diff --git a/src/Controller/Booking/Create/Step2Controller.php b/src/Controller/Booking/Create/Step2Controller.php
index ab75508c..3e6dc1bc 100644
--- a/src/Controller/Booking/Create/Step2Controller.php
+++ b/src/Controller/Booking/Create/Step2Controller.php
@@ -13,6 +13,7 @@ use App\Form\BookingCreateStep2Type;
 use App\Form\Model\BookingDto;
 use App\Form\Model\ParticipantDto;
 use App\Form\Model\RoomSelectionDto;
+use App\Model\RoomSelectionAdjustment;
 use App\Service\BookingConfigurator;
 use App\Service\BookingCreateContextFactory;
 use App\Service\BookingSessionManager;
@@ -66,6 +67,12 @@ class Step2Controller extends AbstractBookingCreateController
         // Enrich with fresh availability data
         $this->travelDataService->enrichWithFreshAvailabilities($bookingCreateDto->travel);
 
+        // Resync the room selections with the refreshed travel before any participant count is derived
+        // from them, and send the customer back to step 1 when a room they picked is gone.
+        if (null !== $redirect = $this->guardAgainstUnavailableRoomSelection($request, $bookingCreateDto)) {
+            return $redirect;
+        }
+
         // Ensure correct number of participants first, then prepopulate the applicant if needed.
         if (null !== $redirect = $this->guardAgainstEmptyParticipantList($bookingCreateDto)) {
             return $redirect;
@@ -120,6 +127,79 @@ class Step2Controller extends AbstractBookingCreateController
         return $this->render('booking/create/step_2.html.twig', $templateData);
     }
 
+    /**
+     * Sends the customer back to step 1 when the refreshed travel data no longer covers their rooms.
+     *
+     * Deriving the participant count from a room that has disappeared would silently fall back to a
+     * capacity of one and render too few participants, and a quantity the contingent no longer covers
+     * reaches the BusPro submit unchecked, because no later step re-reads room availability. The
+     * selections are resynced and saved before every exit, so step 1 and the neighbouring guard both
+     * see a collection that matches the rooms the template renders: leaving a stale entry behind would
+     * fail step 1's own validation on a field it never shows.
+     */
+    private function guardAgainstUnavailableRoomSelection(Request $request, BookingDto $bookingCreateDto): ?RedirectResponse
+    {
+        $adjustments = $this->bookingService->reconcileRoomSelections($bookingCreateDto);
+
+        $this->bookingSessionService->saveBookingDto($request, $bookingCreateDto, BookingDto::MODE_CREATE);
+
+        if ([] === $bookingCreateDto->travel->getAvailableRooms()) {
+            $this->handleApiError(
+                $this->logger,
+                'Travel has no bookable rooms left while a booking is in progress.',
+                $this->createRoomContext($bookingCreateDto, $adjustments),
+                'Für diese Reise sind leider keine Zimmer mehr verfügbar.'
+            );
+
+            return $this->redirectToRoute('app_booking_create_error');
+        }
+
+        if ([] === $adjustments) {
+            return null;
+        }
+
+        $hasDroppedRoom = false;
+        foreach ($adjustments as $adjustment) {
+            $hasDroppedRoom = $hasDroppedRoom || $adjustment->isDropped();
+        }
+
+        $this->handleApiError(
+            $this->logger,
+            'Room selection exceeds what the current travel data still offers.',
+            $this->createRoomContext($bookingCreateDto, $adjustments),
+            true === $hasDroppedRoom
+                ? 'Dein gewähltes Zimmer ist leider nicht mehr verfügbar. Bitte wähle dein Zimmer erneut.'
+                : 'Für dein gewähltes Zimmer sind nicht mehr genügend Plätze verfügbar. Wir haben deine Auswahl angepasst, bitte überprüfe sie.'
+        );
+
+        return $this->redirectToRoute('app_booking_create_step_1');
+    }
+
+    /**
+     * @param list<RoomSelectionAdjustment> $adjustments
+     *
+     * @return array<string, mixed>
+     */
+    private function createRoomContext(BookingDto $bookingCreateDto, array $adjustments): array
+    {
+        $rooms = [];
+        foreach ($adjustments as $adjustment) {
+            $rooms[] = [
+                'roomId' => $adjustment->roomId,
+                'label' => $adjustment->label,
+                'requestedQuantity' => $adjustment->requestedQuantity,
+                'bookableQuantity' => $adjustment->bookableQuantity,
+            ];
+        }
+
+        return [
+            'travelId' => $bookingCreateDto->travel->id,
+            'travelCode' => $bookingCreateDto->travel->code,
+            'hotelId' => $bookingCreateDto->hotelId,
+            'rooms' => $rooms,
+        ];
+    }
+
     /**
      * Rejects a room selection that resolves to no participants at all.
