feat(inventory): Inventur durchführen – Ist-Mengen erfassen (US-6.2)

Implementiert startCounting() und updateCountItem() auf dem InventoryCount-
Aggregate, zwei neue Use Cases (StartInventoryCount, RecordCountItem) mit
zugehörigen Controller-Endpoints (PATCH /{id}/start, PATCH /{id}/items/{itemId}).
Inkl. Domain-, Application-, Integrations- und Gatling-Lasttests.

backend/src/main/java/de/effigenix/application/inventory/RecordCountItem.java

@@ -0,0 +1,102 @@
+package de.effigenix.application.inventory;
+
+import de.effigenix.application.inventory.command.RecordCountItemCommand;
+import de.effigenix.domain.inventory.CountItemId;
+import de.effigenix.domain.inventory.InventoryAction;
+import de.effigenix.domain.inventory.InventoryCount;
+import de.effigenix.domain.inventory.InventoryCountError;
+import de.effigenix.domain.inventory.InventoryCountId;
+import de.effigenix.domain.inventory.InventoryCountRepository;
+import de.effigenix.shared.common.Quantity;
+import de.effigenix.shared.common.Result;
+import de.effigenix.shared.common.UnitOfMeasure;
+import de.effigenix.shared.persistence.UnitOfWork;
+import de.effigenix.shared.security.ActorId;
+import de.effigenix.shared.security.AuthorizationPort;
+
+import java.math.BigDecimal;
+
+public class RecordCountItem {
+
+    private final InventoryCountRepository inventoryCountRepository;
+    private final UnitOfWork unitOfWork;
+    private final AuthorizationPort authPort;
+
+    public RecordCountItem(InventoryCountRepository inventoryCountRepository,
+                            UnitOfWork unitOfWork,
+                            AuthorizationPort authPort) {
+        this.inventoryCountRepository = inventoryCountRepository;
+        this.unitOfWork = unitOfWork;
+        this.authPort = authPort;
+    }
+
+    public Result<InventoryCountError, InventoryCount> execute(RecordCountItemCommand cmd, ActorId actorId) {
+        if (!authPort.can(actorId, InventoryAction.INVENTORY_COUNT_WRITE)) {
+            return Result.failure(new InventoryCountError.Unauthorized("Not authorized to record count items"));
+        }
+
+        if (cmd.inventoryCountId() == null || cmd.inventoryCountId().isBlank()) {
+            return Result.failure(new InventoryCountError.InvalidInventoryCountId("must not be blank"));
+        }
+
+        InventoryCountId id;
+        try {
+            id = InventoryCountId.of(cmd.inventoryCountId());
+        } catch (IllegalArgumentException e) {
+            return Result.failure(new InventoryCountError.InvalidInventoryCountId(e.getMessage()));
+        }
+
+        CountItemId itemId;
+        try {
+            itemId = CountItemId.of(cmd.countItemId());
+        } catch (IllegalArgumentException e) {
+            return Result.failure(new InventoryCountError.InvalidCountItemId(e.getMessage()));
+        }
+
+        // Parse quantity
+        BigDecimal amount;
+        try {
+            amount = new BigDecimal(cmd.actualQuantityAmount());
+        } catch (NumberFormatException | NullPointerException e) {
+            return Result.failure(new InventoryCountError.InvalidQuantity(
+                    "Invalid quantity amount: " + cmd.actualQuantityAmount()));
+        }
+
+        UnitOfMeasure uom;
+        try {
+            uom = UnitOfMeasure.valueOf(cmd.actualQuantityUnit());
+        } catch (IllegalArgumentException | NullPointerException e) {
+            return Result.failure(new InventoryCountError.InvalidQuantity(
+                    "Invalid unit: " + cmd.actualQuantityUnit()));
+        }
+
+        Quantity actualQuantity = Quantity.reconstitute(amount, uom);
+
+        // Load aggregate
+        InventoryCount count;
+        switch (inventoryCountRepository.findById(id)) {
+            case Result.Failure(var err) ->
+                    { return Result.failure(new InventoryCountError.RepositoryFailure(err.message())); }
+            case Result.Success(var optCount) -> {
+                if (optCount.isEmpty()) {
+                    return Result.failure(new InventoryCountError.InventoryCountNotFound(cmd.inventoryCountId()));
+                }
+                count = optCount.get();
+            }
+        }
+
+        switch (count.updateCountItem(itemId, actualQuantity)) {
+            case Result.Failure(var err) -> { return Result.failure(err); }
+            case Result.Success(var ignored) -> { }
+        }
+
+        return unitOfWork.executeAtomically(() -> {
+            switch (inventoryCountRepository.save(count)) {
+                case Result.Failure(var err) ->
+                        { return Result.failure(new InventoryCountError.RepositoryFailure(err.message())); }
+                case Result.Success(var ignored) -> { }
+            }
+            return Result.success(count);
+        });
+    }
+}

backend/src/main/java/de/effigenix/application/inventory/StartInventoryCount.java

@@ -0,0 +1,69 @@
+package de.effigenix.application.inventory;
+
+import de.effigenix.domain.inventory.InventoryAction;
+import de.effigenix.domain.inventory.InventoryCount;
+import de.effigenix.domain.inventory.InventoryCountError;
+import de.effigenix.domain.inventory.InventoryCountId;
+import de.effigenix.domain.inventory.InventoryCountRepository;
+import de.effigenix.shared.common.Result;
+import de.effigenix.shared.persistence.UnitOfWork;
+import de.effigenix.shared.security.ActorId;
+import de.effigenix.shared.security.AuthorizationPort;
+
+public class StartInventoryCount {
+
+    private final InventoryCountRepository inventoryCountRepository;
+    private final UnitOfWork unitOfWork;
+    private final AuthorizationPort authPort;
+
+    public StartInventoryCount(InventoryCountRepository inventoryCountRepository,
+                                UnitOfWork unitOfWork,
+                                AuthorizationPort authPort) {
+        this.inventoryCountRepository = inventoryCountRepository;
+        this.unitOfWork = unitOfWork;
+        this.authPort = authPort;
+    }
+
+    public Result<InventoryCountError, InventoryCount> execute(String inventoryCountId, ActorId actorId) {
+        if (!authPort.can(actorId, InventoryAction.INVENTORY_COUNT_WRITE)) {
+            return Result.failure(new InventoryCountError.Unauthorized("Not authorized to start inventory count"));
+        }
+
+        if (inventoryCountId == null || inventoryCountId.isBlank()) {
+            return Result.failure(new InventoryCountError.InvalidInventoryCountId("must not be blank"));
+        }
+
+        InventoryCountId id;
+        try {
+            id = InventoryCountId.of(inventoryCountId);
+        } catch (IllegalArgumentException e) {
+            return Result.failure(new InventoryCountError.InvalidInventoryCountId(e.getMessage()));
+        }
+
+        InventoryCount count;
+        switch (inventoryCountRepository.findById(id)) {
+            case Result.Failure(var err) ->
+                    { return Result.failure(new InventoryCountError.RepositoryFailure(err.message())); }
+            case Result.Success(var optCount) -> {
+                if (optCount.isEmpty()) {
+                    return Result.failure(new InventoryCountError.InventoryCountNotFound(inventoryCountId));
+                }
+                count = optCount.get();
+            }
+        }
+
+        switch (count.startCounting()) {
+            case Result.Failure(var err) -> { return Result.failure(err); }
+            case Result.Success(var ignored) -> { }
+        }
+
+        return unitOfWork.executeAtomically(() -> {
+            switch (inventoryCountRepository.save(count)) {
+                case Result.Failure(var err) ->
+                        { return Result.failure(new InventoryCountError.RepositoryFailure(err.message())); }
+                case Result.Success(var ignored) -> { }
+            }
+            return Result.success(count);
+        });
+    }
+}

backend/src/main/java/de/effigenix/application/inventory/command/RecordCountItemCommand.java

