diff --git a/SVSimLoader/CaptureWriter.cs b/SVSimLoader/CaptureWriter.cs
index 977f12f..03a7afa 100644
--- a/SVSimLoader/CaptureWriter.cs
+++ b/SVSimLoader/CaptureWriter.cs
@@ -15,6 +15,7 @@ internal static class CaptureWriter
     private static string _cardsPath;
     private static string _userDataPath;
     private static string _sbsPath;
+    private static string _spinProbePath;
     private static readonly HashSet<string> _seenSbsIds = new HashSet<string>();
     private static ulong _lastSeenSteamId;
 
@@ -33,6 +34,32 @@ internal static class CaptureWriter
         _cardsPath = Path.Combine(_sessionDir, "cards.json");
         _userDataPath = Path.Combine(_sessionDir, "user-data.json");
         _sbsPath = Path.Combine(_sessionDir, "special-battle-settings.ndjson");
+        _spinProbePath = Path.Combine(_sessionDir, "spin-rng.ndjson");
+    }
+
+    /// <summary>
+    /// Append one row of the spin/shared-RNG diagnostic (EnableSpinProbe). `event` is "receive"
+    /// (OperateReceive.StartOperate, logged before the spin crank) or "send" (NetworkBattleSender.EmitMsg).
+    /// `spin` is the inbound crank count (receive only; -1 when not applicable) and `count` is the
+    /// cumulative BattleManagerBase.stableRandomCount tally (-1 when no battle manager is live).
+    /// Offline, per-turn `count` deltas vs `spin` settle whether prod authors spin from server-side
+    /// simulation or it is derivable from the active player's wire data. See
+    /// docs/audits/battle-node-spin-rng-model-2026-06-04.md.
+    /// </summary>
+    public static void AppendSpinProbe(string eventKind, string uri, int spin, int count)
+    {
+        string line = JsonMapper.ToJson(new Dictionary<string, object>
+        {
+            { "ts", DateTime.UtcNow.ToString("o") },
+            { "event", eventKind },
+            { "uri", uri },
+            { "spin", spin },
+            { "count", count },
+        });
+        lock (_lock)
+        {
+            File.AppendAllText(_spinProbePath, line + "\n");
+        }
     }
 
     private static string ExtractHost(string url)
diff --git a/SVSimLoader/Patches/SpinProbe.cs b/SVSimLoader/Patches/SpinProbe.cs
new file mode 100644
index 0000000..3eef597
--- /dev/null
+++ b/SVSimLoader/Patches/SpinProbe.cs
@@ -0,0 +1,80 @@
+using System;
+using System.Collections.Generic;
+using System.Reflection;
+using HarmonyLib;
+
+namespace SVSimLoader.Patches;
+
+/// <summary>
+/// Diagnostic for the battle-node `spin` investigation (gated on EnableSpinProbe). It records, per
+/// battle frame, the cumulative shared-RNG draw tally (BattleManagerBase.stableRandomCount) alongside
+/// the inbound `spin` crank count, so per-turn `count` deltas can be compared against `spin` offline.
+///
+/// Why this settles the open question: every shared-RNG draw funnels through StableRandom/
+/// StableRandomDouble, which increment stableRandomCount — so that field is an EXACT local draw count
+/// (decomp: BattleManagerBase.cs:1581,1592; the only `_stableRandom.` callers). The receiver cranks the
+/// shared RNG `spin` times before dispatching each frame (OperateReceive.StartOperate:80-84). If our own
+/// turns advance `count` by ~the magnitude of the `spin` the opponent's turns hand us, the draws are
+/// client-side and `spin` is plausibly wire-derivable; if our local deltas stay near zero while inbound
+/// `spin` is in the tens–hundreds, prod authors `spin` from server-side simulation (≈ an engine).
+///
+/// Run against PROD (servers live until end of June 2026) to capture real `spin`; a local Bot/AI battle
+/// only ever sends spin=0. Output: spin-rng.ndjson in the capture session dir.
+/// See docs/audits/battle-node-spin-rng-model-2026-06-04.md.
+/// </summary>
+[HarmonyPatch]
+public static class SpinProbe
+{
+    // stableRandomCount is a private int on BattleManagerBase — read it reflectively.