      *
@@ -190,7 +270,14 @@ class Step2Controller extends AbstractBookingCreateController
         $rooms = $travelData->getAvailableRooms();
 
         foreach ($roomSelections as $roomSelection) {
-            $room = $rooms[$roomSelection->id];
+            $room = $rooms[$roomSelection->id] ?? null;
+
+            if (null === $room) {
+                // Unreachable from index(), which reconciles the selections first. Kept so a future
+                // caller cannot silently fall back to a capacity of one and book too few people.
+                continue;
+            }
+
             $participantsCount += ($room->minPax ?? 1) * $roomSelection->quantity;
         }
 
diff --git a/src/Model/RoomSelectionAdjustment.php b/src/Model/RoomSelectionAdjustment.php
new file mode 100644
index 00000000..8ae6cef2
--- /dev/null
+++ b/src/Model/RoomSelectionAdjustment.php
@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Model;
+
+/**
+ * A room selection the current travel data no longer supports in full.
+ *
+ * A room that disappeared altogether is the same case with nothing left to book, so it is reported
+ * as an adjustment down to zero rather than as a second kind of result.
+ */
+final readonly class RoomSelectionAdjustment
+{
+    public function __construct(
+        public int $roomId,
+        public ?string $label,
+        public int $requestedQuantity,
+        public int $bookableQuantity,
+    ) {
+    }
+
+    /**
+     * Whether the room is gone entirely rather than merely reduced.
+     */
+    public function isDropped(): bool
+    {
+        return 0 === $this->bookableQuantity;
+    }
+}
diff --git a/src/Service/BookingConfigurator.php b/src/Service/BookingConfigurator.php
index af3db09c..97251464 100644
--- a/src/Service/BookingConfigurator.php
+++ b/src/Service/BookingConfigurator.php
@@ -15,6 +15,7 @@ use App\Form\Model\BookingDto;
 use App\Form\Model\ParticipantDto;
 use App\Form\Model\RoomSelectionDto;
 use App\Form\Service\ServiceAgeEvaluator;
+use App\Model\RoomSelectionAdjustment;
 use Symfony\Component\DependencyInjection\Attribute\Autowire;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
@@ -91,6 +92,54 @@ class BookingConfigurator
         return $bookingCreateDto;
     }
 
+    /**
+     * Brings the room selections back in line with the rooms the travel currently offers.
+     *
+     * Room selections are built once when the booking session starts, while the Travel is re-fetched
+     * from BusPro on every request. A room that sells out or leaves a bookable status in between drops
+     * out of getAvailableRooms() and leaves a selection behind that points at a room that no longer
+     * exists; one that only loses part of its contingent leaves a quantity nothing else re-checks
+     * before the booking is submitted; and one that becomes bookable again has no selection at all.
+     * Rebuilding the selections from the current rooms, with the chosen quantities carried over and
+     * capped at what is left, keeps both sides in step.
+     *
+     * @return list<RoomSelectionAdjustment> The selections that had to give up quantity, in selection order
+     */
+    public function reconcileRoomSelections(BookingDto $bookingDto): array
+    {
+        $availableRooms = $bookingDto->travel->getAvailableRooms();
+
+        $adjustments = [];
+        $quantitiesByRoomId = [];
+
+        foreach ($bookingDto->roomSelections as $roomSelection) {
+            if (null === $roomSelection->id) {
+                continue;
+            }
+
+            $requestedQuantity = $roomSelection->quantity ?? 0;
+            $bookableQuantity = min($requestedQuantity, $availableRooms[$roomSelection->id]->available ?? 0);
+
+            $quantitiesByRoomId[$roomSelection->id] = $bookableQuantity;
+
+            if (0 < $requestedQuantity && $bookableQuantity < $requestedQuantity) {
+                $adjustments[] = new RoomSelectionAdjustment(
+                    $roomSelection->id,
+                    $roomSelection->label,
+                    $requestedQuantity,
+                    $bookableQuantity
+                );
+            }
+        }
+
+        $bookingDto->roomSelections = array_map(
+            fn (Room $room) => $this->createRoomSelection($room, $quantitiesByRoomId),
+            $availableRooms
+        );
+
+        return $adjustments;
+    }
+
     /**
      * Creates a room selection DTO from room data and quantities.
