diff --git a/SVSim.EmulatedEntrypoint/Middlewares/ShadowverseTranslationMiddleware.cs b/SVSim.EmulatedEntrypoint/Middlewares/ShadowverseTranslationMiddleware.cs
index ef0b64a..5d4269e 100644
--- a/SVSim.EmulatedEntrypoint/Middlewares/ShadowverseTranslationMiddleware.cs
+++ b/SVSim.EmulatedEntrypoint/Middlewares/ShadowverseTranslationMiddleware.cs
@@ -1,10 +1,13 @@
 using System.Text;
+using System.Text.Json;
+using System.Text.Json.Serialization;
 using MessagePack;
+using MessagePack.Resolvers;
 using Microsoft.AspNetCore.Mvc.Abstractions;
 using Microsoft.AspNetCore.Mvc.Controllers;
 using Microsoft.AspNetCore.Mvc.Infrastructure;
 using Microsoft.Extensions.Primitives;
-using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
 using SVSim.Database.Models;
 using SVSim.EmulatedEntrypoint.Constants;
 using SVSim.EmulatedEntrypoint.Extensions;
@@ -23,6 +26,16 @@ public class ShadowverseTranslationMiddleware : IMiddleware
     private readonly IActionDescriptorCollectionProvider _actionDescriptorCollectionProvider;
     private readonly ShadowverseSessionService _sessionService;
 
+    // Serialization policy MUST match what AddJsonOptions configured on the controllers, or the
+    // model binder won't find the snake_case keys we write into the synthetic request body and
+    // every request 400s with empty ModelState. WhenWritingNull is irrelevant for request
+    // serialization but kept here for symmetry.
+    private static readonly JsonSerializerOptions ControllerJsonOptions = new()
+    {
+        PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower,
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
+    };
+
     public ShadowverseTranslationMiddleware(IActionDescriptorCollectionProvider actionDescriptorCollectionProvider, ShadowverseSessionService sessionService)
     {
         _actionDescriptorCollectionProvider = actionDescriptorCollectionProvider;
@@ -58,9 +71,12 @@ public class ShadowverseTranslationMiddleware : IMiddleware
         
         // Decrypt incoming data.
         requestBytes = Encryption.Decrypt(requestBytes, udid);
-        object? data = MessagePackSerializer.Deserialize(endpointDescriptor.Parameters.FirstOrDefault().ParameterType,
-            requestBytes);
-        string json = JsonConvert.SerializeObject(data);
+        Type requestType = endpointDescriptor.Parameters.FirstOrDefault().ParameterType;
+        object? data = MessagePackSerializer.Deserialize(requestType, requestBytes);
+        // Re-serialize via System.Text.Json with the SAME options the controllers use, so the
+        // model binder sees snake_case keys it can match. Using JsonConvert here writes the
+        // CLR property names (PascalCase) and every property silently binds to default → 400.
+        string json = JsonSerializer.Serialize(data, requestType, ControllerJsonOptions);
         StringContent newStream = new StringContent(json, Encoding.UTF8, "application/json");
         context.Request.Body = newStream.ReadAsStream();
         context.Request.Headers.ContentType = new StringValues("application/json");
@@ -68,19 +84,20 @@ public class ShadowverseTranslationMiddleware : IMiddleware
         await next.Invoke(context);
 
         Viewer? viewer = context.GetViewer();
-        
-        // Grab the response object
-        Type responseType = ((ControllerActionDescriptor)endpointDescriptor).MethodInfo.ReturnType;
-        if (responseType.IsGenericType && responseType.GetGenericTypeDefinition() == typeof(Task<>))
-        {
-            responseType = responseType.GetGenericArguments()[0];
-        }
+
+        // Read the controller's JSON response body. System.Text.Json was configured with
+        // SnakeCaseLower + WhenWritingNull, so the JSON keys are already in the wire shape and
+        // null/optional properties have been omitted. Parse to a JToken tree to preserve that
+        // "absent vs null" information — going back through a typed DTO via JsonConvert would
+        // re-introduce nulls for missing properties and they'd reach the client as msgpack Nil.
         using MemoryStream responseBytesStream = new MemoryStream();
         context.Response.Body.Seek(0, SeekOrigin.Begin);
         await context.Response.Body.CopyToAsync(responseBytesStream);
-        byte[] responseBytes = responseBytesStream.ToArray();
-        object? responseData = JsonConvert.DeserializeObject(Encoding.UTF8.GetString(responseBytes), responseType);
-        
+        string responseJson = Encoding.UTF8.GetString(responseBytesStream.ToArray());
+        object? responseData = string.IsNullOrEmpty(responseJson)
+            ? null
+            : ConvertJsonTreeToPlainObject(JToken.Parse(responseJson));
+
         // Wrap the response in a datawrapper
         DataWrapper wrappedResponseData = new DataWrapper
         {
@@ -88,8 +105,14 @@ public class ShadowverseTranslationMiddleware : IMiddleware
             DataHeaders = new DataHeaders
             {
                 Servertime = DateTime.UtcNow.Ticks,
-                Sid =
-                    context.Request.Headers[NetworkConstants.SessionIdHeaderName].FirstOrDefault(),
+                // SID intentionally empty. See docs/api-spec/common/envelope.md §"SID
+                // rotation" — the client's SessionId is a hash-on-read property, so echoing
+                // the request's SID poisons its backing field and the next request hashes
+                // the hash, missing our SID→UDID dict and crashing decryption. To rotate
+                // sessions in the future, use the "stable-prefix + counter" pattern from
+                // that doc (Option B), and pre-hash the rotated value to index the map by
+                // what the client will actually send back on the next request.
+                Sid = "",
                 // TODO error handling
                 ResultCode = 1,
                 ShortUdid = viewer.ShortUdid,
@@ -97,10 +120,39 @@ public class ShadowverseTranslationMiddleware : IMiddleware
             }
         };
 
