diff --git a/SVSim.UnitTests/Controllers/FreeplayInvariantTests.cs b/SVSim.UnitTests/Controllers/FreeplayInvariantTests.cs
new file mode 100644
index 0000000..0e97276
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+++ b/SVSim.UnitTests/Controllers/FreeplayInvariantTests.cs
@@ -0,0 +1,72 @@
+using System.Net;
+using System.Text;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.DependencyInjection;
+using SVSim.Database;
+using SVSim.Database.Enums;
+using SVSim.Database.Models;
+using SVSim.UnitTests.Infrastructure;
+
+namespace SVSim.UnitTests.Controllers;
+
+public class FreeplayInvariantTests
+{
+    private static StringContent JsonBody(string json) => new(json, Encoding.UTF8, "application/json");
+
+    [Test]
+    public async Task Freeplay_pack_open_leaves_viewer_currency_unchanged()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+        await SeedCrystalPack(factory, viewerId);
+        await factory.EnableFreeplayAsync();
+
+        ulong before;
+        using (var scope = factory.Services.CreateScope())
+        {
+            var db = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+            before = (await db.Viewers.FirstAsync(v => v.Id == viewerId)).Currency.Crystals;
+        }
+
+        // Verify the precondition: viewer has 0 crystals, so without freeplay this would be rejected.
+        Assert.That(before, Is.EqualTo(0UL), "precondition: viewer must be broke before the open");
+
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+        // gacha_type:1 is the parent pack's gacha_type — see project_wire_pack_gacha_type memory.
+        var json = """{"viewer_id":"0","steam_id":0,"steam_session_ticket":"","parent_gacha_id":10001,"gacha_id":100002,"gacha_type":1,"pack_number":1,"exclude_card_ids":[]}""";
+        var resp = await client.PostAsync("/pack/open", JsonBody(json));
+        Assert.That(resp.StatusCode, Is.EqualTo(HttpStatusCode.OK), await resp.Content.ReadAsStringAsync());
+
+        ulong after;
+        using (var scope2 = factory.Services.CreateScope())
+        {
+            var db2 = scope2.ServiceProvider.GetRequiredService<SVSimDbContext>();
+            after = (await db2.Viewers.FirstAsync(v => v.Id == viewerId)).Currency.Crystals;
+        }
+
+        Assert.That(after, Is.EqualTo(before), "freeplay must not write currency to the DB");
+    }
+
+    /// <summary>
+    /// Seeds a crystal pack (parent gacha 10001, child gacha_id 100002, TypeDetail=2, cost=100)
+    /// with the viewer broke (0 crystals). Mirrors the pack shape from
+    /// PackControllerOpenTests.Open_with_crystals_deducts_crystals — the only difference is
+    /// Crystals=0 instead of 250, so without freeplay this open would be refused.
+    /// </summary>
+    private static async Task SeedCrystalPack(SVSimTestFactory f, long viewerId)
+    {
+        using var scope = f.Services.CreateScope();
+        var db = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+        int baseId = await db.CardSets.Where(s => s.Cards.Count > 0).Select(s => s.Id).FirstAsync();
+        db.Packs.Add(new PackConfigEntry
+        {
+            Id = 10001, BasePackId = baseId, PackCategory = PackCategory.None,
+            CommenceDate = DateTime.UtcNow.AddDays(-1), CompleteDate = DateTime.UtcNow.AddDays(30),
+            GachaType = 1, GachaDetail = "test",
+            ChildGachas = { new PackChildGachaEntry { GachaId = 100002, TypeDetail = 2, Cost = 100, CardCount = 8 } },
+        });
+        var v = await db.Viewers.FirstAsync(x => x.Id == viewerId);
+        v.Currency.Crystals = 0;
+        await db.SaveChangesAsync();
+    }
+}