diff --git a/SVSim.EmulatedEntrypoint/Services/IMatchContextBuilder.cs b/SVSim.EmulatedEntrypoint/Services/IMatchContextBuilder.cs
index 41f93a9..a9da5d0 100644
--- a/SVSim.EmulatedEntrypoint/Services/IMatchContextBuilder.cs
+++ b/SVSim.EmulatedEntrypoint/Services/IMatchContextBuilder.cs
@@ -1,4 +1,5 @@
 using SVSim.BattleNode.Bridge;
+using SVSim.Database.Enums;
 
 namespace SVSim.EmulatedEntrypoint.Services;
 
@@ -15,4 +16,15 @@ public interface IMatchContextBuilder
     /// Throws <see cref="ArenaTwoPickException"/> on missing run / incomplete draft.
     /// </summary>
     Task<MatchContext> BuildForTwoPickAsync(long viewerId);
+
+    /// <summary>
+    /// Build a context for a rank-battle viewer + format (rotation / unlimited). Pulls the
+    /// viewer's deck #1 for that format + viewer cosmetics. Throws if the viewer has no
+    /// deck registered for the format.
+    /// </summary>
+    /// <remarks>
+    /// Deck-selection persistence (which deck number is "current" for this format) is a
+    /// separate concern; deck #1 is a placeholder until that lands.
+    /// </remarks>
+    Task<MatchContext> BuildForRankBattleAsync(long viewerId, Format format);
 }
diff --git a/SVSim.EmulatedEntrypoint/Services/MatchContextBuilder.cs b/SVSim.EmulatedEntrypoint/Services/MatchContextBuilder.cs
index ef62467..c7a1e1a 100644
--- a/SVSim.EmulatedEntrypoint/Services/MatchContextBuilder.cs
+++ b/SVSim.EmulatedEntrypoint/Services/MatchContextBuilder.cs
@@ -1,6 +1,8 @@
 using System.Text.Json;
 using SVSim.BattleNode.Bridge;
+using SVSim.Database.Enums;
 using SVSim.Database.Models.Config;
+using SVSim.Database.Repositories.Deck;
 using SVSim.Database.Repositories.Viewer;
 using SVSim.Database.Services;
 
@@ -10,15 +12,18 @@ public class MatchContextBuilder : IMatchContextBuilder
 {
     private readonly IArenaTwoPickRunRepository _runs;
     private readonly IViewerRepository _viewers;
+    private readonly IDeckRepository _decks;
     private readonly IGameConfigService _config;
 
     public MatchContextBuilder(
         IArenaTwoPickRunRepository runs,
         IViewerRepository viewers,
+        IDeckRepository decks,
         IGameConfigService config)
     {
         _runs = runs;
         _viewers = viewers;
+        _decks = decks;
         _config = config;
     }
 