-        // Convert the response into a messagepack, encrypt it
-        byte[] packedData = MessagePackSerializer.Serialize<DataWrapper>(wrappedResponseData);
+        // Convert the response into a messagepack, encrypt it. ContractlessStandardResolver
+        // walks the DataWrapper's typed properties (DataHeaders) AND the boxed object/list/
+        // primitive tree under Data — emitting only the keys present in the dictionary.
+        var msgPackOptions = MessagePackSerializerOptions.Standard
+            .WithResolver(ContractlessStandardResolver.Instance);
+        byte[] packedData = MessagePackSerializer.Serialize(wrappedResponseData, msgPackOptions);
         packedData = Encryption.Encrypt(packedData, udid);
         await originalResponsebody.WriteAsync(Encoding.UTF8.GetBytes(Convert.ToBase64String(packedData)));
         context.Response.Body = originalResponsebody;
     }
+
+    /// <summary>
+    /// Walks a parsed JSON tree into the plain CLR shape MessagePack-CSharp's contractless
+    /// resolver understands: objects → <c>Dictionary&lt;string, object?&gt;</c>, arrays →
+    /// <c>List&lt;object?&gt;</c>, scalars unboxed to their nearest primitive. Crucially, JSON
+    /// objects that lacked a key DON'T get one in the dictionary — preserving "absent" as a
+    /// distinct state from "null" all the way to the msgpack writer.
+    /// </summary>
+    internal static object? ConvertJsonTreeToPlainObject(JToken? token)
+    {
+        if (token is null || token.Type == JTokenType.Null) return null;
+        return token.Type switch
+        {
+            JTokenType.Object => token.Children<JProperty>()
+                .ToDictionary(p => p.Name, p => ConvertJsonTreeToPlainObject(p.Value)),
+            JTokenType.Array => token.Children().Select(ConvertJsonTreeToPlainObject).ToList(),
+            JTokenType.Integer => token.Value<long>(),
+            JTokenType.Float => token.Value<double>(),
+            JTokenType.String => token.Value<string>(),
+            JTokenType.Boolean => token.Value<bool>(),
+            JTokenType.Date => token.Value<DateTime>(),
+            JTokenType.Bytes => token.Value<byte[]>(),
+            _ => token.ToString()
+        };
+    }
 }
\ No newline at end of file
diff --git a/SVSim.EmulatedEntrypoint/Models/Dtos/Responses/GameStartResponse.cs b/SVSim.EmulatedEntrypoint/Models/Dtos/Responses/GameStartResponse.cs
index a568078..d2b3282 100644
--- a/SVSim.EmulatedEntrypoint/Models/Dtos/Responses/GameStartResponse.cs
+++ b/SVSim.EmulatedEntrypoint/Models/Dtos/Responses/GameStartResponse.cs
@@ -48,13 +48,20 @@ public class GameStartResponse
     [Key("transition_account_data")]
     public List<TransitionAccountData> TransitionAccountData { get; set; } = new();
 
-    // INTENTIONALLY OMITTED: `rewrite_viewer_id` and `account_delete_reservation_status`.
-    // Both are presence-checked by the client via `Keys.Contains(...)` + `.ToInt()` with no
-    // null guard. MessagePack-CSharp writes [Key] properties unconditionally (null → Nil),
-    // and the System.Text.Json `WhenWritingNull` ignore only affects the plain-JSON path.
-    // So including these as nullable properties is a guaranteed NRE on the encrypted client
-    // path. We don't need them — the client tolerates their absence — so don't declare them.
-    // Re-add only if we have a real value to send.
+    /// <summary>
+    /// When present, client overwrites <c>Certification.ViewerId</c> with this value. Optional
+    /// — leave null to omit. The serialization pipeline (JSON + msgpack via the translation
+    /// middleware) drops null properties end-to-end, so the client sees the key as absent.
+    /// </summary>
+    [Key("rewrite_viewer_id")]
+    public long? RewriteViewerId { get; set; }
+
+    /// <summary>
+    /// Presence indicates the user has applied for account deletion (value ignored by client at
+    /// this stage). Optional — leave null to omit.
+    /// </summary>
+    [Key("account_delete_reservation_status")]
+    public int? AccountDeleteReservationStatus { get; set; }
 
     // --- Agreement / consent state (all required) ---
 
diff --git a/SVSim.EmulatedEntrypoint/Program.cs b/SVSim.EmulatedEntrypoint/Program.cs
index 9a5eab5..4f2a365 100644
--- a/SVSim.EmulatedEntrypoint/Program.cs
+++ b/SVSim.EmulatedEntrypoint/Program.cs
@@ -1,3 +1,4 @@
+using System.Text.Json;
 using System.Text.Json.Serialization;
 using Microsoft.EntityFrameworkCore;
 using SVSim.Database;
@@ -22,10 +23,16 @@ public class Program
 
         builder.Services.AddControllers().AddJsonOptions(opt =>
         {
+            // Wire-format congruence: the encrypted msgpack path uses snake_case [Key("...")]
+            // names; the plain-JSON path runs through System.Text.Json. Match them by using
+            // SnakeCaseLower naming policy here so both paths emit identical key names — and
+            // so the translation middleware can hand JSON keys straight through to msgpack
+            // without per-property name remapping.
+            opt.JsonSerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower;
             // Production omits null/optional fields entirely; the client uses
             // `Keys.Contains(name)` as a presence check and calls `.ToInt()` (etc.) on the
             // value without a null guard. Emitting `"key":null` makes Contains return true and
-            // crashes the client. Match prod by dropping nulls during serialization.
+            // crashes the client. Drop nulls during serialization so missing == absent.
             opt.JsonSerializerOptions.DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull;
         });
         // Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
diff --git a/SVSim.EmulatedEntrypoint/SVSim.EmulatedEntrypoint.csproj b/SVSim.EmulatedEntrypoint/SVSim.EmulatedEntrypoint.csproj
index 869ba35..f607413 100644
--- a/SVSim.EmulatedEntrypoint/SVSim.EmulatedEntrypoint.csproj
+++ b/SVSim.EmulatedEntrypoint/SVSim.EmulatedEntrypoint.csproj
@@ -28,6 +28,10 @@
     </Content>
   </ItemGroup>
 
+  <ItemGroup>
+    <InternalsVisibleTo Include="SVSim.UnitTests" />
+  </ItemGroup>
+
   <ItemGroup>
     <ProjectReference Include="..\SVSim.Database\SVSim.Database.csproj" />
   </ItemGroup>
diff --git a/SVSim.EmulatedEntrypoint/Security/SteamSessionAuthentication/SteamSessionAuthenticationHandler.cs b/SVSim.EmulatedEntrypoint/Security/SteamSessionAuthentication/SteamSessionAuthenticationHandler.cs
index 2c3b800..83165e4 100644
--- a/SVSim.EmulatedEntrypoint/Security/SteamSessionAuthentication/SteamSessionAuthenticationHandler.cs
+++ b/SVSim.EmulatedEntrypoint/Security/SteamSessionAuthentication/SteamSessionAuthenticationHandler.cs
@@ -1,9 +1,9 @@
 using System.Security.Claims;
 using System.Text;
 using System.Text.Encodings.Web;
+using System.Text.Json;
 using Microsoft.AspNetCore.Authentication;
 using Microsoft.Extensions.Options;
-using Newtonsoft.Json;
 using SVSim.Database.Enums;
 using SVSim.Database.Models;
 using SVSim.Database.Repositories.Viewer;
@@ -16,6 +16,14 @@ namespace SVSim.EmulatedEntrypoint.Security.SteamSessionAuthentication;
 
 public class SteamSessionAuthenticationHandler : AuthenticationHandler<SteamAuthenticationHandlerOptions>
 {
+    // Must mirror the controller-side JSON options — the translation middleware rewrites the
+    // request body in snake_case, and we have to read it back the same way or every property
+    // binds to null and we NRE downstream against the Steam ticket.
+    private static readonly JsonSerializerOptions RequestJsonOptions = new()
+    {
+        PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower
+    };
+
     private readonly SteamSessionService _sessionService;
     private readonly IViewerRepository _viewerRepository;
     public SteamSessionAuthenticationHandler(IOptionsMonitor<SteamAuthenticationHandlerOptions> options, ILoggerFactory logger, UrlEncoder encoder, SteamSessionService sessionService, IViewerRepository viewerRepository) : base(options, logger, encoder)
@@ -48,13 +56,21 @@ public class SteamSessionAuthenticationHandler : AuthenticationHandler<SteamAuth
 