@@ -0,0 +1,8 @@
+package de.effigenix.application.inventory.command;
+
+public record RecordCountItemCommand(
+        String inventoryCountId,
+        String countItemId,
+        String actualQuantityAmount,
+        String actualQuantityUnit
+) {}

backend/src/main/java/de/effigenix/domain/inventory/CountItem.java

@@ -89,6 +89,10 @@ public class CountItem {
         return actualQuantity.amount().subtract(expectedQuantity.amount());
     }
 
+    void recordActualQuantity(Quantity actualQuantity) {
+        this.actualQuantity = actualQuantity;
+    }
+
     public boolean isCounted() {
         return actualQuantity != null;
     }

backend/src/main/java/de/effigenix/domain/inventory/InventoryCount.java

@@ -1,8 +1,10 @@
 package de.effigenix.domain.inventory;
 
 import de.effigenix.domain.masterdata.article.ArticleId;
+import de.effigenix.shared.common.Quantity;
 import de.effigenix.shared.common.Result;
 
+import java.math.BigDecimal;
 import java.time.Instant;
 import java.time.LocalDate;
 import java.time.format.DateTimeParseException;
@@ -21,6 +23,8 @@ import java.util.Objects;
  * - Max. one active count (OPEN/COUNTING) per StorageLocation (Application Layer)
  * - ArticleId unique within countItems
  * - addCountItem only in status OPEN
+ * - startCounting only in status OPEN, requires non-empty countItems
+ * - updateCountItem only in status COUNTING
  */
 public class InventoryCount {
 
@@ -28,7 +32,7 @@ public class InventoryCount {
     private final StorageLocationId storageLocationId;
     private final LocalDate countDate;
     private final String initiatedBy;
-    private final InventoryCountStatus status;
+    private InventoryCountStatus status;
     private final Instant createdAt;
     private final List<CountItem> countItems;
 
@@ -110,7 +114,7 @@ public class InventoryCount {
     public Result<InventoryCountError, CountItem> addCountItem(CountItemDraft draft) {
         if (status != InventoryCountStatus.OPEN) {
             return Result.failure(new InventoryCountError.InvalidStatusTransition(
-                    status.name(), "addCountItem requires OPEN"));
+                    status.name(), InventoryCountStatus.OPEN.name()));
         }
 
         CountItem item;
@@ -127,6 +131,50 @@ public class InventoryCount {
         return Result.success(item);
     }
 
+    // ==================== Status Transitions ====================
+
+    public Result<InventoryCountError, Void> startCounting() {
+        if (status != InventoryCountStatus.OPEN) {
+            return Result.failure(new InventoryCountError.InvalidStatusTransition(
+                    status.name(), InventoryCountStatus.COUNTING.name()));
+        }
+        if (countItems.isEmpty()) {
+            return Result.failure(new InventoryCountError.NoCountItems());
+        }
+        this.status = InventoryCountStatus.COUNTING;
+        return Result.success(null);
+    }
+
+    public Result<InventoryCountError, Void> updateCountItem(CountItemId itemId, Quantity actualQuantity) {
+        if (status != InventoryCountStatus.COUNTING) {
+            return Result.failure(new InventoryCountError.InvalidStatusTransition(
+                    status.name(), InventoryCountStatus.COUNTING.name()));
+        }
+
+        var item = countItems.stream()
+                .filter(ci -> ci.id().equals(itemId))
+                .findFirst()
+                .orElse(null);
+
+        if (item == null) {
+            return Result.failure(new InventoryCountError.CountItemNotFound(itemId.value()));
+        }
+
+        if (actualQuantity.uom() != item.expectedQuantity().uom()) {
+            return Result.failure(new InventoryCountError.InvalidQuantity(
+                    "Unit of measure mismatch: expected " + item.expectedQuantity().uom()
+                            + " but got " + actualQuantity.uom()));
+        }
+
+        if (actualQuantity.amount().compareTo(BigDecimal.ZERO) < 0) {
+            return Result.failure(new InventoryCountError.InvalidQuantity(
+                    "Actual quantity must be >= 0"));
+        }
+
+        item.recordActualQuantity(actualQuantity);
+        return Result.success(null);
+    }
+
     // ==================== Queries ====================
 
     public boolean isActive() {

backend/src/main/java/de/effigenix/domain/inventory/InventoryCountError.java

@@ -55,6 +55,11 @@ public sealed interface InventoryCountError {
         @Override public String message() { return "Not all count items have been counted"; }
     }
 
+    record InvalidCountItemId(String reason) implements InventoryCountError {
+        @Override public String code() { return "INVALID_COUNT_ITEM_ID"; }
+        @Override public String message() { return "Invalid count item ID: " + reason; }
+    }
+
     record CountItemNotFound(String id) implements InventoryCountError {
         @Override public String code() { return "COUNT_ITEM_NOT_FOUND"; }
         @Override public String message() { return "Count item not found: " + id; }

backend/src/main/java/de/effigenix/infrastructure/config/InventoryUseCaseConfiguration.java

@@ -3,6 +3,8 @@ package de.effigenix.infrastructure.config;
 import de.effigenix.application.inventory.CreateInventoryCount;
 import de.effigenix.application.inventory.GetInventoryCount;
 import de.effigenix.application.inventory.ListInventoryCounts;
+import de.effigenix.application.inventory.RecordCountItem;
+import de.effigenix.application.inventory.StartInventoryCount;
 import de.effigenix.application.inventory.GetStockMovement;
 import de.effigenix.application.inventory.ListStockMovements;
 import de.effigenix.application.inventory.ConfirmReservation;
@@ -172,4 +174,14 @@ public class InventoryUseCaseConfiguration {
     public ListInventoryCounts listInventoryCounts(InventoryCountRepository inventoryCountRepository, AuthorizationPort authorizationPort) {
         return new ListInventoryCounts(inventoryCountRepository, authorizationPort);
     }
+
+    @Bean
+    public StartInventoryCount startInventoryCount(InventoryCountRepository inventoryCountRepository, UnitOfWork unitOfWork, AuthorizationPort authorizationPort) {
+        return new StartInventoryCount(inventoryCountRepository, unitOfWork, authorizationPort);
+    }
+
+    @Bean
+    public RecordCountItem recordCountItem(InventoryCountRepository inventoryCountRepository, UnitOfWork unitOfWork, AuthorizationPort authorizationPort) {
+        return new RecordCountItem(inventoryCountRepository, unitOfWork, authorizationPort);
+    }
 }

backend/src/main/java/de/effigenix/infrastructure/inventory/web/controller/InventoryCountController.java