@@ -64,4 +69,46 @@ public class MatchContextBuilder : IMatchContextBuilder
             IsOfficial: viewer.Info.IsOfficial ? 1 : 0,
             BattleType: 11);
     }
+
+    public async Task<MatchContext> BuildForRankBattleAsync(long viewerId, Format format)
+    {
+        var viewer = await _viewers.LoadForMatchContextAsync(viewerId)
+            ?? throw new InvalidOperationException($"viewer {viewerId} not found");
+
+        // Per spec, deck-selection persistence (which deck number is "current" for this
+        // format) is a separate concern; #1 is a placeholder until that lands. IDeckRepository
+        // is the right path here — viewer-graph nav refs (DeckCard.Card) don't auto-load
+        // (see project_ef_nav_include_pitfall memory), which would silently ship card_id=0.
+        var deck = await _decks.GetDeck(viewerId, format, deckNo: 1)
+            ?? throw new InvalidOperationException($"viewer {viewerId} has no deck for format {format}");
+
+        var defaults = _config.Get<DefaultLoadoutConfig>();
+        var emblemId = viewer.Info.SelectedEmblem.Id != 0
+            ? viewer.Info.SelectedEmblem.Id.ToString()
+            : defaults.EmblemId.ToString();
+        var degreeId = viewer.Info.SelectedDegree.Id != 0
+            ? viewer.Info.SelectedDegree.Id.ToString()
+            : defaults.DegreeId.ToString();
+        var charaId = deck.LeaderSkin.Id != 0
+            ? deck.LeaderSkin.Id.ToString()
+            : deck.Class.Id.ToString();
+        var sleeveId = deck.Sleeve.Id != 0
+            ? deck.Sleeve.Id.ToString()
+            : defaults.SleeveId.ToString();
+        var deckCardIds = deck.Cards.Select(c => c.Card.Id).ToList();
+
+        return new MatchContext(
+            SelfDeckCardIds: deckCardIds,
+            ClassId: deck.Class.Id.ToString(),
+            CharaId: charaId,
+            CardMasterName: "card_master_node_10015",
+            CountryCode: viewer.Info.CountryCode ?? string.Empty,
+            UserName: viewer.DisplayName,
+            SleeveId: sleeveId,
+            EmblemId: emblemId,
+            DegreeId: degreeId,
+            FieldId: 43,
+            IsOfficial: viewer.Info.IsOfficial ? 1 : 0,
+            BattleType: 11);
+    }
 }
diff --git a/SVSim.UnitTests/Services/MatchContextBuilderTests.cs b/SVSim.UnitTests/Services/MatchContextBuilderTests.cs
index c84e425..a2018c6 100644
--- a/SVSim.UnitTests/Services/MatchContextBuilderTests.cs
+++ b/SVSim.UnitTests/Services/MatchContextBuilderTests.cs
@@ -3,6 +3,7 @@ using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.DependencyInjection;
 using SVSim.BattleNode.Bridge;
 using SVSim.Database;
+using SVSim.Database.Enums;
 using SVSim.Database.Models;
 using SVSim.EmulatedEntrypoint.Services;
 using SVSim.UnitTests.Infrastructure;
@@ -116,6 +117,41 @@ public class MatchContextBuilderTests
         Assert.That(ex!.ErrorCode, Is.EqualTo("arena_two_pick_draft_incomplete"));
     }
 
+    [Test]
+    public async Task BuildForRankBattle_returns_MatchContext_with_format_specific_deck()
+    {
+        await using var factory = new SVSimTestFactory();
+        var viewerId = await factory.SeedViewerAsync(displayName: "Ranker");
+        await factory.SeedGlobalsAsync();
+        await factory.SeedDeckAsync(viewerId, Format.Rotation, 1, "Rank Rotation Deck");
+
+        using var scope = factory.Services.CreateScope();
+        var builder = scope.ServiceProvider.GetRequiredService<IMatchContextBuilder>();
+
+        var ctx = await builder.BuildForRankBattleAsync(viewerId, Format.Rotation);
+
+        Assert.That(ctx.UserName, Is.EqualTo("Ranker"));
+        Assert.That(ctx.BattleType, Is.EqualTo(11), "BattleType=11 matches the prod rank-battle wire value (same as TK2).");
+        Assert.That(ctx.ClassId, Is.Not.Null.And.Not.Empty, "ClassId from the selected deck's class.");
+        Assert.That(ctx.CardMasterName, Is.EqualTo("card_master_node_10015"));
+        Assert.That(ctx.FieldId, Is.EqualTo(43));
+    }
+
+    [Test]
+    public async Task BuildForRankBattle_throws_when_no_deck_for_format()
+    {
+        await using var factory = new SVSimTestFactory();
+        var viewerId = await factory.SeedViewerAsync();
+        await factory.SeedGlobalsAsync();
+        // Intentionally no SeedDeckAsync for Rotation.
+
+        using var scope = factory.Services.CreateScope();
+        var builder = scope.ServiceProvider.GetRequiredService<IMatchContextBuilder>();
+
+        Assert.That(async () => await builder.BuildForRankBattleAsync(viewerId, Format.Rotation),
+            Throws.Exception);
+    }
+
     [Test]
     public async Task BuildForTwoPick_falls_back_to_default_loadout_when_unequipped()
     {