diff --git a/SVSim.Database/Services/Inventory/InventoryTransaction.cs b/SVSim.Database/Services/Inventory/InventoryTransaction.cs
index a6337db..f812ba3 100644
--- a/SVSim.Database/Services/Inventory/InventoryTransaction.cs
+++ b/SVSim.Database/Services/Inventory/InventoryTransaction.cs
@@ -200,8 +200,42 @@ internal sealed class InventoryTransaction : IInventoryTransaction
         }
     }
 
-    public Task<int> BackfillCardCosmeticsAsync(CancellationToken ct = default)
-        => throw new NotImplementedException();
+    public async Task<int> BackfillCardCosmeticsAsync(CancellationToken ct = default)
+    {
+        ThrowIfCommitted();
+
+        var lookupIds = Viewer.Cards
+            .Select(c => c.Card.IsFoil ? c.Card.Id - 1 : c.Card.Id)
+            .Distinct()
+            .ToList();
+
+        var cascade = await _db.CardCosmeticRewards
+            .Where(r => lookupIds.Contains(r.CardId))
+            .ToListAsync(ct);
+
+        int granted = 0;
+        foreach (var reward in cascade)
+        {
+            if (AlreadyOwnsCosmetic(reward.Type, reward.CosmeticId)) continue;
+            if (TryAddCascadeCosmetic(reward, reward.CardId))
+            {
+                granted++;
+                _ops.Add(new GrantOp((UserGoodsType)(int)reward.Type, reward.CosmeticId, 1, 1, true));
+            }
+        }
+
+        return granted;
+    }
+
+    private bool AlreadyOwnsCosmetic(CosmeticType type, long id) => type switch
+    {
+        CosmeticType.Sleeve   => Viewer.Sleeves.Any(s => s.Id == id),
+        CosmeticType.Emblem   => Viewer.Emblems.Any(e => e.Id == id),
+        CosmeticType.Skin     => Viewer.LeaderSkins.Any(s => s.Id == id),
+        CosmeticType.Degree   => Viewer.Degrees.Any(d => d.Id == id),
+        CosmeticType.MyPageBG => Viewer.MyPageBackgrounds.Any(b => b.Id == id),
+        _ => false,
+    };
 
     public long EffectiveBalance(SpendCurrency currency) => throw new NotImplementedException();
     public bool OwnsCard(long cardId) => throw new NotImplementedException();
diff --git a/SVSim.UnitTests/Services/Inventory/InventoryBackfillTests.cs b/SVSim.UnitTests/Services/Inventory/InventoryBackfillTests.cs
new file mode 100644
index 0000000..e9edd00
--- /dev/null
+++ b/SVSim.UnitTests/Services/Inventory/InventoryBackfillTests.cs
@@ -0,0 +1,66 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.DependencyInjection;
+using SVSim.Database;
+using SVSim.Database.Enums;
+using SVSim.Database.Models;
+using SVSim.Database.Services.Inventory;
+using SVSim.UnitTests.Infrastructure;
+
+namespace SVSim.UnitTests.Services.Inventory;
+
+public class InventoryBackfillTests
+{
+    [Test]
+    public async Task Backfill_grants_missing_cosmetic_for_already_owned_card()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+        long cardId = await factory.SeedCardAsync();
+        using var scope = factory.Services.CreateScope();
+        var ctx = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+
+        const int sleeveId = 2_000_020_000;
+        ctx.Sleeves.Add(new SleeveEntry { Id = sleeveId });
+        ctx.CardCosmeticRewards.Add(new CardCosmeticReward { CardId = cardId, CosmeticId = sleeveId, Type = CosmeticType.Sleeve });
+        var card = await ctx.Cards.FirstAsync(c => c.Id == cardId);
+        var v = await ctx.Viewers.Include(x => x.Cards).ThenInclude(c => c.Card).FirstAsync(x => x.Id == viewerId);
+        v.Cards.Add(new OwnedCardEntry { Card = card, Count = 3, IsProtected = false });
+        await ctx.SaveChangesAsync();
+
+        var inv = scope.ServiceProvider.GetRequiredService<IInventoryService>();
+        await using var tx = await inv.BeginAsync(viewerId);
+        int granted = await tx.BackfillCardCosmeticsAsync();
+
+        Assert.That(granted, Is.EqualTo(1));
+        Assert.That(tx.Viewer.Sleeves.Any(s => s.Id == sleeveId), Is.True);
+    }
+
+    [Test]
+    public async Task Backfill_idempotent_on_already_owned_cosmetic()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+        long cardId = await factory.SeedCardAsync();
+        using var scope = factory.Services.CreateScope();
+        var ctx = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+
+        const int sleeveId = 2_000_020_001;
+        var sleeve = new SleeveEntry { Id = sleeveId };
+        ctx.Sleeves.Add(sleeve);
+        ctx.CardCosmeticRewards.Add(new CardCosmeticReward { CardId = cardId, CosmeticId = sleeveId, Type = CosmeticType.Sleeve });
+        var card = await ctx.Cards.FirstAsync(c => c.Id == cardId);
+        var v = await ctx.Viewers
+            .Include(x => x.Cards).ThenInclude(c => c.Card)
+            .Include(x => x.Sleeves)
+            .FirstAsync(x => x.Id == viewerId);
+        v.Cards.Add(new OwnedCardEntry { Card = card, Count = 3, IsProtected = false });
+        v.Sleeves.Add(sleeve);
+        await ctx.SaveChangesAsync();
+
+        var inv = scope.ServiceProvider.GetRequiredService<IInventoryService>();
+        await using var tx = await inv.BeginAsync(viewerId);
+        int granted = await tx.BackfillCardCosmeticsAsync();
+
+        Assert.That(granted, Is.EqualTo(0));
+    }
+}