@@ -3,10 +3,14 @@ package de.effigenix.infrastructure.inventory.web.controller;
 import de.effigenix.application.inventory.CreateInventoryCount;
 import de.effigenix.application.inventory.GetInventoryCount;
 import de.effigenix.application.inventory.ListInventoryCounts;
+import de.effigenix.application.inventory.RecordCountItem;
+import de.effigenix.application.inventory.StartInventoryCount;
 import de.effigenix.application.inventory.command.CreateInventoryCountCommand;
+import de.effigenix.application.inventory.command.RecordCountItemCommand;
 import de.effigenix.domain.inventory.InventoryCountError;
 import de.effigenix.infrastructure.inventory.web.dto.CreateInventoryCountRequest;
 import de.effigenix.infrastructure.inventory.web.dto.InventoryCountResponse;
+import de.effigenix.infrastructure.inventory.web.dto.RecordCountItemRequest;
 import de.effigenix.shared.security.ActorId;
 import io.swagger.v3.oas.annotations.security.SecurityRequirement;
 import io.swagger.v3.oas.annotations.tags.Tag;
@@ -28,13 +32,19 @@ public class InventoryCountController {
     private final CreateInventoryCount createInventoryCount;
     private final GetInventoryCount getInventoryCount;
     private final ListInventoryCounts listInventoryCounts;
+    private final StartInventoryCount startInventoryCount;
+    private final RecordCountItem recordCountItem;
 
     public InventoryCountController(CreateInventoryCount createInventoryCount,
                                      GetInventoryCount getInventoryCount,
-                                     ListInventoryCounts listInventoryCounts) {
+                                     ListInventoryCounts listInventoryCounts,
+                                     StartInventoryCount startInventoryCount,
+                                     RecordCountItem recordCountItem) {
         this.createInventoryCount = createInventoryCount;
         this.getInventoryCount = getInventoryCount;
         this.listInventoryCounts = listInventoryCounts;
+        this.startInventoryCount = startInventoryCount;
+        this.recordCountItem = recordCountItem;
     }
 
     @PostMapping
@@ -92,6 +102,39 @@ public class InventoryCountController {
         return ResponseEntity.ok(responses);
     }
 
+    @PatchMapping("/{id}/start")
+    @PreAuthorize("hasAuthority('INVENTORY_COUNT_WRITE')")
+    public ResponseEntity<InventoryCountResponse> startInventoryCount(
+            @PathVariable String id,
+            Authentication authentication
+    ) {
+        var result = startInventoryCount.execute(id, ActorId.of(authentication.getName()));
+
+        if (result.isFailure()) {
+            throw new InventoryCountDomainErrorException(result.unsafeGetError());
+        }
+
+        return ResponseEntity.ok(InventoryCountResponse.from(result.unsafeGetValue()));
+    }
+
+    @PatchMapping("/{id}/items/{itemId}")
+    @PreAuthorize("hasAuthority('INVENTORY_COUNT_WRITE')")
+    public ResponseEntity<InventoryCountResponse> recordCountItem(
+            @PathVariable String id,
+            @PathVariable String itemId,
+            @Valid @RequestBody RecordCountItemRequest request,
+            Authentication authentication
+    ) {
+        var cmd = new RecordCountItemCommand(id, itemId, request.actualQuantityAmount(), request.actualQuantityUnit());
+        var result = recordCountItem.execute(cmd, ActorId.of(authentication.getName()));
+
+        if (result.isFailure()) {
+            throw new InventoryCountDomainErrorException(result.unsafeGetError());
+        }
+
+        return ResponseEntity.ok(InventoryCountResponse.from(result.unsafeGetValue()));
+    }
+
     // ==================== Exception Wrapper ====================
 
     public static class InventoryCountDomainErrorException extends RuntimeException {

backend/src/main/java/de/effigenix/infrastructure/inventory/web/dto/RecordCountItemRequest.java

@@ -0,0 +1,8 @@
+package de.effigenix.infrastructure.inventory.web.dto;
+
+import jakarta.validation.constraints.NotBlank;
+
+public record RecordCountItemRequest(
+        @NotBlank String actualQuantityAmount,
+        @NotBlank String actualQuantityUnit
+) {}

backend/src/main/java/de/effigenix/infrastructure/inventory/web/exception/InventoryErrorHttpStatusMapper.java

@@ -56,6 +56,7 @@ public final class InventoryErrorHttpStatusMapper {
     public static int toHttpStatus(InventoryCountError error) {
         return switch (error) {
             case InventoryCountError.InventoryCountNotFound e -> 404;
+            case InventoryCountError.InvalidCountItemId e -> 400;
             case InventoryCountError.CountItemNotFound e -> 404;
             case InventoryCountError.ActiveCountExists e -> 409;
             case InventoryCountError.DuplicateArticle e -> 409;

backend/src/test/java/de/effigenix/application/inventory/RecordCountItemTest.java