         // Convert bytes to json
         string requestString = Encoding.UTF8.GetString(requestBytes);
-        BaseRequest? requestJson = JsonConvert.DeserializeObject<BaseRequest>(requestString);
-
-        if (requestJson is null)
+        BaseRequest? requestJson;
+        try
+        {
+            requestJson = JsonSerializer.Deserialize<BaseRequest>(requestString, RequestJsonOptions);
+        }
+        catch (JsonException)
         {
             return AuthenticateResult.Fail("Invalid request body.");
         }
-        
+
+        if (requestJson is null || string.IsNullOrEmpty(requestJson.SteamSessionTicket))
+        {
+            return AuthenticateResult.Fail("Invalid request body.");
+        }
+
         // Check steam session validity
         bool sessionIsValid = _sessionService.IsTicketValidForUser(requestJson.SteamSessionTicket, requestJson.SteamId);
         if (!sessionIsValid)
diff --git a/SVSim.EmulatedEntrypoint/Services/SteamSessionService.cs b/SVSim.EmulatedEntrypoint/Services/SteamSessionService.cs
index a97f176..f0dc884 100644
--- a/SVSim.EmulatedEntrypoint/Services/SteamSessionService.cs
+++ b/SVSim.EmulatedEntrypoint/Services/SteamSessionService.cs
@@ -19,6 +19,14 @@ public class SteamSessionService : IDisposable
     /// <returns>whether the ticket is valid for the given steamid</returns>
     public bool IsTicketValidForUser(string ticket, ulong steamId)
     {
+        if (string.IsNullOrEmpty(ticket))
+        {
+            // Caller already shouldn't pass null/empty here, but a misshaped request body
+            // (e.g. wrong casing) used to NRE on the ConcurrentDictionary lookup below.
+            // Fail cleanly so the auth pipeline returns 401 instead of crashing the request.
+            return false;
+        }
+
         if (_validatedSessionTickets.TryGetValue(ticket, out ulong storedSteamId))
         {
             return storedSteamId == steamId;
diff --git a/SVSim.UnitTests/Controllers/CheckControllerTests.cs b/SVSim.UnitTests/Controllers/CheckControllerTests.cs
index 71ee1fe..ed83fda 100644
--- a/SVSim.UnitTests/Controllers/CheckControllerTests.cs
+++ b/SVSim.UnitTests/Controllers/CheckControllerTests.cs
@@ -13,10 +13,10 @@ namespace SVSim.UnitTests.Controllers;
 public class CheckControllerTests
 {
     private const string BaseRequestJson =
-        """{"viewerId":"0","steamId":0,"steamSessionTicket":""}""";
+        """{"viewer_id":"0","steam_id":0,"steam_session_ticket":""}""";
 
     private const string GameStartRequestJson =
-        """{"viewerId":"0","steamId":0,"steamSessionTicket":"","appType":0,"campaignData":"","campaignSign":"","campaignUser":0}""";
+        """{"viewer_id":"0","steam_id":0,"steam_session_ticket":"","app_type":0,"campaign_data":"","campaign_sign":"","campaign_user":0}""";
 
     [Test]
     public async Task SpecialTitle_returns_default_title_id()
@@ -31,7 +31,7 @@ public class CheckControllerTests
 
         var body = await response.Content.ReadAsStringAsync();
         using var doc = JsonDocument.Parse(body);
-        Assert.That(doc.RootElement.GetProperty("titleImageId").GetString(), Is.EqualTo("0"));
+        Assert.That(doc.RootElement.GetProperty("title_image_id").GetString(), Is.EqualTo("0"));
     }
 
     [Test]
@@ -51,30 +51,30 @@ public class CheckControllerTests
         var root = doc.RootElement;
 
         // now_tutorial_step is a STRING on the wire (prod sends "100"); client calls .ToInt().
-        Assert.That(root.GetProperty("nowTutorialStep").GetString(), Is.EqualTo("100"),
+        Assert.That(root.GetProperty("now_tutorial_step").GetString(), Is.EqualTo("100"),
             "RegisterViewer's seed-config default sets tutorial_state=100 (tutorial complete).");
-        Assert.That(root.GetProperty("tosState").GetInt32(), Is.EqualTo(1));
-        Assert.That(root.GetProperty("policyState").GetInt32(), Is.EqualTo(1));
-        Assert.That(root.GetProperty("korAuthorityState").GetInt32(), Is.EqualTo(0));
-        Assert.That(root.GetProperty("tosId").GetInt32(), Is.EqualTo(1));
-        Assert.That(root.GetProperty("policyId").GetInt32(), Is.EqualTo(1));
-        Assert.That(root.GetProperty("korAuthorityId").GetInt32(), Is.EqualTo(0));
+        Assert.That(root.GetProperty("tos_state").GetInt32(), Is.EqualTo(1));
+        Assert.That(root.GetProperty("policy_state").GetInt32(), Is.EqualTo(1));
+        Assert.That(root.GetProperty("kor_authority_state").GetInt32(), Is.EqualTo(0));
+        Assert.That(root.GetProperty("tos_id").GetInt32(), Is.EqualTo(1));
+        Assert.That(root.GetProperty("policy_id").GetInt32(), Is.EqualTo(1));
+        Assert.That(root.GetProperty("kor_authority_id").GetInt32(), Is.EqualTo(0));
 
         // Prod-shape fields (not strictly read by GameStartCheckTask.Parse but sent by prod).
-        Assert.That(root.GetProperty("nowViewerId").GetInt64(), Is.GreaterThan(0));
-        Assert.That(root.GetProperty("nowName").GetString(), Is.Not.Empty);
-        Assert.That(root.GetProperty("nowRank").ValueKind, Is.EqualTo(JsonValueKind.Object));
+        Assert.That(root.GetProperty("now_viewer_id").GetInt64(), Is.GreaterThan(0));
+        Assert.That(root.GetProperty("now_name").GetString(), Is.Not.Empty);
+        Assert.That(root.GetProperty("now_rank").ValueKind, Is.EqualTo(JsonValueKind.Object));
 
