diff --git a/SVSim.Database/Services/Inventory/IInventoryService.cs b/SVSim.Database/Services/Inventory/IInventoryService.cs
new file mode 100644
index 0000000..e1d6ce3
--- /dev/null
+++ b/SVSim.Database/Services/Inventory/IInventoryService.cs
@@ -0,0 +1,28 @@
+using SVSim.Database.Models;
+using SVSim.Database.Services;
+
+namespace SVSim.Database.Services.Inventory;
+
+public interface IInventoryService
+{
+    /// <summary>
+    /// Loads the viewer with the canonical inventory graph (Cards.Card, Sleeves, Emblems,
+    /// LeaderSkins, Degrees, MyPageBackgrounds, Items.Item under AsSplitQuery), opens a DB
+    /// transaction, and returns a builder for queueing operations. Throws
+    /// <see cref="InventoryViewerNotFoundException"/> if the viewer does not exist.
+    /// </summary>
+    Task<IInventoryTransaction> BeginAsync(
+        long viewerId,
+        CancellationToken ct = default,
+        Action<InventoryLoadConfig>? configure = null);
+
+    Task<IReadOnlyList<OwnedCardEntry>> EffectiveOwnedCardsAsync(Viewer viewer, CancellationToken ct = default);
+    Task<EffectiveCosmetics> EffectiveCosmeticsAsync(Viewer viewer, CancellationToken ct = default);
+    long EffectiveBalance(Viewer viewer, SpendCurrency currency);
+}
+
+public sealed class InventoryViewerNotFoundException : Exception
+{
+    public InventoryViewerNotFoundException(long viewerId)
+        : base($"Viewer {viewerId} not found") { }
+}
diff --git a/SVSim.Database/Services/Inventory/IInventoryTransaction.cs b/SVSim.Database/Services/Inventory/IInventoryTransaction.cs
new file mode 100644
index 0000000..1ba2059
--- /dev/null
+++ b/SVSim.Database/Services/Inventory/IInventoryTransaction.cs
@@ -0,0 +1,30 @@
+using SVSim.Database.Enums;
+using SVSim.Database.Models;
+using SVSim.Database.Services;
+
+namespace SVSim.Database.Services.Inventory;
+
+/// <summary>
+/// Scoped builder returned by <see cref="IInventoryService.BeginAsync"/>. Queue spend +
+/// grant operations; commit to save and assemble the <see cref="InventoryCommitResult"/>.
+/// <para>
+/// Dispose without committing rolls back the underlying DB transaction and detaches any
+/// in-memory mutations. <b>Always</b> wrap in <c>await using</c>.
+/// </para>
+/// </summary>
+public interface IInventoryTransaction : IAsyncDisposable
+{
+    Viewer Viewer { get; }
+    bool IsFreeplay { get; }
+
+    Task<SpendResult> TrySpendAsync(SpendCurrency currency, long cost, CancellationToken ct = default);
+    Task<SpendResult> TryDebitAsync(UserGoodsType type, long detailId, int num, CancellationToken ct = default);
+    Task<IReadOnlyList<GrantedReward>> GrantAsync(UserGoodsType type, long detailId, int num, CancellationToken ct = default);
+    Task<int> BackfillCardCosmeticsAsync(CancellationToken ct = default);
+
+    long EffectiveBalance(SpendCurrency currency);
+    bool OwnsCard(long cardId);
+    bool OwnsCosmetic(CosmeticType type, int id);
+
+    Task<InventoryCommitResult> CommitAsync(CancellationToken ct = default);
+}
diff --git a/SVSim.Database/Services/Inventory/InventoryCatalogException.cs b/SVSim.Database/Services/Inventory/InventoryCatalogException.cs
new file mode 100644
index 0000000..d01943f
--- /dev/null
+++ b/SVSim.Database/Services/Inventory/InventoryCatalogException.cs
@@ -0,0 +1,10 @@
+namespace SVSim.Database.Services.Inventory;
+
+/// <summary>
+/// Thrown when an inventory operation references a catalog id that doesn't exist
+/// (unknown card / item / cosmetic). Programmer error — bubbles to the global error handler.
+/// </summary>
+public sealed class InventoryCatalogException : Exception
+{
+    public InventoryCatalogException(string message) : base(message) { }
+}
diff --git a/SVSim.Database/Services/Inventory/InventoryCommitResult.cs b/SVSim.Database/Services/Inventory/InventoryCommitResult.cs
new file mode 100644
index 0000000..e058901
--- /dev/null
+++ b/SVSim.Database/Services/Inventory/InventoryCommitResult.cs
@@ -0,0 +1,20 @@
+using SVSim.Database.Services;
+
+namespace SVSim.Database.Services.Inventory;
+
+/// <summary>
+/// Result of <see cref="IInventoryTransaction.CommitAsync"/>.
+/// <para>
+/// <see cref="RewardList"/> — wire-shape entries with currency-collision resolved (one entry per
+/// (type, id); for currencies that were both spent and granted, the last post-state in op order
+/// wins). Use this for response <c>reward_list</c> fields.
+/// </para>
+/// <para>
+/// <see cref="Deltas"/> — verbatim ordered (type, id, num) sequence the caller queued. No
+/// collapse, no cosmetic-cascade entries. Use this for BP <c>achieved_info</c> and Story
+/// <c>story_reward_list</c> popups.
+/// </para>
+/// </summary>
+public sealed record InventoryCommitResult(
+    IReadOnlyList<GrantedReward> RewardList,
+    IReadOnlyList<GrantedReward> Deltas);
diff --git a/SVSim.Database/Services/Inventory/InventoryLoadConfig.cs b/SVSim.Database/Services/Inventory/InventoryLoadConfig.cs
new file mode 100644
index 0000000..b11997c
--- /dev/null
+++ b/SVSim.Database/Services/Inventory/InventoryLoadConfig.cs
@@ -0,0 +1,31 @@
+using System.Linq.Expressions;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Query;
+using SVSim.Database.Models;
+
+namespace SVSim.Database.Services.Inventory;
+
+/// <summary>
+/// Caller-supplied extra <c>.Include</c> chains on top of the canonical viewer-inventory query
+/// in <see cref="IInventoryService.BeginAsync"/>. Use to bring in extra collections needed by
+/// the calling controller (e.g. <c>MissionData</c>, <c>BuildDeckPurchases</c>).
+/// </summary>
+public sealed class InventoryLoadConfig
+{
+    internal List<Func<IQueryable<Viewer>, IQueryable<Viewer>>> Includes { get; } = new();
+
+    public InventoryLoadConfig WithInclude<TProperty>(
+        Expression<Func<Viewer, TProperty>> path)
+    {
+        Includes.Add(q => q.Include(path));
+        return this;
+    }
+
+    public InventoryLoadConfig WithInclude<TProperty, TThen>(
+        Expression<Func<Viewer, IEnumerable<TProperty>>> collectionPath,
+        Expression<Func<TProperty, TThen>> thenPath)
+    {
+        Includes.Add(q => q.Include(collectionPath).ThenInclude(thenPath));
+        return this;
+    }
+}