@@ -0,0 +1,309 @@
+package de.effigenix.application.inventory;
+
+import de.effigenix.application.inventory.command.RecordCountItemCommand;
+import de.effigenix.domain.inventory.*;
+import de.effigenix.domain.masterdata.article.ArticleId;
+import de.effigenix.shared.common.Quantity;
+import de.effigenix.shared.common.RepositoryError;
+import de.effigenix.shared.common.Result;
+import de.effigenix.shared.common.UnitOfMeasure;
+import de.effigenix.shared.persistence.UnitOfWork;
+import de.effigenix.shared.security.ActorId;
+import de.effigenix.shared.security.AuthorizationPort;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import java.math.BigDecimal;
+import java.time.Instant;
+import java.time.LocalDate;
+import java.util.List;
+import java.util.Optional;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.*;
+
+@ExtendWith(MockitoExtension.class)
+@DisplayName("RecordCountItem Use Case")
+class RecordCountItemTest {
+
+    @Mock private InventoryCountRepository inventoryCountRepository;
+    @Mock private UnitOfWork unitOfWork;
+    @Mock private AuthorizationPort authPort;
+
+    private RecordCountItem recordCountItem;
+    private final ActorId actorId = ActorId.of("user-1");
+
+    @BeforeEach
+    void setUp() {
+        recordCountItem = new RecordCountItem(inventoryCountRepository, unitOfWork, authPort);
+        lenient().when(authPort.can(any(ActorId.class), any())).thenReturn(true);
+    }
+
+    private void stubUnitOfWork() {
+        when(unitOfWork.executeAtomically(any())).thenAnswer(invocation -> {
+            @SuppressWarnings("unchecked")
+            var supplier = (java.util.function.Supplier<Result<?, ?>>) invocation.getArgument(0);
+            return supplier.get();
+        });
+    }
+
+    private InventoryCount createCountingCount() {
+        return InventoryCount.reconstitute(
+                InventoryCountId.of("count-1"), StorageLocationId.of("location-1"),
+                LocalDate.now(), "user-1", InventoryCountStatus.COUNTING,
+                Instant.now(), List.of(
+                        CountItem.reconstitute(CountItemId.of("item-1"), ArticleId.of("article-1"),
+                                Quantity.reconstitute(new BigDecimal("10.0"), UnitOfMeasure.KILOGRAM), null),
+                        CountItem.reconstitute(CountItemId.of("item-2"), ArticleId.of("article-2"),
+                                Quantity.reconstitute(new BigDecimal("5.0"), UnitOfMeasure.LITER), null)
+                )
+        );
+    }
+
+    @Test
+    @DisplayName("should record actual quantity on count item")
+    void shouldRecordActualQuantity() {
+        stubUnitOfWork();
+        var count = createCountingCount();
+        var cmd = new RecordCountItemCommand("count-1", "item-1", "8.5", "KILOGRAM");
+
+        when(inventoryCountRepository.findById(InventoryCountId.of("count-1")))
+                .thenReturn(Result.success(Optional.of(count)));
+        when(inventoryCountRepository.save(any(InventoryCount.class)))
+                .thenReturn(Result.success(null));
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isSuccess()).isTrue();
+        var item = result.unsafeGetValue().countItems().stream()
+                .filter(ci -> ci.id().equals(CountItemId.of("item-1")))
+                .findFirst().orElseThrow();
+        assertThat(item.actualQuantity().amount()).isEqualByComparingTo(new BigDecimal("8.5"));
+        assertThat(item.isCounted()).isTrue();
+    }
+
+    @Test
+    @DisplayName("should fail when not authorized")
+    void shouldFailWhenNotAuthorized() {
+        when(authPort.can(any(ActorId.class), any())).thenReturn(false);
+        var cmd = new RecordCountItemCommand("count-1", "item-1", "8.0", "KILOGRAM");
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.Unauthorized.class);
+        verify(inventoryCountRepository, never()).findById(any());
+    }
+
+    @Test
+    @DisplayName("should fail when inventory count not found")
+    void shouldFailWhenNotFound() {
+        var cmd = new RecordCountItemCommand("count-1", "item-1", "8.0", "KILOGRAM");
+
+        when(inventoryCountRepository.findById(InventoryCountId.of("count-1")))
+                .thenReturn(Result.success(Optional.empty()));
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InventoryCountNotFound.class);
+    }
+
+    @Test
+    @DisplayName("should fail when count item not found")
+    void shouldFailWhenCountItemNotFound() {
+        var count = createCountingCount();
+        var cmd = new RecordCountItemCommand("count-1", "nonexistent-but-valid", "8.0", "KILOGRAM");
+
+        when(inventoryCountRepository.findById(InventoryCountId.of("count-1")))
+                .thenReturn(Result.success(Optional.of(count)));
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.CountItemNotFound.class);
+    }
+
+    @Test
+    @DisplayName("should fail when countItemId is blank")
+    void shouldFailWhenCountItemIdBlank() {
+        var cmd = new RecordCountItemCommand("count-1", "  ", "8.0", "KILOGRAM");
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidCountItemId.class);
+    }
+
+    @Test
+    @DisplayName("should fail when countItemId is null")
+    void shouldFailWhenCountItemIdNull() {
+        var cmd = new RecordCountItemCommand("count-1", null, "8.0", "KILOGRAM");
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidCountItemId.class);
+    }
+
+    @Test
+    @DisplayName("should fail when quantity amount is invalid")
+    void shouldFailWhenQuantityAmountInvalid() {
+        var cmd = new RecordCountItemCommand("count-1", "item-1", "not-a-number", "KILOGRAM");
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidQuantity.class);
+    }
+
+    @Test
+    @DisplayName("should fail when quantity unit is invalid")
+    void shouldFailWhenUnitInvalid() {
+        var cmd = new RecordCountItemCommand("count-1", "item-1", "8.0", "INVALID_UNIT");
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidQuantity.class);
+    }
+
+    @Test
+    @DisplayName("should fail when UOM does not match expected")
+    void shouldFailWhenUomMismatch() {
+        var count = createCountingCount();
+        var cmd = new RecordCountItemCommand("count-1", "item-1", "8.0", "LITER");
+
+        when(inventoryCountRepository.findById(InventoryCountId.of("count-1")))
+                .thenReturn(Result.success(Optional.of(count)));
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidQuantity.class);
+    }
+
+    @Test
+    @DisplayName("should fail when inventoryCountId is blank")
+    void shouldFailWhenIdBlank() {
+        var cmd = new RecordCountItemCommand("", "item-1", "8.0", "KILOGRAM");
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidInventoryCountId.class);
+    }
+
+    @Test
+    @DisplayName("should fail when quantity amount is null")
+    void shouldFailWhenQuantityAmountNull() {
+        var cmd = new RecordCountItemCommand("count-1", "item-1", null, "KILOGRAM");
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidQuantity.class);
+    }
+
+    @Test
+    @DisplayName("should fail when quantity unit is null")
+    void shouldFailWhenUnitNull() {
+        var cmd = new RecordCountItemCommand("count-1", "item-1", "8.0", null);
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidQuantity.class);
+    }
+
+    @Test
+    @DisplayName("should fail when status is OPEN")
+    void shouldFailWhenStatusIsOpen() {
+        var openCount = InventoryCount.reconstitute(
+                InventoryCountId.of("count-1"), StorageLocationId.of("location-1"),
+                LocalDate.now(), "user-1", InventoryCountStatus.OPEN,
+                Instant.now(), List.of(
+                        CountItem.reconstitute(CountItemId.of("item-1"), ArticleId.of("article-1"),
+                                Quantity.reconstitute(new BigDecimal("10.0"), UnitOfMeasure.KILOGRAM), null)
+                )
+        );
+        var cmd = new RecordCountItemCommand("count-1", "item-1", "8.0", "KILOGRAM");
+
+        when(inventoryCountRepository.findById(InventoryCountId.of("count-1")))
+                .thenReturn(Result.success(Optional.of(openCount)));
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidStatusTransition.class);
+    }
+
+    @Test
+    @DisplayName("should fail when repository findById fails")
+    void shouldFailWhenRepositoryFails() {
+        var cmd = new RecordCountItemCommand("count-1", "item-1", "8.0", "KILOGRAM");
+
+        when(inventoryCountRepository.findById(InventoryCountId.of("count-1")))
+                .thenReturn(Result.failure(new RepositoryError.DatabaseError("connection lost")));
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.RepositoryFailure.class);
+    }
+
+    @Test
+    @DisplayName("should fail when save fails")
+    void shouldFailWhenSaveFails() {
+        stubUnitOfWork();
+        var count = createCountingCount();
+        var cmd = new RecordCountItemCommand("count-1", "item-1", "8.0", "KILOGRAM");
+
+        when(inventoryCountRepository.findById(InventoryCountId.of("count-1")))
+                .thenReturn(Result.success(Optional.of(count)));
+        when(inventoryCountRepository.save(any(InventoryCount.class)))
+                .thenReturn(Result.failure(new RepositoryError.DatabaseError("save failed")));
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.RepositoryFailure.class);
+    }
+
+    @Test
+    @DisplayName("should allow zero actual quantity")
+    void shouldAllowZeroActualQuantity() {
+        stubUnitOfWork();
+        var count = createCountingCount();
+        var cmd = new RecordCountItemCommand("count-1", "item-1", "0", "KILOGRAM");
+
+        when(inventoryCountRepository.findById(InventoryCountId.of("count-1")))
+                .thenReturn(Result.success(Optional.of(count)));
+        when(inventoryCountRepository.save(any(InventoryCount.class)))
+                .thenReturn(Result.success(null));
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isSuccess()).isTrue();
+    }
+
+    @Test
+    @DisplayName("should fail when negative actual quantity")
+    void shouldFailWhenNegativeQuantity() {
+        var count = createCountingCount();
+        var cmd = new RecordCountItemCommand("count-1", "item-1", "-5", "KILOGRAM");
+
+        when(inventoryCountRepository.findById(InventoryCountId.of("count-1")))
+                .thenReturn(Result.success(Optional.of(count)));
+
+        var result = recordCountItem.execute(cmd, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidQuantity.class);
+    }
+}

backend/src/test/java/de/effigenix/application/inventory/StartInventoryCountTest.java