         // Steam connection should round-trip into transition_account_data — all three fields
         // serialized as strings (matches prod wire shape).
-        var transitions = root.GetProperty("transitionAccountData");
+        var transitions = root.GetProperty("transition_account_data");
         Assert.That(transitions.ValueKind, Is.EqualTo(JsonValueKind.Array));
         Assert.That(transitions.GetArrayLength(), Is.EqualTo(1),
             "Seeded viewer has exactly one Steam social account connection.");
-        Assert.That(transitions[0].GetProperty("socialAccountType").GetString(),
+        Assert.That(transitions[0].GetProperty("social_account_type").GetString(),
             Is.EqualTo(((int)SVSim.Database.Enums.SocialAccountType.Steam).ToString()));
-        Assert.That(transitions[0].GetProperty("socialAccountId").GetString(), Is.Not.Empty);
-        Assert.That(transitions[0].GetProperty("connectedViewerId").GetString(), Is.Not.Empty);
+        Assert.That(transitions[0].GetProperty("social_account_id").GetString(), Is.Not.Empty);
+        Assert.That(transitions[0].GetProperty("connected_viewer_id").GetString(), Is.Not.Empty);
     }
 
     [Test]
@@ -97,9 +97,9 @@ public class CheckControllerTests
 
         using var doc = JsonDocument.Parse(body);
         var root = doc.RootElement;
-        Assert.That(root.TryGetProperty("rewriteViewerId", out _), Is.False,
+        Assert.That(root.TryGetProperty("rewrite_viewer_id", out _), Is.False,
             "rewrite_viewer_id must NOT be present in the response — client NREs on null .ToInt().");
-        Assert.That(root.TryGetProperty("accountDeleteReservationStatus", out _), Is.False,
+        Assert.That(root.TryGetProperty("account_delete_reservation_status", out _), Is.False,
             "account_delete_reservation_status must NOT be present — presence triggers client behavior.");
     }
 
diff --git a/SVSim.UnitTests/Controllers/DeckControllerTests.cs b/SVSim.UnitTests/Controllers/DeckControllerTests.cs
index 55c3670..2b9ebd5 100644
--- a/SVSim.UnitTests/Controllers/DeckControllerTests.cs
+++ b/SVSim.UnitTests/Controllers/DeckControllerTests.cs
@@ -17,7 +17,7 @@ namespace SVSim.UnitTests.Controllers;
 public class DeckControllerTests
 {
     private static string DeckFormatRequestJson(Format f) =>
-        $$"""{"viewerId":"0","steamId":0,"steamSessionTicket":"","deckFormat":{{(int)f}}}""";
+        $$"""{"viewer_id":"0","steam_id":0,"steam_session_ticket":"","deck_format":{{(int)f}}}""";
 
     private static StringContent JsonBody(string json) => new(json, Encoding.UTF8, "application/json");
 
@@ -49,7 +49,7 @@ public class DeckControllerTests
         Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK), body);
 
         using var doc = JsonDocument.Parse(body);
-        var decks = doc.RootElement.GetProperty("userDeckList");
+        var decks = doc.RootElement.GetProperty("user_deck_list");
         Assert.That(decks.GetArrayLength(), Is.EqualTo(2),
             "Only Rotation-format decks should be returned for a Rotation request.");
         var names = Enumerable.Range(0, decks.GetArrayLength())
@@ -72,7 +72,7 @@ public class DeckControllerTests
         Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK), body);
 
         using var doc = JsonDocument.Parse(body);
-        var decks = doc.RootElement.GetProperty("userDeckList");
+        var decks = doc.RootElement.GetProperty("user_deck_list");
         Assert.That(decks.GetArrayLength(), Is.EqualTo(1));
         Assert.That(decks[0].GetProperty("name").GetString(), Is.EqualTo("Unlimited Deck"));
     }
@@ -90,7 +90,7 @@ public class DeckControllerTests
         Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK), body);
 
         using var doc = JsonDocument.Parse(body);
-        var decks = doc.RootElement.GetProperty("userDeckList");
+        var decks = doc.RootElement.GetProperty("user_deck_list");
         Assert.That(decks.GetArrayLength(), Is.EqualTo(0));
     }
 
@@ -108,7 +108,7 @@ public class DeckControllerTests
 
         var body = await response.Content.ReadAsStringAsync();
         using var doc = JsonDocument.Parse(body);
-        Assert.That(doc.RootElement.GetProperty("emptyDeckNum").GetInt32(), Is.EqualTo(1));
+        Assert.That(doc.RootElement.GetProperty("empty_deck_num").GetInt32(), Is.EqualTo(1));
     }
 
     [Test]
@@ -127,7 +127,7 @@ public class DeckControllerTests
 
         var body = await response.Content.ReadAsStringAsync();
         using var doc = JsonDocument.Parse(body);
-        Assert.That(doc.RootElement.GetProperty("emptyDeckNum").GetInt32(), Is.EqualTo(3),
+        Assert.That(doc.RootElement.GetProperty("empty_deck_num").GetInt32(), Is.EqualTo(3),
             "Algorithm must return the smallest free slot, not just one past the highest used.");
     }
 
@@ -142,10 +142,10 @@ public class DeckControllerTests
         using var client = factory.CreateAuthenticatedClient(viewerId);
 
         var updateJson = $$"""
-            {"viewerId":"0","steamId":0,"steamSessionTicket":"",
-             "deckNo":1,"classId":{{classId}},"leaderSkinId":{{leaderSkinId}},
-             "isRandomLeaderSkin":false,"sleeveId":{{sleeveId}},"deckName":"Fresh Deck",
-             "isDelete":0,"deckFormat":0}
+            {"viewer_id":"0","steam_id":0,"steam_session_ticket":"",
+             "deck_no":1,"class_id":{{classId}},"leader_skin_id":{{leaderSkinId}},
+             "is_random_leader_skin":false,"sleeve_id":{{sleeveId}},"deck_name":"Fresh Deck",
+             "is_delete":0,"deck_format":0}
             """;
         var response = await client.PostAsync("/deck/update", JsonBody(updateJson));
 
@@ -170,10 +170,10 @@ public class DeckControllerTests
         using var client = factory.CreateAuthenticatedClient(viewerId);
 
         var updateJson = $$"""
-            {"viewerId":"0","steamId":0,"steamSessionTicket":"",
-             "deckNo":1,"classId":{{classId}},"leaderSkinId":{{leaderSkinId}},
-             "isRandomLeaderSkin":false,"sleeveId":{{sleeveId}},"deckName":"Renamed",
-             "isDelete":0,"deckFormat":0}
+            {"viewer_id":"0","steam_id":0,"steam_session_ticket":"",
+             "deck_no":1,"class_id":{{classId}},"leader_skin_id":{{leaderSkinId}},
+             "is_random_leader_skin":false,"sleeve_id":{{sleeveId}},"deck_name":"Renamed",
+             "is_delete":0,"deck_format":0}
             """;
         await client.PostAsync("/deck/update", JsonBody(updateJson));
 
