ class MultiServerRecordingDeploymentJob extends ApplicationDeploymentJob
 
     public function __construct() {}
 
+    /** Output that `docker images -q` returns; empty means the image is missing. */
+    public string $localImageOutput = 'sha256:0123456789ab';
+
     public function execute_remote_command(...$commands): void
     {
         foreach ($commands as $command) {
             $this->recordedCommands[] = $command['command'] ?? $command[0];
+            if (isset($command['save'])) {
+                (new ReflectionProperty(ApplicationDeploymentJob::class, 'saved_outputs'))->getValue($this)->put($command['save'], $this->localImageOutput);
+            }
         }
     }
 }
 
-function makeInlineDockerfileJob(Application $application, bool $isAdditionalServer): array
+function makeInlineDockerfileJob(Application $application, bool $isAdditionalServer, ?Server $server = null): array
 {
     $reflection = new ReflectionClass(ApplicationDeploymentJob::class);
     $job = new MultiServerRecordingDeploymentJob;
@@ -316,6 +362,8 @@ function makeInlineDockerfileJob(Application $application, bool $isAdditionalSer
         'application_deployment_queue' => $queue,
         'is_this_additional_server' => $isAdditionalServer,
         'pull_request_id' => 0,
+        'saved_outputs' => collect(),
+        'server' => $server ?? $application->destination->server,
     ] as $name => $value) {
         $reflection->getProperty($name)->setValue($job, $value);
     }
@@ -336,7 +384,19 @@ describe('inline Dockerfile image on additional servers', function () {
         [$job, $reflection] = makeInlineDockerfileJob($this->application->fresh(), true);
 
         expect($reflection->getMethod('pull_image_for_additional_server')->invoke($job))->toBeTrue()
-            ->and($job->recordedCommands)->toBe(["docker pull 'registry.example.com/nginx-multi:latest'"]);
+            ->and($job->recordedCommands)->toBe([
+                "docker pull 'registry.example.com/nginx-multi:latest'",
+                "docker images -q 'registry.example.com/nginx-multi:latest' 2>/dev/null",
+            ]);
+    });
+
+    test('a deployments only server explains that the main server must push a missing image', function () {
+        $this->extraServer->settings->update(['server_role' => 'deployment']);
+        [$job, $reflection] = makeInlineDockerfileJob($this->application->fresh(), true, $this->extraServer->fresh());
+        $job->localImageOutput = '';
+
+        expect(fn () => $reflection->getMethod('pull_image_for_additional_server')->invoke($job))
+            ->toThrow(DeploymentException::class, 'Image (registry.example.com/nginx-multi:latest) not found in the registry; the main server must push it first. The server (extra-server) is set to deployments only and cannot build it.');
     });
 
     test('the main server builds the image', function () {
diff --git a/tests/Feature/QueueApplicationDeploymentCommitTest.php b/tests/Feature/QueueApplicationDeploymentCommitTest.php
index 992856bae5..422047979b 100644
--- a/tests/Feature/QueueApplicationDeploymentCommitTest.php
+++ b/tests/Feature/QueueApplicationDeploymentCommitTest.php
@@ -61,6 +61,62 @@ describe('queue_application_deployment commit resolution', function () {
         expect($creationLevel)->toBeGreaterThan($initialLevel);
     });
 
+    test('starts a deployment only while the admission transaction is open', function () {
+        $application = makeApplication($this->environment->id, $this->destination->id, 'HEAD');
+        $initialLevel = DB::transactionLevel();
+        $startLevel = null;
+
+        ApplicationDeploymentQueue::updated(function (ApplicationDeploymentQueue $deployment) use (&$startLevel): void {
+            if ($deployment->status === ApplicationDeploymentStatus::IN_PROGRESS->value) {
+                $startLevel = DB::transactionLevel();
+            }
+        });
+
+        $result = queue_application_deployment($application, 'atomic-start');
+
+        expect($result['status'])->toBe('queued')
+            ->and($startLevel)->toBeGreaterThan($initialLevel);
+        Bus::assertDispatchedTimes(ApplicationDeploymentJob::class, 1);
+    });
+
+    test('starts the next queued deployment only while a transaction is open', function (string $trigger) {
+        $application = makeApplication($this->environment->id, $this->destination->id, 'HEAD');
+        $this->server->settings->update(['concurrent_builds' => 0]);
+        queue_application_deployment($application, 'waiting-deployment');
+        $this->server->settings->update(['concurrent_builds' => 1]);
+        $initialLevel = DB::transactionLevel();
+        $startLevel = null;
+
+        ApplicationDeploymentQueue::updated(function (ApplicationDeploymentQueue $deployment) use (&$startLevel): void {
+            if ($deployment->status === ApplicationDeploymentStatus::IN_PROGRESS->value) {
+                $startLevel = DB::transactionLevel();
+            }
+        });
+
+        $trigger === 'queue_next_deployment'
+            ? queue_next_deployment($application)
+            : next_after_cancel($this->server);
+
+        expect($startLevel)->toBeGreaterThan($initialLevel)
+            ->and(ApplicationDeploymentQueue::query()->sole()->status)->toBe(ApplicationDeploymentStatus::IN_PROGRESS->value);
+        Bus::assertDispatchedTimes(ApplicationDeploymentJob::class, 1);
+    })->with(['queue_next_deployment', 'next_after_cancel']);
+
+    test('does not start a queued deployment that another worker already started', function () {
+        $application = makeApplication($this->environment->id, $this->destination->id, 'HEAD');
+        $this->server->settings->update(['concurrent_builds' => 0]);
+        queue_application_deployment($application, 'stale-deployment');
+        $this->server->settings->update(['concurrent_builds' => 5]);
+        $stale = ApplicationDeploymentQueue::query()->sole();
+
+        ApplicationDeploymentQueue::query()->whereKey($stale->id)->update([
+            'status' => ApplicationDeploymentStatus::IN_PROGRESS->value,
+        ]);
+
+        expect(start_queued_deployment($stale))->toBeFalse();
+        Bus::assertNotDispatched(ApplicationDeploymentJob::class);
+    });
+
     test('skips a matching commit without creating or dispatching a second deployment', function () {
         $application = makeApplication($this->environment->id, $this->destination->id, 'HEAD');