@@ -0,0 +1,193 @@
+package de.effigenix.application.inventory;
+
+import de.effigenix.domain.inventory.*;
+import de.effigenix.domain.masterdata.article.ArticleId;
+import de.effigenix.shared.common.Quantity;
+import de.effigenix.shared.common.RepositoryError;
+import de.effigenix.shared.common.Result;
+import de.effigenix.shared.common.UnitOfMeasure;
+import de.effigenix.shared.persistence.UnitOfWork;
+import de.effigenix.shared.security.ActorId;
+import de.effigenix.shared.security.AuthorizationPort;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import java.math.BigDecimal;
+import java.time.Instant;
+import java.time.LocalDate;
+import java.util.List;
+import java.util.Optional;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.*;
+
+@ExtendWith(MockitoExtension.class)
+@DisplayName("StartInventoryCount Use Case")
+class StartInventoryCountTest {
+
+    @Mock private InventoryCountRepository inventoryCountRepository;
+    @Mock private UnitOfWork unitOfWork;
+    @Mock private AuthorizationPort authPort;
+
+    private StartInventoryCount startInventoryCount;
+    private final ActorId actorId = ActorId.of("user-1");
+
+    @BeforeEach
+    void setUp() {
+        startInventoryCount = new StartInventoryCount(inventoryCountRepository, unitOfWork, authPort);
+        lenient().when(authPort.can(any(ActorId.class), any())).thenReturn(true);
+    }
+
+    private void stubUnitOfWork() {
+        when(unitOfWork.executeAtomically(any())).thenAnswer(invocation -> {
+            @SuppressWarnings("unchecked")
+            var supplier = (java.util.function.Supplier<Result<?, ?>>) invocation.getArgument(0);
+            return supplier.get();
+        });
+    }
+
+    private InventoryCount createOpenCountWithItems() {
+        var items = List.of(
+                CountItem.reconstitute(CountItemId.of("item-1"), ArticleId.of("article-1"),
+                        Quantity.reconstitute(new BigDecimal("10.0"), UnitOfMeasure.KILOGRAM), null)
+        );
+        return InventoryCount.reconstitute(
+                InventoryCountId.of("count-1"), StorageLocationId.of("location-1"),
+                LocalDate.now(), "user-1", InventoryCountStatus.OPEN,
+                Instant.now(), items
+        );
+    }
+
+    @Test
+    @DisplayName("should start counting for OPEN count with items")
+    void shouldStartCounting() {
+        stubUnitOfWork();
+        var count = createOpenCountWithItems();
+
+        when(inventoryCountRepository.findById(InventoryCountId.of("count-1")))
+                .thenReturn(Result.success(Optional.of(count)));
+        when(inventoryCountRepository.save(any(InventoryCount.class)))
+                .thenReturn(Result.success(null));
+
+        var result = startInventoryCount.execute("count-1", actorId);
+
+        assertThat(result.isSuccess()).isTrue();
+        assertThat(result.unsafeGetValue().status()).isEqualTo(InventoryCountStatus.COUNTING);
+        verify(inventoryCountRepository).save(any(InventoryCount.class));
+    }
+
+    @Test
+    @DisplayName("should fail when not authorized")
+    void shouldFailWhenNotAuthorized() {
+        when(authPort.can(any(ActorId.class), any())).thenReturn(false);
+
+        var result = startInventoryCount.execute("count-1", actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.Unauthorized.class);
+        verify(inventoryCountRepository, never()).findById(any());
+    }
+
+    @Test
+    @DisplayName("should fail when inventory count not found")
+    void shouldFailWhenNotFound() {
+        when(inventoryCountRepository.findById(InventoryCountId.of("count-1")))
+                .thenReturn(Result.success(Optional.empty()));
+
+        var result = startInventoryCount.execute("count-1", actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InventoryCountNotFound.class);
+    }
+
+    @Test
+    @DisplayName("should fail when count has no items")
+    void shouldFailWhenNoItems() {
+        var emptyCount = InventoryCount.reconstitute(
+                InventoryCountId.of("count-1"), StorageLocationId.of("location-1"),
+                LocalDate.now(), "user-1", InventoryCountStatus.OPEN,
+                Instant.now(), List.of()
+        );
+
+        when(inventoryCountRepository.findById(InventoryCountId.of("count-1")))
+                .thenReturn(Result.success(Optional.of(emptyCount)));
+
+        var result = startInventoryCount.execute("count-1", actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.NoCountItems.class);
+    }
+
+    @Test
+    @DisplayName("should fail when count is already COUNTING")
+    void shouldFailWhenAlreadyCounting() {
+        var countingCount = InventoryCount.reconstitute(
+                InventoryCountId.of("count-1"), StorageLocationId.of("location-1"),
+                LocalDate.now(), "user-1", InventoryCountStatus.COUNTING,
+                Instant.now(), List.of(
+                        CountItem.reconstitute(CountItemId.of("item-1"), ArticleId.of("article-1"),
+                                Quantity.reconstitute(new BigDecimal("10.0"), UnitOfMeasure.KILOGRAM), null)
+                )
+        );
+
+        when(inventoryCountRepository.findById(InventoryCountId.of("count-1")))
+                .thenReturn(Result.success(Optional.of(countingCount)));
+
+        var result = startInventoryCount.execute("count-1", actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidStatusTransition.class);
+    }
+
+    @Test
+    @DisplayName("should fail when inventoryCountId is blank")
+    void shouldFailWhenIdBlank() {
+        var result = startInventoryCount.execute("", actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidInventoryCountId.class);
+    }
+
+    @Test
+    @DisplayName("should fail when inventoryCountId is null")
+    void shouldFailWhenIdNull() {
+        var result = startInventoryCount.execute(null, actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidInventoryCountId.class);
+    }
+
+    @Test
+    @DisplayName("should fail when repository findById fails")
+    void shouldFailWhenRepositoryFails() {
+        when(inventoryCountRepository.findById(InventoryCountId.of("count-1")))
+                .thenReturn(Result.failure(new RepositoryError.DatabaseError("connection lost")));
+
+        var result = startInventoryCount.execute("count-1", actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.RepositoryFailure.class);
+    }
+
+    @Test
+    @DisplayName("should fail when save fails")
+    void shouldFailWhenSaveFails() {
+        stubUnitOfWork();
+        var count = createOpenCountWithItems();
+
+        when(inventoryCountRepository.findById(InventoryCountId.of("count-1")))
+                .thenReturn(Result.success(Optional.of(count)));
+        when(inventoryCountRepository.save(any(InventoryCount.class)))
+                .thenReturn(Result.failure(new RepositoryError.DatabaseError("save failed")));
+
+        var result = startInventoryCount.execute("count-1", actorId);
+
+        assertThat(result.isFailure()).isTrue();
+        assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.RepositoryFailure.class);
+    }
+}