@@ -194,10 +194,10 @@ public class DeckControllerTests
         using var client = factory.CreateAuthenticatedClient(viewerId);
 
         var deleteJson = $$"""
-            {"viewerId":"0","steamId":0,"steamSessionTicket":"",
-             "deckNo":1,"classId":{{classId}},"leaderSkinId":{{leaderSkinId}},
-             "isRandomLeaderSkin":false,"sleeveId":{{sleeveId}},"deckName":null,
-             "isDelete":1,"deckFormat":0}
+            {"viewer_id":"0","steam_id":0,"steam_session_ticket":"",
+             "deck_no":1,"class_id":{{classId}},"leader_skin_id":{{leaderSkinId}},
+             "is_random_leader_skin":false,"sleeve_id":{{sleeveId}},"deck_name":null,
+             "is_delete":1,"deck_format":0}
             """;
         var response = await client.PostAsync("/deck/update", JsonBody(deleteJson));
 
@@ -219,16 +219,16 @@ public class DeckControllerTests
         using var client = factory.CreateAuthenticatedClient(viewerId);
 
         var updateJson = $$"""
-            {"viewerId":"0","steamId":0,"steamSessionTicket":"",
-             "deckNo":2,"classId":{{classId}},"leaderSkinId":{{leaderSkinId}},
-             "isRandomLeaderSkin":false,"sleeveId":{{sleeveId}},"deckName":"Second",
-             "isDelete":0,"deckFormat":0}
+            {"viewer_id":"0","steam_id":0,"steam_session_ticket":"",
+             "deck_no":2,"class_id":{{classId}},"leader_skin_id":{{leaderSkinId}},
+             "is_random_leader_skin":false,"sleeve_id":{{sleeveId}},"deck_name":"Second",
+             "is_delete":0,"deck_format":0}
             """;
         var response = await client.PostAsync("/deck/update", JsonBody(updateJson));
 
         var body = await response.Content.ReadAsStringAsync();
         using var doc = JsonDocument.Parse(body);
-        var decks = doc.RootElement.GetProperty("userDeckList");
+        var decks = doc.RootElement.GetProperty("user_deck_list");
         Assert.That(decks.GetArrayLength(), Is.EqualTo(2),
             "/deck/update should hand back the full refreshed list, saving the client a follow-up.");
         var names = Enumerable.Range(0, decks.GetArrayLength())
@@ -247,12 +247,12 @@ public class DeckControllerTests
         await factory.SeedDeckAsync(viewerId, Format.Rotation, 1, name: "Old Name");
         using var client = factory.CreateAuthenticatedClient(viewerId);
 
-        var json = """{"viewerId":"0","steamId":0,"steamSessionTicket":"","deckNo":1,"deckName":"New Name","deckFormat":0}""";
+        var json = """{"viewer_id":"0","steam_id":0,"steam_session_ticket":"","deck_no":1,"deck_name":"New Name","deck_format":0}""";
         var response = await client.PostAsync("/deck/update_name", JsonBody(json));
 
         var body = await response.Content.ReadAsStringAsync();
         using var doc = JsonDocument.Parse(body);
-        Assert.That(doc.RootElement.GetProperty("userDeck").GetProperty("name").GetString(),
+        Assert.That(doc.RootElement.GetProperty("user_deck").GetProperty("name").GetString(),
             Is.EqualTo("New Name"));
 
         using var scope = factory.Services.CreateScope();
@@ -276,12 +276,12 @@ public class DeckControllerTests
         }
         using var client = factory.CreateAuthenticatedClient(viewerId);
 
-        var json = $$"""{"viewerId":"0","steamId":0,"steamSessionTicket":"","deckNo":1,"sleeveId":{{sleeveId}},"deckFormat":0}""";
+        var json = $$"""{"viewer_id":"0","steam_id":0,"steam_session_ticket":"","deckNo":1,"sleeve_id":{{sleeveId}},"deckFormat":0}""";
         var response = await client.PostAsync("/deck/update_sleeve", JsonBody(json));
 
         var body = await response.Content.ReadAsStringAsync();
         using var doc = JsonDocument.Parse(body);
-        Assert.That(doc.RootElement.GetProperty("userDeck").GetProperty("sleeveId").GetInt32(),
+        Assert.That(doc.RootElement.GetProperty("user_deck").GetProperty("sleeve_id").GetInt32(),
             Is.EqualTo(sleeveId));
     }
 
@@ -299,14 +299,14 @@ public class DeckControllerTests
         }
         using var client = factory.CreateAuthenticatedClient(viewerId);
 
-        var json = $$"""{"viewerId":"0","steamId":0,"steamSessionTicket":"","deckNo":1,"leaderSkinId":{{skinId}},"deckFormat":0}""";
+        var json = $$"""{"viewer_id":"0","steam_id":0,"steam_session_ticket":"","deck_no":1,"leader_skin_id":{{skinId}},"deck_format":0}""";
         var response = await client.PostAsync("/deck/update_leader_skin", JsonBody(json));
 
         var body = await response.Content.ReadAsStringAsync();
         using var doc = JsonDocument.Parse(body);
-        var userDeck = doc.RootElement.GetProperty("userDeck");
-        Assert.That(userDeck.GetProperty("leaderSkinId").GetInt32(), Is.EqualTo(skinId));
-        Assert.That(userDeck.GetProperty("isRandomLeaderSkin").GetInt32(), Is.EqualTo(0),
+        var userDeck = doc.RootElement.GetProperty("user_deck");
+        Assert.That(userDeck.GetProperty("leader_skin_id").GetInt32(), Is.EqualTo(skinId));
+        Assert.That(userDeck.GetProperty("is_random_leader_skin").GetInt32(), Is.EqualTo(0),
             "Selecting a specific leader skin clears the random-skin flag.");
     }
 
@@ -325,17 +325,17 @@ public class DeckControllerTests
         using var client = factory.CreateAuthenticatedClient(viewerId);
 
         var json =
-            $$"""{"viewerId":"0","steamId":0,"steamSessionTicket":"","deckNo":1,"deckFormat":0,"leaderSkinIdList":[{{string.Join(',', pool)}}]}""";
+            $$"""{"viewer_id":"0","steam_id":0,"steam_session_ticket":"","deck_no":1,"deck_format":0,"leader_skin_id_list":[{{string.Join(',', pool)}}]}""";
         var response = await client.PostAsync("/deck/update_random_leader_skin", JsonBody(json));
 
         var body = await response.Content.ReadAsStringAsync();
         Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK), body);
 
         using var doc = JsonDocument.Parse(body);
-        var userDeck = doc.RootElement.GetProperty("userDeck");
-        Assert.That(pool, Contains.Item(userDeck.GetProperty("leaderSkinId").GetInt32()),
+        var userDeck = doc.RootElement.GetProperty("user_deck");
+        Assert.That(pool, Contains.Item(userDeck.GetProperty("leader_skin_id").GetInt32()),
             "Chosen skin must come from the supplied pool.");
-        Assert.That(userDeck.GetProperty("isRandomLeaderSkin").GetInt32(), Is.EqualTo(1));
+        Assert.That(userDeck.GetProperty("is_random_leader_skin").GetInt32(), Is.EqualTo(1));
     }
 
