Distribution/Features/Stock/IStockRepository.cs

@@ -28,5 +28,7 @@ public interface IStockRepository
 
     // Serial numbers
     Task<bool> SerialNumberExistsAsync(Guid itemId, string serialNumber);
+    /// <summary>One past the highest numeric suffix among the item's existing serials that start with <paramref name="prefix"/> (1 if none).</summary>
+    Task<int> GetNextSerialSequenceAsync(Guid itemId, string prefix);
     Task AddItemSerialNumberAsync(ItemSerialNumber serial);
 }

Distribution/Features/Stock/StockController.cs

@@ -37,6 +37,14 @@ public class StockController(StockService service, ICurrentUserService currentUs
         return Ok(result);
     }
 
+    /// <summary>Next free number for auto-generated serials (e.g. after ...-0005 the answer is 6).</summary>
+    [HttpGet("serials/next-sequence")]
+    public async Task<IActionResult> GetNextSerialSequence([FromQuery] Guid itemId, [FromQuery] string prefix = "")
+    {
+        var next = await service.GetNextSerialSequenceAsync(itemId, prefix);
+        return Ok(new { next });
+    }
+
     [HttpGet("movements")]
     public async Task<IActionResult> GetMovements(
         [FromQuery] Guid? locationId,

Distribution/Features/Stock/StockRepository.cs

@@ -107,6 +107,26 @@ public class StockRepository(AppDBContext db) : IStockRepository
     public Task<bool> SerialNumberExistsAsync(Guid itemId, string serialNumber) =>
         db.ItemSerialNumbers.AnyAsync(s => s.FK_ItemId == itemId && s.SerialNumber == serialNumber);
 
+    public async Task<int> GetNextSerialSequenceAsync(Guid itemId, string prefix)
+    {
+        // Only serials of the form <prefix><digits> count; anything else (a supplier's own serial that
+        // happens to start with the prefix) is ignored for numbering.
+        var serials = await db.ItemSerialNumbers
+            .AsNoTracking()
+            .Where(s => s.FK_ItemId == itemId && s.SerialNumber.StartsWith(prefix))
+            .Select(s => s.SerialNumber)
+            .ToListAsync();
+
+        var max = 0L;
+        foreach (var serial in serials)
+        {
+            var suffix = serial[prefix.Length..];
+            if (suffix.Length is > 0 and <= 9 && suffix.All(char.IsDigit) && long.TryParse(suffix, out var n))
+                max = Math.Max(max, n);
+        }
+        return (int)max + 1;
+    }
+
     public async Task AddItemSerialNumberAsync(ItemSerialNumber serial) =>
         await db.ItemSerialNumbers.AddAsync(serial);
 }

Distribution/Features/Stock/StockService.cs

@@ -96,6 +96,10 @@ public class StockService(
             b.Quantity, b.Item.ReorderLevel!.Value)).ToList();
     }
 
+    /// <summary>The next unused sequence number for generating serials like "&lt;prefix&gt;0001" for an item.</summary>
+    public Task<int> GetNextSerialSequenceAsync(Guid itemId, string prefix) =>
+        repo.GetNextSerialSequenceAsync(itemId, prefix ?? string.Empty);
+
     public async Task<StockTotalResponse?> GetItemTotalAsync(Guid itemId)
     {
         var item = await itemRepo.GetByIdAsync(itemId);