backend/src/test/java/de/effigenix/domain/inventory/InventoryCountTest.java

@@ -10,6 +10,7 @@ import org.junit.jupiter.api.Test;
 import java.math.BigDecimal;
 import java.time.Instant;
 import java.time.LocalDate;
+import java.util.ArrayList;
 import java.util.List;
 
 import static org.assertj.core.api.Assertions.*;
@@ -386,6 +387,231 @@ class InventoryCountTest {
         }
     }
 
+    // ==================== startCounting ====================
+
+    @Nested
+    @DisplayName("startCounting()")
+    class StartCounting {
+
+        @Test
+        @DisplayName("should transition from OPEN to COUNTING when countItems exist")
+        void shouldTransitionToCountingWhenItemsExist() {
+            var count = createOpenCount();
+            count.addCountItem(new CountItemDraft("article-1", "10.0", "KILOGRAM"));
+
+            var result = count.startCounting();
+
+            assertThat(result.isSuccess()).isTrue();
+            assertThat(count.status()).isEqualTo(InventoryCountStatus.COUNTING);
+        }
+
+        @Test
+        @DisplayName("should fail when status is COUNTING")
+        void shouldFailWhenStatusIsCounting() {
+            var count = createCountingCount();
+
+            var result = count.startCounting();
+
+            assertThat(result.isFailure()).isTrue();
+            assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidStatusTransition.class);
+        }
+
+        @Test
+        @DisplayName("should fail when status is COMPLETED")
+        void shouldFailWhenStatusIsCompleted() {
+            var count = reconstitute(InventoryCountStatus.COMPLETED, createCountItems());
+
+            var result = count.startCounting();
+
+            assertThat(result.isFailure()).isTrue();
+            assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidStatusTransition.class);
+        }
+
+        @Test
+        @DisplayName("should fail when status is CANCELLED")
+        void shouldFailWhenStatusIsCancelled() {
+            var count = reconstitute(InventoryCountStatus.CANCELLED, createCountItems());
+
+            var result = count.startCounting();
+
+            assertThat(result.isFailure()).isTrue();
+            assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidStatusTransition.class);
+        }
+
+        @Test
+        @DisplayName("should fail when countItems is empty")
+        void shouldFailWhenNoCountItems() {
+            var count = createOpenCount();
+
+            var result = count.startCounting();
+
+            assertThat(result.isFailure()).isTrue();
+            assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.NoCountItems.class);
+        }
+
+        private InventoryCount reconstitute(InventoryCountStatus status, List<CountItem> items) {
+            return InventoryCount.reconstitute(
+                    InventoryCountId.of("count-1"), StorageLocationId.of("location-1"),
+                    LocalDate.now(), "user-1", status,
+                    Instant.now(), items
+            );
+        }
+    }
+
+    // ==================== updateCountItem ====================
+
+    @Nested
+    @DisplayName("updateCountItem()")
+    class UpdateCountItem {
+
+        @Test
+        @DisplayName("should record actual quantity on count item")
+        void shouldRecordActualQuantity() {
+            var count = createCountingCount();
+            var itemId = count.countItems().getFirst().id();
+            var actualQty = Quantity.reconstitute(new BigDecimal("8.0"), UnitOfMeasure.KILOGRAM);
+
+            var result = count.updateCountItem(itemId, actualQty);
+
+            assertThat(result.isSuccess()).isTrue();
+            assertThat(count.countItems().getFirst().actualQuantity()).isEqualTo(actualQty);
+            assertThat(count.countItems().getFirst().isCounted()).isTrue();
+        }
+
+        @Test
+        @DisplayName("should allow zero actual quantity")
+        void shouldAllowZeroActualQuantity() {
+            var count = createCountingCount();
+            var itemId = count.countItems().getFirst().id();
+            var actualQty = Quantity.reconstitute(BigDecimal.ZERO, UnitOfMeasure.KILOGRAM);
+
+            var result = count.updateCountItem(itemId, actualQty);
+
+            assertThat(result.isSuccess()).isTrue();
+            assertThat(count.countItems().getFirst().actualQuantity().amount())
+                    .isEqualByComparingTo(BigDecimal.ZERO);
+        }
+
+        @Test
+        @DisplayName("should compute deviation after recording")
+        void shouldComputeDeviationAfterRecording() {
+            var count = createCountingCount();
+            var itemId = count.countItems().getFirst().id();
+            var actualQty = Quantity.reconstitute(new BigDecimal("7.0"), UnitOfMeasure.KILOGRAM);
+
+            count.updateCountItem(itemId, actualQty);
+
+            assertThat(count.countItems().getFirst().deviation())
+                    .isEqualByComparingTo(new BigDecimal("-3.0"));
+        }
+
+        @Test
+        @DisplayName("should fail when status is OPEN")
+        void shouldFailWhenStatusIsOpen() {
+            var count = createOpenCount();
+            count.addCountItem(new CountItemDraft("article-1", "10.0", "KILOGRAM"));
+            var itemId = count.countItems().getFirst().id();
+            var actualQty = Quantity.reconstitute(new BigDecimal("8.0"), UnitOfMeasure.KILOGRAM);
+
+            var result = count.updateCountItem(itemId, actualQty);
+
+            assertThat(result.isFailure()).isTrue();
+            assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidStatusTransition.class);
+        }
+
+        @Test
+        @DisplayName("should fail when status is COMPLETED")
+        void shouldFailWhenStatusIsCompleted() {
+            var items = createCountItems();
+            var count = InventoryCount.reconstitute(
+                    InventoryCountId.of("count-1"), StorageLocationId.of("location-1"),
+                    LocalDate.now(), "user-1", InventoryCountStatus.COMPLETED,
+                    Instant.now(), items
+            );
+            var actualQty = Quantity.reconstitute(new BigDecimal("8.0"), UnitOfMeasure.KILOGRAM);
+
+            var result = count.updateCountItem(items.getFirst().id(), actualQty);
+
+            assertThat(result.isFailure()).isTrue();
+            assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidStatusTransition.class);
+        }
+
+        @Test
+        @DisplayName("should fail when countItem not found")
+        void shouldFailWhenCountItemNotFound() {
+            var count = createCountingCount();
+            var actualQty = Quantity.reconstitute(new BigDecimal("8.0"), UnitOfMeasure.KILOGRAM);
+
+            var result = count.updateCountItem(CountItemId.of("nonexistent"), actualQty);
+
+            assertThat(result.isFailure()).isTrue();
+            assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.CountItemNotFound.class);
+        }
+
+        @Test
+        @DisplayName("should fail when UOM does not match expected quantity")
+        void shouldFailWhenUomMismatch() {
+            var count = createCountingCount();
+            var itemId = count.countItems().getFirst().id();
+            var actualQty = Quantity.reconstitute(new BigDecimal("8.0"), UnitOfMeasure.LITER);
+
+            var result = count.updateCountItem(itemId, actualQty);
+
+            assertThat(result.isFailure()).isTrue();
+            assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidQuantity.class);
+        }
+
+        @Test
+        @DisplayName("should fail when actual quantity is negative")
+        void shouldFailWhenNegativeQuantity() {
+            var count = createCountingCount();
+            var itemId = count.countItems().getFirst().id();
+            var actualQty = Quantity.reconstitute(new BigDecimal("-1.0"), UnitOfMeasure.KILOGRAM);
+
+            var result = count.updateCountItem(itemId, actualQty);
+
+            assertThat(result.isFailure()).isTrue();
+            assertThat(result.unsafeGetError()).isInstanceOf(InventoryCountError.InvalidQuantity.class);
+        }
+
+        @Test
+        @DisplayName("should allow overwriting previously recorded quantity")
+        void shouldAllowOverwritingRecordedQuantity() {
+            var count = createCountingCount();
+            var itemId = count.countItems().getFirst().id();
+
+            count.updateCountItem(itemId, Quantity.reconstitute(new BigDecimal("8.0"), UnitOfMeasure.KILOGRAM));
+            var result = count.updateCountItem(itemId, Quantity.reconstitute(new BigDecimal("12.0"), UnitOfMeasure.KILOGRAM));
+
+            assertThat(result.isSuccess()).isTrue();
+            assertThat(count.countItems().getFirst().actualQuantity().amount())
+                    .isEqualByComparingTo(new BigDecimal("12.0"));
+        }
+
+        @Test
+        @DisplayName("should update specific item in multi-item count")
+        void shouldUpdateSpecificItemInMultiItemCount() {
+            var items = List.of(
+                    CountItem.reconstitute(CountItemId.of("item-1"), ArticleId.of("article-1"),
+                            Quantity.reconstitute(new BigDecimal("10.0"), UnitOfMeasure.KILOGRAM), null),
+                    CountItem.reconstitute(CountItemId.of("item-2"), ArticleId.of("article-2"),
+                            Quantity.reconstitute(new BigDecimal("5.0"), UnitOfMeasure.LITER), null)
+            );
+            var count = InventoryCount.reconstitute(
+                    InventoryCountId.of("count-1"), StorageLocationId.of("location-1"),
+                    LocalDate.now(), "user-1", InventoryCountStatus.COUNTING,
+                    Instant.now(), items
+            );
+            var actualQty = Quantity.reconstitute(new BigDecimal("3.0"), UnitOfMeasure.LITER);
+
+            var result = count.updateCountItem(CountItemId.of("item-2"), actualQty);
+
+            assertThat(result.isSuccess()).isTrue();
+            assertThat(count.countItems().get(1).actualQuantity()).isEqualTo(actualQty);
+            assertThat(count.countItems().getFirst().actualQuantity()).isNull();
+        }
+    }
+
     // ==================== CountItem deviation ====================
 