+    private static readonly FieldInfo CountField =
+        AccessTools.Field(typeof(BattleManagerBase), "stableRandomCount");
+
+    /// <summary>Current cumulative shared-RNG draw tally, or -1 when no battle manager is live / unreadable.</summary>
+    private static int ReadCount()
+    {
+        try
+        {
+            var mgr = BattleManagerBase.GetIns();
+            if (mgr == null || CountField == null) return -1;
+            return (int)CountField.GetValue(mgr);
+        }
+        catch
+        {
+            return -1;
+        }
+    }
+
+    // Receive side: logged BEFORE the spin crank runs, so `count` is the tally entering this frame and
+    // `spin` is the value about to be applied.
+    [HarmonyPatch(typeof(OperateReceive), nameof(OperateReceive.StartOperate))]
+    [HarmonyPrefix]
+    public static void OnReceiveFrame(NetworkBattleReceiver.ReceiveData receivedData)
+    {
+        if (!SvSimConfig.EnableSpinProbe || receivedData == null) return;
+        try
+        {
+            CaptureWriter.AppendSpinProbe("receive", $"{receivedData.dataUri}", receivedData.spin, ReadCount());
+        }
+        catch (Exception e)
+        {
+            Plugin.Log.LogError(e);
+        }
+    }
+
+    // Send side: our own emitted frames. `count` shows how much our turn's actions advanced the tally;
+    // `spin` is not applicable on a send (-1).
+    [HarmonyPatch(typeof(NetworkBattleSender), "EmitMsg")]
+    [HarmonyPrefix]
+    public static void OnSendFrame(NetworkBattleDefine.NetworkBattleURI uri, Dictionary<string, object> dataList = null)
+    {
+        if (!SvSimConfig.EnableSpinProbe) return;
+        try
+        {
+            CaptureWriter.AppendSpinProbe("send", $"{uri}", -1, ReadCount());
+        }
+        catch (Exception e)
+        {
+            Plugin.Log.LogError(e);
+        }
+    }
+}
diff --git a/SVSimLoader/Plugin.cs b/SVSimLoader/Plugin.cs
index a6bbd34..1463e43 100644
--- a/SVSimLoader/Plugin.cs
+++ b/SVSimLoader/Plugin.cs
@@ -49,6 +49,9 @@ namespace SVSimLoader
             SvSimConfig.EnableBattleCapture =
                 Config.Bind("Capture", "EnableBattleCapture", true,
                     "Write each Socket.IO battle send/receive body to battle-traffic.ndjson in the current capture session directory").Value;
+            SvSimConfig.EnableSpinProbe =
+                Config.Bind("Capture", "EnableSpinProbe", false,
+                    "Diagnostic for the battle-node `spin` investigation. On every received battle frame (OperateReceive.StartOperate) and every emitted frame (NetworkBattleSender.EmitMsg), append {event, uri, spin, count} to spin-rng.ndjson, where `count` is BattleManagerBase.stableRandomCount (the cumulative shared-RNG draw tally, read before the spin crank). Lets us compare per-turn local draw deltas against the inbound spin to settle whether prod authors spin from server-side simulation or it is wire-derivable. Run against PROD (servers live until end of June 2026) to capture real spin values. See docs/audits/battle-node-spin-rng-model-2026-06-04.md.").Value;
             SvSimConfig.DumpCardDB =
                 Config.Bind("Capture", "DumpCardDB", false,
                     "Dumps the loaded card master to cards.json in the current capture session directory").Value;
diff --git a/SVSimLoader/SvSimConfig.cs b/SVSimLoader/SvSimConfig.cs
index 7712aa8..87fb5cf 100644
--- a/SVSimLoader/SvSimConfig.cs
+++ b/SVSimLoader/SvSimConfig.cs
@@ -7,6 +7,7 @@ public static class SvSimConfig
     public static bool DumpCardDB { get; set; }
     public static bool EnableTrafficCapture { get; set; }
     public static bool EnableBattleCapture { get; set; }
+    public static bool EnableSpinProbe { get; set; }
     public static bool DumpUserData { get; set; }
     public static bool SweepGachaExchange { get; set; }
     public static float GachaExchangeSweepPacingSeconds { get; set; }