      *
diff --git a/tests/Controller/Booking/Create/Step2ControllerTest.php b/tests/Controller/Booking/Create/Step2ControllerTest.php
new file mode 100644
index 00000000..6bc52444
--- /dev/null
+++ b/tests/Controller/Booking/Create/Step2ControllerTest.php
@@ -0,0 +1,327 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Controller\Booking\Create;
+
+use App\BusProNet\Constants;
+use App\BusProNet\Model\Room;
+use App\BusProNet\Model\Travel;
+use App\Controller\Booking\Create\Step2Controller;
+use App\Form\Model\BookingDto;
+use App\Form\Model\RoomSelectionDto;
+use App\Model\RoomSelectionAdjustment;
+use App\Service\BookingConfigurator;
+use App\Service\BookingCreateContextFactory;
+use App\Service\BookingSessionManager;
+use App\Service\ParticipantDataPrefiller;
+use App\Service\RoomAssigner;
+use App\Service\TravelDataProvider;
+use PHPUnit\Framework\TestCase;
+use Psr\Log\LoggerInterface;
+use Symfony\Component\Form\FormInterface;
+use Symfony\Component\HttpFoundation\RedirectResponse;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\Response;
+use Symfony\Component\Security\Core\User\UserInterface;
+
+class Step2ControllerTest extends TestCase
+{
+    public function testSendsCustomerBackToStep1WhenASelectedRoomIsNoLongerBookable(): void
+    {
+        $request = Request::create('/bookings/create/participants', 'GET');
+        $travel = $this->createTravel([$this->createRoom(1409, 'Einzelzimmer', minPax: 1)]);
+        $bookingDto = $this->createBookingDto($travel);
+
+        $bookingService = $this->createMock(BookingConfigurator::class);
+        $bookingService->expects($this->once())
+            ->method('reconcileRoomSelections')
+            ->with($bookingDto)
+            ->willReturn([new RoomSelectionAdjustment(1408, 'Doppelzimmer', 2, 0)]);
+        $bookingService->expects($this->never())->method('preselectDefaultServices');
+
+        $bookingSessionService = $this->createMock(BookingSessionManager::class);
+        $bookingSessionService->method('getOrCreateBookingCreateDto')->willReturn($bookingDto);
+        $bookingSessionService->expects($this->once())
+            ->method('saveBookingDto')
+            ->with($request, $bookingDto, BookingDto::MODE_CREATE);
+
+        $logger = $this->createMock(LoggerInterface::class);
+        $logger->expects($this->once())
+            ->method('error')
+            ->with(
+                'Room selection exceeds what the current travel data still offers.',
+                [
+                    'travelId' => 4711,
+                    'travelCode' => 'TRV-4711',
+                    'hotelId' => 1,
+                    'rooms' => [[
+                        'roomId' => 1408,
+                        'label' => 'Doppelzimmer',
+                        'requestedQuantity' => 2,
+                        'bookableQuantity' => 0,
+                    ]],
+                ]
+            );
+
+        $controller = $this->createController($bookingService, $bookingSessionService, $logger);
+
+        $response = $controller->index($request);
+
+        $this->assertInstanceOf(RedirectResponse::class, $response);
+        $this->assertSame('/app_booking_create_step_1', $response->getTargetUrl());
+        $this->assertSame(
+            [['error', 'Dein gewähltes Zimmer ist leider nicht mehr verfügbar. Bitte wähle dein Zimmer erneut.']],
+            $controller->flashes
+        );
+    }
+
+    public function testSendsCustomerBackToStep1WithTheCappedQuantityPrefilled(): void
+    {
+        $request = Request::create('/bookings/create/participants', 'GET');
+        $travel = $this->createTravel([$this->createRoom(1408, 'Doppelzimmer', minPax: 2)]);
+        $bookingDto = $this->createBookingDto($travel);
+        $bookingDto->roomSelections = [
+            1408 => $this->createRoomSelection(1408, 'Doppelzimmer', quantity: 1, capacity: 2),
+        ];
+
+        $bookingService = $this->createMock(BookingConfigurator::class);
+        $bookingService->expects($this->once())
+            ->method('reconcileRoomSelections')
+            ->with($bookingDto)
+            ->willReturn([new RoomSelectionAdjustment(1408, 'Doppelzimmer', 3, 1)]);