     @Nested
@@ -599,4 +825,20 @@ class InventoryCountTest {
                 new InventoryCountDraft("location-1", LocalDate.now().toString(), "user-1")
         ).unsafeGetValue();
     }
+
+    private InventoryCount createCountingCount() {
+        var items = createCountItems();
+        return InventoryCount.reconstitute(
+                InventoryCountId.of("count-1"), StorageLocationId.of("location-1"),
+                LocalDate.now(), "user-1", InventoryCountStatus.COUNTING,
+                Instant.now(), items
+        );
+    }
+
+    private List<CountItem> createCountItems() {
+        return new ArrayList<>(List.of(
+                CountItem.reconstitute(CountItemId.of("item-1"), ArticleId.of("article-1"),
+                        Quantity.reconstitute(new BigDecimal("10.0"), UnitOfMeasure.KILOGRAM), null)
+        ));
+    }
 }

backend/src/test/java/de/effigenix/infrastructure/inventory/web/InventoryCountControllerIntegrationTest.java

@@ -12,7 +12,7 @@ import java.time.LocalDate;
 import java.util.Set;
 import java.util.UUID;
 
-import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.post;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;
 
@@ -21,6 +21,7 @@ import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.
  *
  * Abgedeckte Testfälle:
  * - US-6.1 – Inventur anlegen und Zählpositionen befüllen
+ * - US-6.2 – Inventur durchführen (Ist-Mengen erfassen)
  */
 @DisplayName("InventoryCount Controller Integration Tests")
 class InventoryCountControllerIntegrationTest extends AbstractIntegrationTest {
@@ -295,8 +296,253 @@ class InventoryCountControllerIntegrationTest extends AbstractIntegrationTest {
                 .andExpect(jsonPath("$.length()").value(0));
     }
 