     [Test]
@@ -347,7 +347,7 @@ public class DeckControllerTests
         using var client = factory.CreateAuthenticatedClient(viewerId);
 
         var json =
-            """{"viewerId":"0","steamId":0,"steamSessionTicket":"","deckNo":1,"deckFormat":0,"leaderSkinIdList":[]}""";
+            """{"viewer_id":"0","steam_id":0,"steam_session_ticket":"","deck_no":1,"deck_format":0,"leader_skin_id_list":[]}""";
         var response = await client.PostAsync("/deck/update_random_leader_skin", JsonBody(json));
 
         Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.BadRequest));
@@ -365,7 +365,7 @@ public class DeckControllerTests
         using var client = factory.CreateAuthenticatedClient(viewerId);
 
         var json =
-            """{"viewerId":"0","steamId":0,"steamSessionTicket":"","deckOrder":[2,1],"deckFormat":0}""";
+            """{"viewer_id":"0","steam_id":0,"steam_session_ticket":"","deck_order":[2,1],"deck_format":0}""";
         var response = await client.PostAsync("/deck/update_order", JsonBody(json));
 
         Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK));
@@ -382,7 +382,7 @@ public class DeckControllerTests
         using var client = factory.CreateAuthenticatedClient(viewerId);
 
         var json =
-            """{"viewerId":"0","steamId":0,"steamSessionTicket":"","deckNoList":[1,3],"deckFormat":0}""";
+            """{"viewer_id":"0","steam_id":0,"steam_session_ticket":"","deck_no_list":[1,3],"deck_format":0}""";
         var response = await client.PostAsync("/deck/delete_deck_list", JsonBody(json));
 
         Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK));
@@ -401,7 +401,7 @@ public class DeckControllerTests
         long viewerId = await factory.SeedViewerAsync();
         using var client = factory.CreateAuthenticatedClient(viewerId);
 
-        var json = """{"viewerId":"0","steamId":0,"steamSessionTicket":"","deckNo":1,"classId":1}""";
+        var json = """{"viewer_id":"0","steam_id":0,"steam_session_ticket":"","deck_no":1,"class_id":1}""";
         var response = await client.PostAsync("/deck/set_deck_redis", JsonBody(json));
 
         Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK));
diff --git a/SVSim.UnitTests/Controllers/LoadControllerTests.cs b/SVSim.UnitTests/Controllers/LoadControllerTests.cs
index 6700a46..ebce139 100644
--- a/SVSim.UnitTests/Controllers/LoadControllerTests.cs
+++ b/SVSim.UnitTests/Controllers/LoadControllerTests.cs
@@ -14,7 +14,7 @@ namespace SVSim.UnitTests.Controllers;
 public class LoadControllerTests
 {
     private const string IndexRequestJson =
-        """{"viewerId":"0","steamId":0,"steamSessionTicket":"","carrier":"steam","cardMasterHash":""}""";
+        """{"viewer_id":"0","steam_id":0,"steam_session_ticket":"","carrier":"steam","card_master_hash":""}""";
 
     /// <summary>
     /// JSON keys (camelCased C# property names) for fields the client reads unconditionally.
@@ -25,17 +25,17 @@ public class LoadControllerTests
     /// </summary>
     private static readonly string[] RequiredIndexKeys =
     {
-        "userTutorial", "userInfo", "userCurrency", "userItems",
-        "userRotationDecks", "userUnlimitedDecks", "userMyRotationDecks",
-        "userCards", "userClasses", "sleeves", "userEmblems",
-        "userDegrees", "leaderSkins", "myPageBackgrounds",
-        "userRankInfo", "userRankedMatches", "dailyLoginBonus", "arenaConfig",
-        "redEtherOverrides", "maintenanceCards", "arenaInfos", "rankInfo",
-        "classExp", "loadingTipCardExclusions", "defaultSettings",
-        "unlimitedBanList", "rotationSets",
-        "reprintedCards", "spotCards", "featureMaintenances",
-        "specialCrystalInfos", "openBattlefieldIds", "lootBoxRegulations",
-        "gatheringInfo", "userConfig", "deckFormat", "cardSetIdForResourceDlView"
+        "user_tutorial", "user_info", "user_currency", "user_items",
+        "user_rotation_decks", "user_unlimited_decks", "user_my_rotation_decks",
+        "user_cards", "user_classes", "sleeves", "user_emblems",
+        "user_degrees", "leader_skins", "my_page_backgrounds",
+        "user_rank_info", "user_ranked_matches", "daily_login_bonus", "arena_config",
+        "red_ether_overrides", "maintenance_cards", "arena_infos", "rank_info",
+        "class_exp", "loading_tip_card_exclusions", "default_settings",
+        "unlimited_ban_list", "rotation_sets",
+        "reprinted_cards", "spot_cards", "feature_maintenances",
+        "special_crystal_infos", "open_battlefield_ids", "loot_box_regulations",
+        "gathering_info", "user_config", "deck_format", "card_set_id_for_resource_dl_view"
     };
 
     private static async Task<JsonElement> PostIndexAndReadBody(SVSimTestFactory factory, long viewerId)
@@ -100,7 +100,7 @@ public class LoadControllerTests
 
         var root = await PostIndexAndReadBody(factory, viewerId);
 
-        Assert.That(root.GetProperty("userRankInfo").ValueKind, Is.EqualTo(JsonValueKind.Array));
+        Assert.That(root.GetProperty("user_rank_info").ValueKind, Is.EqualTo(JsonValueKind.Array));
     }
 
     [Test]
@@ -114,7 +114,7 @@ public class LoadControllerTests
 
         var root = await PostIndexAndReadBody(factory, viewerId);
 
-        Assert.That(root.GetProperty("userRankInfo").GetArrayLength(), Is.EqualTo(5));
+        Assert.That(root.GetProperty("user_rank_info").GetArrayLength(), Is.EqualTo(5));
     }
 
     [Test]
@@ -127,7 +127,7 @@ public class LoadControllerTests
 
         var root = await PostIndexAndReadBody(factory, viewerId);
 
-        Assert.That(root.GetProperty("rotationSets").GetArrayLength(),
+        Assert.That(root.GetProperty("rotation_sets").GetArrayLength(),
             Is.GreaterThanOrEqualTo(2));
     }
 
@@ -141,12 +141,12 @@ public class LoadControllerTests
 
         var root = await PostIndexAndReadBody(factory, viewerId);
 