+
+        $bookingSessionService = $this->createMock(BookingSessionManager::class);
+        $bookingSessionService->method('getOrCreateBookingCreateDto')->willReturn($bookingDto);
+        $bookingSessionService->expects($this->once())
+            ->method('saveBookingDto')
+            ->with($request, $bookingDto, BookingDto::MODE_CREATE);
+
+        $controller = $this->createController(
+            $bookingService,
+            $bookingSessionService,
+            $this->createStub(LoggerInterface::class)
+        );
+
+        $response = $controller->index($request);
+
+        $this->assertInstanceOf(RedirectResponse::class, $response);
+        $this->assertSame('/app_booking_create_step_1', $response->getTargetUrl());
+        $this->assertSame(
+            [['error', 'Für dein gewähltes Zimmer sind nicht mehr genügend Plätze verfügbar. Wir haben deine Auswahl angepasst, bitte überprüfe sie.']],
+            $controller->flashes
+        );
+        $this->assertSame(1, $bookingDto->roomSelections[1408]->quantity, 'Step 1 reopens on the capped selection');
+    }
+
+    public function testSendsCustomerToTheErrorPageWhenTheTravelSoldOutCompletely(): void
+    {
+        $request = Request::create('/bookings/create/participants', 'GET');
+        $bookingDto = $this->createBookingDto($this->createTravel([]));
+
+        $bookingService = $this->createMock(BookingConfigurator::class);
+        $bookingService->expects($this->once())
+            ->method('reconcileRoomSelections')
+            ->with($bookingDto)
+            ->willReturn([new RoomSelectionAdjustment(1408, 'Doppelzimmer', 2, 0)]);
+
+        $bookingSessionService = $this->createMock(BookingSessionManager::class);
+        $bookingSessionService->method('getOrCreateBookingCreateDto')->willReturn($bookingDto);
+        $bookingSessionService->expects($this->once())
+            ->method('saveBookingDto')
+            ->with($request, $bookingDto, BookingDto::MODE_CREATE);
+
+        $logger = $this->createMock(LoggerInterface::class);
+        $logger->expects($this->once())
+            ->method('error')
+            ->with('Travel has no bookable rooms left while a booking is in progress.', $this->anything());
+
+        $controller = $this->createController($bookingService, $bookingSessionService, $logger);
+
+        $response = $controller->index($request);
+
+        $this->assertInstanceOf(RedirectResponse::class, $response);
+        $this->assertSame('/app_booking_create_error', $response->getTargetUrl());
+        $this->assertSame(
+            [['error', 'Für diese Reise sind leider keine Zimmer mehr verfügbar.']],
+            $controller->flashes
+        );
+    }
+
+    public function testContinuesWhenOnlyUnselectedRoomsWereDropped(): void
+    {
+        $request = Request::create('/bookings/create/participants', 'GET');
+        $travel = $this->createTravel([$this->createRoom(1409, 'Einzelzimmer', minPax: 3)]);
+        $bookingDto = $this->createBookingDto($travel);
+        $bookingDto->roomSelections = [
+            1409 => $this->createRoomSelection(1409, 'Einzelzimmer', quantity: 2, capacity: 3),
+        ];
+
+        $bookingService = $this->createMock(BookingConfigurator::class);
+        $bookingService->expects($this->once())
+            ->method('reconcileRoomSelections')
+            ->with($bookingDto)
+            ->willReturn([]);
+
+        $bookingSessionService = $this->createMock(BookingSessionManager::class);
+        $bookingSessionService->method('getOrCreateBookingCreateDto')->willReturn($bookingDto);
+        $bookingSessionService->expects($this->exactly(2))
+            ->method('saveBookingDto')
+            ->with($request, $bookingDto, BookingDto::MODE_CREATE);
+
+        $logger = $this->createMock(LoggerInterface::class);
+        $logger->expects($this->never())->method('error');
+
+        $controller = $this->createController($bookingService, $bookingSessionService, $logger);
+
+        $response = $controller->index($request);
+
+        $this->assertSame([], $controller->flashes);
+        $this->assertInstanceOf(Response::class, $response);
