diff --git a/SVSim.EmulatedEntrypoint/Models/Dtos/UserMyPageInfo.cs b/SVSim.EmulatedEntrypoint/Models/Dtos/UserMyPageInfo.cs
index 1656925..7bc178a 100644
--- a/SVSim.EmulatedEntrypoint/Models/Dtos/UserMyPageInfo.cs
+++ b/SVSim.EmulatedEntrypoint/Models/Dtos/UserMyPageInfo.cs
@@ -17,21 +17,22 @@ public class UserMyPageInfo
 }
 
 /// <summary>
-/// Active mypage background selection. Shape from prod 2026-05-23.
+/// Active mypage background selection. Shape from prod 2026-06-09 (capture line 12/56).
+/// All three fields ship as strings on the wire even though the underlying ids are integers.
 /// </summary>
 [MessagePackObject]
 public class MyPageBgSetting
 {
     [JsonPropertyName("mypage_id")]
     [Key("mypage_id")]
-    public int MyPageId { get; set; }
+    public string MyPageId { get; set; } = "0";
 
-    /// <summary>0 = single selection (mypage_id), 1+ = random rotation across mypage_id_list.</summary>
+    /// <summary>BGType enum as decimal string: "0"=Deck, "1"=CustomBG, "2"=RandomBG.</summary>
     [JsonPropertyName("select_type")]
     [Key("select_type")]
-    public int SelectType { get; set; }
+    public string SelectType { get; set; } = "0";
 
     [JsonPropertyName("mypage_id_list")]
     [Key("mypage_id_list")]
-    public List<int> MyPageIdList { get; set; } = new();
+    public List<string> MyPageIdList { get; set; } = new();
 }
diff --git a/SVSim.UnitTests/Wire/UserMyPageWireShape.cs b/SVSim.UnitTests/Wire/UserMyPageWireShape.cs
new file mode 100644
index 0000000..6e64018
--- /dev/null
+++ b/SVSim.UnitTests/Wire/UserMyPageWireShape.cs
@@ -0,0 +1,40 @@
+using System.Text.Json;
+using SVSim.EmulatedEntrypoint.Models.Dtos;
+
+namespace SVSim.UnitTests.Wire;
+
+public class UserMyPageWireShape
+{
+    [Test]
+    public void MyPageBgSetting_serialization_emits_strings_matching_prod_capture()
+    {
+        var setting = new MyPageBgSetting
+        {
+            MyPageId = "1213410310",
+            SelectType = "1",
+            MyPageIdList = new List<string>
+            {
+                "1211410310", "1212410310", "1213410310", "1214410310",
+                "1215410310", "1216410310", "1217410310", "1218410310",
+            },
+        };
+
+        var options = new JsonSerializerOptions
+        {
+            PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower,
+            DefaultIgnoreCondition = System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingNull,
+        };
+        var json = JsonSerializer.Serialize(setting, options);
+
+        // Prod capture line 12 / line 56 of traffic_prod_misc_clicking.ndjson, user_mypage_info.user_mypage_setting:
+        var expected =
+            "{\"mypage_id\":\"1213410310\"," +
+            "\"select_type\":\"1\"," +
+            "\"mypage_id_list\":[" +
+            "\"1211410310\",\"1212410310\",\"1213410310\",\"1214410310\"," +
+            "\"1215410310\",\"1216410310\",\"1217410310\",\"1218410310\"" +
+            "]}";
+
+        Assert.That(json, Is.EqualTo(expected));
+    }
+}