-        foreach (var key in new[] { "userRotationDecks", "userUnlimitedDecks", "userMyRotationDecks" })
+        foreach (var key in new[] { "user_rotation_decks", "user_unlimited_decks", "user_my_rotation_decks" })
         {
             var container = root.GetProperty(key);
             Assert.That(container.ValueKind, Is.EqualTo(JsonValueKind.Object),
                 $"{key} should be the UserFormatDeckInfo object wrapper, not a raw array.");
-            var inner = container.GetProperty("userDecks");
+            var inner = container.GetProperty("user_decks");
             Assert.That(inner.ValueKind, Is.EqualTo(JsonValueKind.Array));
             Assert.That(inner.GetArrayLength(), Is.EqualTo(0),
                 $"{key}.userDecks must be an empty array for a deckless viewer, not null.");
diff --git a/SVSim.UnitTests/Controllers/PracticeControllerTests.cs b/SVSim.UnitTests/Controllers/PracticeControllerTests.cs
index 9de6a4d..61744ac 100644
--- a/SVSim.UnitTests/Controllers/PracticeControllerTests.cs
+++ b/SVSim.UnitTests/Controllers/PracticeControllerTests.cs
@@ -14,10 +14,10 @@ namespace SVSim.UnitTests.Controllers;
 public class PracticeControllerTests
 {
     private const string BaseRequestJson =
-        """{"viewerId":"0","steamId":0,"steamSessionTicket":""}""";
+        """{"viewer_id":"0","steam_id":0,"steam_session_ticket":""}""";
 
     private static string DeckFormatRequestJson(Format f) =>
-        $$"""{"viewerId":"0","steamId":0,"steamSessionTicket":"","deckFormat":{{(int)f}}}""";
+        $$"""{"viewer_id":"0","steam_id":0,"steam_session_ticket":"","deck_format":{{(int)f}}}""";
 
     [Test]
     public async Task Info_returns_non_empty_opponent_array()
@@ -36,7 +36,7 @@ public class PracticeControllerTests
         Assert.That(doc.RootElement.ValueKind, Is.EqualTo(JsonValueKind.Array),
             "/practice/info returns a bare array (no wrapper object) per spec.");
         Assert.That(doc.RootElement.GetArrayLength(), Is.GreaterThan(0));
-        Assert.That(doc.RootElement[0].GetProperty("practiceId").GetInt32(), Is.GreaterThan(0));
+        Assert.That(doc.RootElement[0].GetProperty("practice_id").GetInt32(), Is.GreaterThan(0));
     }
 
     [Test]
@@ -55,8 +55,8 @@ public class PracticeControllerTests
         Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK), body);
 
         using var doc = JsonDocument.Parse(body);
-        var rotation = doc.RootElement.GetProperty("userDeckRotation");
-        var unlimited = doc.RootElement.GetProperty("userDeckUnlimited");
+        var rotation = doc.RootElement.GetProperty("user_deck_rotation");
+        var unlimited = doc.RootElement.GetProperty("user_deck_unlimited");
         Assert.That(rotation.GetArrayLength(), Is.EqualTo(1));
         Assert.That(rotation[0].GetProperty("name").GetString(), Is.EqualTo("Rotation Deck"));
         Assert.That(unlimited.GetArrayLength(), Is.EqualTo(1));
@@ -77,8 +77,8 @@ public class PracticeControllerTests
         Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK), body);
 
         using var doc = JsonDocument.Parse(body);
-        Assert.That(doc.RootElement.GetProperty("userDeckRotation").GetArrayLength(), Is.EqualTo(0));
-        Assert.That(doc.RootElement.GetProperty("userDeckUnlimited").GetArrayLength(), Is.EqualTo(0));
+        Assert.That(doc.RootElement.GetProperty("user_deck_rotation").GetArrayLength(), Is.EqualTo(0));
+        Assert.That(doc.RootElement.GetProperty("user_deck_unlimited").GetArrayLength(), Is.EqualTo(0));
     }
 
     [Test]
@@ -104,7 +104,7 @@ public class PracticeControllerTests
         // recoveryData is an opaque JSON blob serialized to string by the client; the server
         // is supposed to accept it without validation. Anything goes.
         var finishJson =
-            """{"viewerId":"0","steamId":0,"steamSessionTicket":"","deckNo":1,"isWin":1,"evolveCount":2,"totalTurn":5,"enemyClassId":3,"difficulty":1,"deckFormat":0,"classId":1,"recoveryData":"{\"opaque\":\"blob\"}"}""";
+            """{"viewer_id":"0","steam_id":0,"steam_session_ticket":"","deck_no":1,"is_win":1,"evolve_count":2,"total_turn":5,"enemy_class_id":3,"difficulty":1,"deck_format":0,"class_id":1,"recovery_data":"{\"opaque\":\"blob\"}"}""";
 
         var response = await client.PostAsync("/practice/finish",
             new StringContent(finishJson, Encoding.UTF8, "application/json"));
@@ -113,8 +113,8 @@ public class PracticeControllerTests
         Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK), body);
 