+        $this->assertCount(6, $bookingDto->participants, 'Two rooms for three people each yield six participants');
+    }
+
+    private function createController(
+        BookingConfigurator $bookingService,
+        BookingSessionManager $bookingSessionService,
+        LoggerInterface $logger,
+    ): TestableStep2Controller {
+        $form = $this->createStub(FormInterface::class);
+        $form->method('isSubmitted')->willReturn(false);
+
+        return new TestableStep2Controller(
+            $bookingService,
+            $bookingSessionService,
+            $this->createStub(BookingCreateContextFactory::class),
+            $this->createStub(TravelDataProvider::class),
+            $this->createStub(RoomAssigner::class),
+            $this->createStub(ParticipantDataPrefiller::class),
+            $logger,
+            $form,
+        );
+    }
+
+    private function createBookingDto(Travel $travel): BookingDto
+    {
+        $bookingDto = new BookingDto($travel, 1);
+        $bookingDto->currentStep = 2;
+
+        return $bookingDto;
+    }
+
+    /**
+     * @param array<int, Room> $rooms
+     */
+    private function createTravel(array $rooms): Travel
+    {
+        $travel = new Travel();
+        $travel->id = 4711;
+        $travel->code = 'TRV-4711';
+        $travel->rooms = $rooms;
+
+        return $travel;
+    }
+
+    private function createRoom(int $id, string $label, int $minPax): Room
+    {
+        $room = new Room();
+        $room->id = $id;
+        $room->label = $label;
+        $room->available = 4;
+        $room->minPax = $minPax;
+        $room->status = Constants::STATUS_AVAILABLE;
+
+        return $room;
+    }
+
+    private function createRoomSelection(int $id, string $label, int $quantity, int $capacity): RoomSelectionDto
+    {
+        $selection = new RoomSelectionDto();
+        $selection->id = $id;
+        $selection->label = $label;
+        $selection->quantity = $quantity;
+        $selection->capacity = $capacity;
+
+        return $selection;
+    }
+}
+
+final class TestableStep2Controller extends Step2Controller
+{
+    /**
+     * @var array<int, array{string, mixed}>
+     */
+    public array $flashes = [];
+
+    /**
+     * @var array<string, mixed>
+     */
+    public array $renderParameters = [];
+
+    /**
+     * @var FormInterface<mixed>
+     */
+    private readonly FormInterface $form;
+
+    /**
+     * @param FormInterface<mixed> $form
+     */
+    public function __construct(
+        BookingConfigurator $bookingService,
+        BookingSessionManager $bookingSessionService,
+        BookingCreateContextFactory $createContextFactory,
+        TravelDataProvider $travelDataService,
+        RoomAssigner $roomAssignmentService,
+        ParticipantDataPrefiller $prepopulationService,
+        LoggerInterface $logger,
+        FormInterface $form,
+    ) {
+        $this->form = $form;
+
+        parent::__construct(
+            $bookingService,
+            $bookingSessionService,
+            $createContextFactory,
+            $travelDataService,
+            $roomAssignmentService,
+            $prepopulationService,
+            $logger,
+        );
+    }
+
+    protected function getUser(): ?UserInterface
+    {
+        return null;
+    }
+
+    /**
+     * @return FormInterface<mixed>
+     */
+    protected function createForm(string $type, mixed $data = null, array $options = []): FormInterface
+    {
+        return $this->form;
+    }
+
+    protected function addFlash(string $type, mixed $message): void
+    {
+        $this->flashes[] = [$type, $message];
+    }
+
+    /**
+     * @param array<string, mixed> $parameters
+     */
+    protected function redirectToRoute(string $route, array $parameters = [], int $status = 302): RedirectResponse
+    {
+        return new RedirectResponse('/'.$route, $status);
+    }
+
+    /**
+     * @param array<string, mixed> $parameters
+     */
+    protected function render(string $view, array $parameters = [], ?Response $response = null): Response
+    {
+        $this->renderParameters = $parameters;
+
+        return new Response('');
+    }
+}