+    // ==================== US-6.2: Inventur starten ====================
+
+    @Test
+    @DisplayName("Inventur starten → 200 mit Status COUNTING")
+    void startInventoryCount_returns200() throws Exception {
+        String countId = createInventoryCountWithStock();
+
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + countId + "/start")
+                        .header("Authorization", "Bearer " + adminToken))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$.id").value(countId))
+                .andExpect(jsonPath("$.status").value("COUNTING"));
+    }
+
+    @Test
+    @DisplayName("Inventur ohne Items starten → 400 NoCountItems")
+    void startInventoryCount_noItems_returns400() throws Exception {
+        // Inventur ohne Stock (= ohne Items) anlegen
+        var request = new CreateInventoryCountRequest(storageLocationId, LocalDate.now().toString());
+        var createResult = mockMvc.perform(post("/api/inventory/inventory-counts")
+                        .header("Authorization", "Bearer " + adminToken)
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content(objectMapper.writeValueAsString(request)))
+                .andExpect(status().isCreated())
+                .andReturn();
+
+        String countId = objectMapper.readTree(createResult.getResponse().getContentAsString()).get("id").asText();
+
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + countId + "/start")
+                        .header("Authorization", "Bearer " + adminToken))
+                .andExpect(status().isConflict())
+                .andExpect(jsonPath("$.code").value("NO_COUNT_ITEMS"));
+    }
+
+    @Test
+    @DisplayName("Bereits gestartete Inventur nochmal starten → 400")
+    void startInventoryCount_alreadyCounting_returns400() throws Exception {
+        String countId = createInventoryCountWithStock();
+
+        // Erster Start
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + countId + "/start")
+                        .header("Authorization", "Bearer " + adminToken))
+                .andExpect(status().isOk());
+
+        // Zweiter Start
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + countId + "/start")
+                        .header("Authorization", "Bearer " + adminToken))
+                .andExpect(status().isConflict())
+                .andExpect(jsonPath("$.code").value("INVALID_STATUS_TRANSITION"));
+    }
+
+    @Test
+    @DisplayName("Inventur starten mit unbekannter ID → 404")
+    void startInventoryCount_notFound_returns404() throws Exception {
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + UUID.randomUUID() + "/start")
+                        .header("Authorization", "Bearer " + adminToken))
+                .andExpect(status().isNotFound())
+                .andExpect(jsonPath("$.code").value("INVENTORY_COUNT_NOT_FOUND"));
+    }
+
+    @Test
+    @DisplayName("Inventur starten ohne Token → 401")
+    void startInventoryCount_noToken_returns401() throws Exception {
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + UUID.randomUUID() + "/start"))
+                .andExpect(status().isUnauthorized());
+    }
+
+    @Test
+    @DisplayName("Inventur starten ohne INVENTORY_COUNT_WRITE → 403")
+    void startInventoryCount_noPermission_returns403() throws Exception {
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + UUID.randomUUID() + "/start")
+                        .header("Authorization", "Bearer " + viewerToken))
+                .andExpect(status().isForbidden());
+    }
+
+    // ==================== US-6.2: Ist-Mengen erfassen ====================
+
+    @Test
+    @DisplayName("Ist-Menge erfassen → 200 mit actualQuantity und Deviation")
+    void recordCountItem_returns200() throws Exception {
+        String countId = createInventoryCountWithStock();
+
+        // Inventur starten
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + countId + "/start")
+                        .header("Authorization", "Bearer " + adminToken))
+                .andExpect(status().isOk());
+
+        // Item-ID holen
+        String itemId = getFirstCountItemId(countId);
+
+        String body = """
+                {"actualQuantityAmount": "20.0", "actualQuantityUnit": "KILOGRAM"}
+                """;
+
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + countId + "/items/" + itemId)
+                        .header("Authorization", "Bearer " + adminToken)
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content(body))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$.status").value("COUNTING"))
+                .andExpect(jsonPath("$.countItems[0].actualQuantityAmount").isNumber())
+                .andExpect(jsonPath("$.countItems[0].deviation").isNumber());
+    }
+
+    @Test
+    @DisplayName("Ist-Menge erfassen in OPEN Status → 409")
+    void recordCountItem_openStatus_returns409() throws Exception {
+        String countId = createInventoryCountWithStock();
+
+        // NICHT starten → bleibt OPEN
+        String itemId = getFirstCountItemId(countId);
+
+        String body = """
+                {"actualQuantityAmount": "20.0", "actualQuantityUnit": "KILOGRAM"}
+                """;
+
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + countId + "/items/" + itemId)
+                        .header("Authorization", "Bearer " + adminToken)
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content(body))
+                .andExpect(status().isConflict())
+                .andExpect(jsonPath("$.code").value("INVALID_STATUS_TRANSITION"));
+    }
+
+    @Test
+    @DisplayName("Ist-Menge mit falscher UOM → 400")
+    void recordCountItem_wrongUom_returns400() throws Exception {
+        String countId = createInventoryCountWithStock();
+
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + countId + "/start")
+                        .header("Authorization", "Bearer " + adminToken))
+                .andExpect(status().isOk());
+
+        String itemId = getFirstCountItemId(countId);
+
+        String body = """
+                {"actualQuantityAmount": "20.0", "actualQuantityUnit": "LITER"}
+                """;
+
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + countId + "/items/" + itemId)
+                        .header("Authorization", "Bearer " + adminToken)
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content(body))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.code").value("INVALID_QUANTITY"));
+    }
+
+    @Test
+    @DisplayName("Ist-Menge für unbekanntes Item → 400")
+    void recordCountItem_itemNotFound_returns400() throws Exception {
+        String countId = createInventoryCountWithStock();
+
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + countId + "/start")
+                        .header("Authorization", "Bearer " + adminToken))
+                .andExpect(status().isOk());
+
+        String body = """
+                {"actualQuantityAmount": "20.0", "actualQuantityUnit": "KILOGRAM"}
+                """;
+
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + countId + "/items/" + UUID.randomUUID())
+                        .header("Authorization", "Bearer " + adminToken)
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content(body))
+                .andExpect(status().isNotFound())
+                .andExpect(jsonPath("$.code").value("COUNT_ITEM_NOT_FOUND"));
+    }
+
+    @Test
+    @DisplayName("Ist-Menge erfassen ohne Token → 401")
+    void recordCountItem_noToken_returns401() throws Exception {
+        String body = """
+                {"actualQuantityAmount": "20.0", "actualQuantityUnit": "KILOGRAM"}
+                """;
+
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + UUID.randomUUID() + "/items/" + UUID.randomUUID())
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content(body))
+                .andExpect(status().isUnauthorized());
+    }
+
+    @Test
+    @DisplayName("Ist-Menge erfassen ohne Permission → 403")
+    void recordCountItem_noPermission_returns403() throws Exception {
+        String body = """
+                {"actualQuantityAmount": "20.0", "actualQuantityUnit": "KILOGRAM"}
+                """;
+
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + UUID.randomUUID() + "/items/" + UUID.randomUUID())
+                        .header("Authorization", "Bearer " + viewerToken)
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content(body))
+                .andExpect(status().isForbidden());
+    }
+
+    @Test
+    @DisplayName("Ist-Menge ohne Body-Felder → 400 (Validation)")
+    void recordCountItem_missingFields_returns400() throws Exception {
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + UUID.randomUUID() + "/items/" + UUID.randomUUID())
+                        .header("Authorization", "Bearer " + adminToken)
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("{}"))
+                .andExpect(status().isBadRequest());
+    }
+
+    @Test
+    @DisplayName("Ist-Menge mit Null-Wert für actualQuantityAmount → 400")
+    void recordCountItem_nullAmount_returns400() throws Exception {
+        String body = """
+                {"actualQuantityAmount": null, "actualQuantityUnit": "KILOGRAM"}
+                """;
+
+        mockMvc.perform(patch("/api/inventory/inventory-counts/" + UUID.randomUUID() + "/items/" + UUID.randomUUID())
+                        .header("Authorization", "Bearer " + adminToken)
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content(body))
+                .andExpect(status().isBadRequest());
+    }
+
     // ==================== Helpers ====================
 
+    private String createInventoryCountWithStock() throws Exception {
+        String articleId = createArticleId();
+        createStockWithBatch(articleId, storageLocationId);
+
+        var request = new CreateInventoryCountRequest(storageLocationId, LocalDate.now().toString());
+
+        var result = mockMvc.perform(post("/api/inventory/inventory-counts")
+                        .header("Authorization", "Bearer " + adminToken)
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content(objectMapper.writeValueAsString(request)))
+                .andExpect(status().isCreated())
+                .andReturn();
+
+        return objectMapper.readTree(result.getResponse().getContentAsString()).get("id").asText();
+    }
+
+    private String getFirstCountItemId(String countId) throws Exception {
+        var getResult = mockMvc.perform(get("/api/inventory/inventory-counts/" + countId)
+                        .header("Authorization", "Bearer " + adminToken))
+                .andExpect(status().isOk())
+                .andReturn();
+
+        return objectMapper.readTree(getResult.getResponse().getContentAsString())
+                .get("countItems").get(0).get("id").asText();
+    }
+
     private String createStorageLocation() throws Exception {
         String json = """
                 {"name": "Testlager-%s", "storageType": "DRY_STORAGE"}