         using var doc = JsonDocument.Parse(body);
-        Assert.That(doc.RootElement.GetProperty("getClassExperience").GetInt32(), Is.EqualTo(0));
-        Assert.That(doc.RootElement.GetProperty("classExperience").GetInt32(), Is.EqualTo(0));
-        Assert.That(doc.RootElement.GetProperty("rewardList").GetArrayLength(), Is.EqualTo(0));
+        Assert.That(doc.RootElement.GetProperty("get_class_experience").GetInt32(), Is.EqualTo(0));
+        Assert.That(doc.RootElement.GetProperty("class_experience").GetInt32(), Is.EqualTo(0));
+        Assert.That(doc.RootElement.GetProperty("reward_list").GetArrayLength(), Is.EqualTo(0));
     }
 }
diff --git a/SVSim.UnitTests/Infrastructure/WireSerializationTests.cs b/SVSim.UnitTests/Infrastructure/WireSerializationTests.cs
new file mode 100644
index 0000000..892dea3
--- /dev/null
+++ b/SVSim.UnitTests/Infrastructure/WireSerializationTests.cs
@@ -0,0 +1,138 @@
+using System.Text;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using MessagePack;
+using MessagePack.Resolvers;
+using Newtonsoft.Json.Linq;
+using SVSim.EmulatedEntrypoint.Middlewares;
+using SVSim.EmulatedEntrypoint.Models.Dtos;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Responses;
+
+namespace SVSim.UnitTests.Infrastructure;
+
+/// <summary>
+/// Exercises the JSON → JTree → msgpack pipeline that runs inside
+/// <see cref="ShadowverseTranslationMiddleware"/>. The middleware itself is hard to unit-test
+/// in isolation because it depends on HttpContext + the encryption layer; instead we re-run
+/// the exact same transformation steps the middleware would and assert on the resulting
+/// msgpack bytes converted back to JSON.
+///
+/// What this guards: that a controller returning an object with mixed null/non-null optional
+/// fields produces a msgpack map containing ONLY the non-null fields, with snake_case keys.
+/// That's the load-bearing property — the Unity client's <c>Keys.Contains</c> + <c>.ToInt()</c>
+/// idiom NREs the moment the wire serializes a present-but-null field.
+/// </summary>
+public class WireSerializationTests
+{
+    private static readonly JsonSerializerOptions ControllerJsonOptions = new()
+    {
+        PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower,
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
+    };
+
+    private static readonly MessagePackSerializerOptions MsgPackOptions =
+        MessagePackSerializerOptions.Standard.WithResolver(ContractlessStandardResolver.Instance);
+
+    /// <summary>
+    /// Reproduces what the middleware does to a controller's response object: serialize via
+    /// our JSON options (snake_case, omit null) → parse JSON tree → convert to plain CLR tree
+    /// → msgpack-serialize → msgpack-to-JSON. Returns the JSON the client would see.
+    /// </summary>
+    private static JsonDocument RoundTripThroughWirePipeline<T>(T response)
+    {
+        var json = JsonSerializer.Serialize(response, ControllerJsonOptions);
+        var tree = ShadowverseTranslationMiddleware.ConvertJsonTreeToPlainObject(JToken.Parse(json));
+        var msgPackBytes = MessagePackSerializer.Serialize(tree, MsgPackOptions);
+        var msgPackJson = MessagePackSerializer.ConvertToJson(msgPackBytes, MsgPackOptions);
+        return JsonDocument.Parse(msgPackJson);
+    }
+
+    [Test]
+    public void GameStartResponse_with_null_optional_fields_omits_them_from_msgpack()
+    {
+        var response = new GameStartResponse
+        {
+            NowViewerId = 42,
+            NowName = "Tester",
+            NowTutorialStep = "100",
+            IsSetTransitionPassword = true,
+            NowRank = new Dictionary<string, string> { { "1", "RankName_010" } },
+            TransitionAccountData = new List<TransitionAccountData>
+            {
+                new() { SocialAccountId = "76561198000000001", SocialAccountType = "5", ConnectedViewerId = "42" }
+            },
+            // Both deliberately left at null — must NOT appear in the msgpack output.
+            RewriteViewerId = null,
+            AccountDeleteReservationStatus = null,
+            TosState = 1, PolicyState = 1, KorAuthorityState = 0,
+            TosId = 1, PolicyId = 1, KorAuthorityId = 0
+        };
+
+        using var doc = RoundTripThroughWirePipeline(response);
+        var root = doc.RootElement;
+
+        Assert.That(root.TryGetProperty("rewrite_viewer_id", out _), Is.False,
+            "Null `rewrite_viewer_id` must not appear in msgpack output — client NREs on Keys.Contains+.ToInt() against Nil.");
+        Assert.That(root.TryGetProperty("account_delete_reservation_status", out _), Is.False,
+            "Null `account_delete_reservation_status` must not appear (presence triggers client behavior).");
+
+        // Sanity: required fields still there, with snake_case keys.
+        Assert.That(root.GetProperty("now_viewer_id").GetInt64(), Is.EqualTo(42));
+        Assert.That(root.GetProperty("now_name").GetString(), Is.EqualTo("Tester"));
+        Assert.That(root.GetProperty("now_tutorial_step").GetString(), Is.EqualTo("100"));
+        Assert.That(root.GetProperty("tos_state").GetInt64(), Is.EqualTo(1));
+    }
+
+    [Test]
+    public void GameStartResponse_with_set_optional_field_emits_it_in_msgpack()
+    {
+        // Mirror image of the omission test: when an optional field IS set, it must reach the
+        // client. Otherwise we'd have papered over the bug by accidentally dropping everything.
+        var response = new GameStartResponse
+        {
+            NowViewerId = 42,
+            NowName = "Tester",
+            NowTutorialStep = "100",
+            IsSetTransitionPassword = false,
+            RewriteViewerId = 999, // explicitly set
+            AccountDeleteReservationStatus = 1,
+            TosState = 1, PolicyState = 1, KorAuthorityState = 0,
+            TosId = 1, PolicyId = 1, KorAuthorityId = 0
+        };
+
+        using var doc = RoundTripThroughWirePipeline(response);
+        var root = doc.RootElement;
+
+        Assert.That(root.GetProperty("rewrite_viewer_id").GetInt64(), Is.EqualTo(999));
+        Assert.That(root.GetProperty("account_delete_reservation_status").GetInt64(), Is.EqualTo(1));
+    }
+
+    [Test]
+    public void GameStartResponse_nested_transition_account_data_round_trips_with_snake_case_keys()
+    {
+        // The nested list of TransitionAccountData entries needs to keep its per-entry shape
+        // (snake_case keys, three string fields) after going JSON → tree → msgpack → JSON.
+        var response = new GameStartResponse
+        {
+            NowViewerId = 42,
+            NowName = "Tester",
+            NowTutorialStep = "100",
+            TransitionAccountData = new List<TransitionAccountData>
+            {
+                new() { SocialAccountId = "111", SocialAccountType = "5", ConnectedViewerId = "42" }
+            },
+            TosState = 1, PolicyState = 1, KorAuthorityState = 0,
+            TosId = 1, PolicyId = 1, KorAuthorityId = 0
+        };
+
+        using var doc = RoundTripThroughWirePipeline(response);
+        var transitions = doc.RootElement.GetProperty("transition_account_data");
+        Assert.That(transitions.ValueKind, Is.EqualTo(JsonValueKind.Array));
+        Assert.That(transitions.GetArrayLength(), Is.EqualTo(1));
+
+        var entry = transitions[0];
+        Assert.That(entry.GetProperty("social_account_id").GetString(), Is.EqualTo("111"));
+        Assert.That(entry.GetProperty("social_account_type").GetString(), Is.EqualTo("5"));
+        Assert.That(entry.GetProperty("connected_viewer_id").GetString(), Is.EqualTo("42"));
+    }
+}
diff --git a/SVSim.UnitTests/RoutingSmokeTests.cs b/SVSim.UnitTests/RoutingSmokeTests.cs
index 8f87224..78b27c7 100644
--- a/SVSim.UnitTests/RoutingSmokeTests.cs
+++ b/SVSim.UnitTests/RoutingSmokeTests.cs
@@ -30,7 +30,7 @@ public class RoutingSmokeTests
     }
 
     private const string ValidBaseRequestJson =
-        """{"viewerId":"0","steamId":0,"steamSessionTicket":""}""";
+        """{"viewer_id":"0","steam_id":0,"steam_session_ticket":""}""";
 
     [Test]
     public async Task CheckSpecialTitle_resolves_to_CheckController()
@@ -48,7 +48,7 @@ public class RoutingSmokeTests
         var body = await response.Content.ReadAsStringAsync();
         // Plain-JSON path uses camelCase (System.Text.Json default); MessagePack [Key] only applies
         // to the Unity-UA encrypted path through ShadowverseTranslationMiddleware.
-        Assert.That(body, Does.Contain("\"titleImageId\":\"0\""),
+        Assert.That(body, Does.Contain("\"title_image_id\":\"0\""),
             "SpecialTitleCheck should return the built-in title id \"0\".");
     }