diff --git a/tests/Service/BookingConfiguratorRoomReconciliationTest.php b/tests/Service/BookingConfiguratorRoomReconciliationTest.php
new file mode 100644
index 00000000..a5e6aef1
--- /dev/null
+++ b/tests/Service/BookingConfiguratorRoomReconciliationTest.php
@@ -0,0 +1,214 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Service;
+
+use App\BusProNet\Constants;
+use App\BusProNet\Model\Room;
+use App\BusProNet\Model\Travel;
+use App\BusProNet\Service\BookingStatusRuleRegistry;
+use App\BusProNet\XmlLoader\AgencyLoader;
+use App\Form\Model\BookingDto;
+use App\Form\Model\RoomSelectionDto;
+use App\Service\BookingConfigurator;
+use App\Service\BookingSessionManager;
+use App\Service\ParticipantEligibilityChecker;
+use App\Service\TravelDataProvider;
+use PHPUnit\Framework\TestCase;
+
+class BookingConfiguratorRoomReconciliationTest extends TestCase
+{
+    public function testKeepsQuantitiesOfRoomsThatAreStillBookable(): void
+    {
+        $travel = $this->createTravel([
+            $this->createRoom(1408, 'Doppelzimmer', available: 4, minPax: 2),
+            $this->createRoom(1409, 'Einzelzimmer', available: 2, minPax: 1),
+        ]);
+
+        $bookingDto = new BookingDto($travel, 1);
+        $bookingDto->roomSelections = [
+            1408 => $this->createRoomSelection(1408, quantity: 2),
+            1409 => $this->createRoomSelection(1409, quantity: 0),
+        ];
+
+        $adjustments = $this->bookingService()->reconcileRoomSelections($bookingDto);
+
+        $this->assertSame([], $adjustments);
+        $this->assertSame([1408, 1409], array_keys($bookingDto->roomSelections));
+        $this->assertSame(2, $bookingDto->roomSelections[1408]->quantity);
+        $this->assertSame(0, $bookingDto->roomSelections[1409]->quantity);
+        $this->assertSame(2, $bookingDto->roomSelections[1408]->capacity);
+        $this->assertSame(4, $bookingDto->roomSelections[1408]->maxQuantity);
+    }
+
+    public function testReportsAndRemovesASelectedRoomThatIsNoLongerBookable(): void
+    {
+        $travel = $this->createTravel([
+            $this->createRoom(1408, 'Doppelzimmer', available: 0, minPax: 2),
+            $this->createRoom(1409, 'Einzelzimmer', available: 2, minPax: 1),
+        ]);
+
+        $bookingDto = new BookingDto($travel, 1);
+        $bookingDto->roomSelections = [
+            1408 => $this->createRoomSelection(1408, quantity: 2),
+            1409 => $this->createRoomSelection(1409, quantity: 0),
+        ];
+
+        $adjustments = $this->bookingService()->reconcileRoomSelections($bookingDto);
+
+        $this->assertCount(1, $adjustments);
+        $this->assertSame(1408, $adjustments[0]->roomId);
+        $this->assertSame(2, $adjustments[0]->requestedQuantity);
+        $this->assertSame(0, $adjustments[0]->bookableQuantity);
+        $this->assertTrue($adjustments[0]->isDropped());
+        $this->assertSame([1409], array_keys($bookingDto->roomSelections));
+    }
+
+    public function testCapsAQuantityThatExceedsTheRemainingContingent(): void
+    {
+        $travel = $this->createTravel([
+            $this->createRoom(1408, 'Doppelzimmer', available: 1, minPax: 2),
+        ]);
+
+        $bookingDto = new BookingDto($travel, 1);
+        $bookingDto->roomSelections = [
+            1408 => $this->createRoomSelection(1408, quantity: 3),
+        ];
+
+        $adjustments = $this->bookingService()->reconcileRoomSelections($bookingDto);
+
+        $this->assertCount(1, $adjustments);
+        $this->assertSame(1408, $adjustments[0]->roomId);
+        $this->assertSame('Aus der Session', $adjustments[0]->label);
+        $this->assertSame(3, $adjustments[0]->requestedQuantity);
+        $this->assertSame(1, $adjustments[0]->bookableQuantity);
+        $this->assertFalse($adjustments[0]->isDropped());
+        $this->assertSame(1, $bookingDto->roomSelections[1408]->quantity, 'The selection carries the capped quantity');
+        $this->assertSame(1, $bookingDto->roomSelections[1408]->maxQuantity);
+    }
+
+    public function testLeavesAQuantityThatStillFitsTheReducedContingent(): void
+    {
+        $travel = $this->createTravel([
+            $this->createRoom(1408, 'Doppelzimmer', available: 2, minPax: 2),
+        ]);
+
+        $bookingDto = new BookingDto($travel, 1);
+        $bookingDto->roomSelections = [
+            1408 => $this->createRoomSelection(1408, quantity: 2),
+        ];
+
+        $adjustments = $this->bookingService()->reconcileRoomSelections($bookingDto);
+
+        $this->assertSame([], $adjustments);
+        $this->assertSame(2, $bookingDto->roomSelections[1408]->quantity);
+    }
+
+    public function testRemovesAnUnselectedRoomThatIsNoLongerBookableWithoutReportingIt(): void
+    {
+        $travel = $this->createTravel([
+            $this->createRoom(1408, 'Doppelzimmer', available: 4, minPax: 2),
+            $this->createRoom(1409, 'Einzelzimmer', available: 0, minPax: 1),
+        ]);
+
+        $bookingDto = new BookingDto($travel, 1);
+        $bookingDto->roomSelections = [
+            1408 => $this->createRoomSelection(1408, quantity: 1),
+            1409 => $this->createRoomSelection(1409, quantity: 0),
+        ];
+
+        $adjustments = $this->bookingService()->reconcileRoomSelections($bookingDto);
+
+        $this->assertSame([], $adjustments);
+        $this->assertSame([1408], array_keys($bookingDto->roomSelections));
+    }
+
+    public function testAddsARoomThatBecameBookableAgain(): void
+    {
+        $travel = $this->createTravel([
+            $this->createRoom(1408, 'Doppelzimmer', available: 4, minPax: 2),
+            $this->createRoom(1409, 'Einzelzimmer', available: 2, minPax: 1),
+        ]);
+
+        $bookingDto = new BookingDto($travel, 1);
+        $bookingDto->roomSelections = [
+            1408 => $this->createRoomSelection(1408, quantity: 1),
+        ];
+
+        $adjustments = $this->bookingService()->reconcileRoomSelections($bookingDto);
+
+        $this->assertSame([], $adjustments);
+        $this->assertSame([1408, 1409], array_keys($bookingDto->roomSelections));
+        $this->assertSame(0, $bookingDto->roomSelections[1409]->quantity);
+        $this->assertSame('Einzelzimmer', $bookingDto->roomSelections[1409]->label);
+    }
+
+    public function testEmptiesTheSelectionsWhenTheTravelSoldOutCompletely(): void
+    {
+        $travel = $this->createTravel([
+            $this->createRoom(1408, 'Doppelzimmer', available: 0, minPax: 2),
+        ]);
+
+        $bookingDto = new BookingDto($travel, 1);
+        $bookingDto->roomSelections = [
+            1408 => $this->createRoomSelection(1408, quantity: 2),
+        ];
+
+        $adjustments = $this->bookingService()->reconcileRoomSelections($bookingDto);
+
+        $this->assertCount(1, $adjustments);
+        $this->assertTrue($adjustments[0]->isDropped());
+        $this->assertSame([], $bookingDto->roomSelections);
+    }
+
+    private function bookingService(): BookingConfigurator
+    {
+        return new BookingConfigurator(
+            $this->createStub(BookingSessionManager::class),
+            $this->createStub(TravelDataProvider::class),
+            $this->createStub(ParticipantEligibilityChecker::class),
+            $this->createStub(BookingStatusRuleRegistry::class),
+            $this->createStub(AgencyLoader::class),
+            'F'
+        );
+    }
+
+    /**
+     * @param array<int, Room> $rooms
+     */
+    private function createTravel(array $rooms): Travel
+    {
+        $travel = new Travel();
+        $travel->id = 4711;
+        $travel->code = 'TRV-4711';
+        $travel->rooms = $rooms;
+
+        return $travel;
+    }
+
+    private function createRoom(int $id, string $label, int $available, int $minPax): Room
+    {
+        $room = new Room();
+        $room->id = $id;
+        $room->label = $label;
+        $room->available = $available;
+        $room->minPax = $minPax;
+        $room->status = Constants::STATUS_AVAILABLE;
+        $room->price = 199.0;
+
+        return $room;
+    }
+
+    private function createRoomSelection(int $id, int $quantity): RoomSelectionDto
+    {
+        $selection = new RoomSelectionDto();
+        $selection->id = $id;
+        $selection->label = 'Aus der Session';
+        $selection->quantity = $quantity;
+        $selection->capacity = 2;
+        $selection->maxQuantity = 9;
+
+        return $selection;
+    }
+}