diff --git a/v2rayN/ServiceLib.Tests/CoreConfig/Singbox/CoreConfigSingboxServiceTests.cs b/v2rayN/ServiceLib.Tests/CoreConfig/Singbox/CoreConfigSingboxServiceTests.cs
index 10b89551..96b4c600 100644
--- a/v2rayN/ServiceLib.Tests/CoreConfig/Singbox/CoreConfigSingboxServiceTests.cs
+++ b/v2rayN/ServiceLib.Tests/CoreConfig/Singbox/CoreConfigSingboxServiceTests.cs
@@ -592,4 +592,38 @@ public class CoreConfigSingboxServiceTests
         proxy.realm.stun_servers.Should().Contain("turn.cloudflare.com:3478");
         proxy.server.Should().BeNull();
     }
+
+    [Fact]
+    public void GenerateClientConfigContent_TunSystemStackWithIpv6_ShouldUsePrefixWithPeerAddress()
+    {
+        // Regression test for #9820: sing-box fails with "need one more IPv6 address in
+        // first prefix for system stack" when the TUN inbound uses a /128 IPv6 prefix.
+        var config = CoreConfigTestFactory.CreateConfig(ECoreType.sing_box);
+        config.TunModeItem.EnableTun = true;
+        config.TunModeItem.Stack = "system";
+        config.TunModeItem.EnableIPv6Address = true;
+        CoreConfigTestFactory.BindAppManagerConfig(config);
+
+        var node = CoreConfigTestFactory.CreateVmessNode(ECoreType.sing_box);
+        var context = CoreConfigTestFactory.CreateContext(config, node, ECoreType.sing_box) with
+        {
+            IsTunEnabled = true,
+        };
+
+        var result = new CoreConfigSingboxService(context).GenerateClientConfigContent();
+
+        result.Success.Should().BeTrue($"ret msg: {result.Msg}");
+        var cfg = JsonUtils.Deserialize<SingboxConfig>(result.Data!.ToString())!;
+        var tun = cfg.inbounds.First(i => i.type == "tun");
+
+        tun.address.Should().NotBeNullOrEmpty();
+        foreach (var address in tun.address!)
+        {
+            var prefixLength = int.Parse(address[(address.LastIndexOf('/') + 1)..]);
+            var isIpv6 = address.Contains(':');
+            prefixLength.Should().BeLessThanOrEqualTo(isIpv6 ? 126 : 30,
+                $"'{address}' must leave room for the peer address the system stack derives");
+        }
+    }
+
 }
diff --git a/v2rayN/ServiceLib.Tests/Manager/CoreManagerTests.cs b/v2rayN/ServiceLib.Tests/Manager/CoreManagerTests.cs
new file mode 100644
index 00000000..d4135758
--- /dev/null
+++ b/v2rayN/ServiceLib.Tests/Manager/CoreManagerTests.cs
@@ -0,0 +1,44 @@
+using AwesomeAssertions;
+using ServiceLib.Enums;
+using ServiceLib.Manager;
+using Xunit;
+
+namespace ServiceLib.Tests.Manager;
+
+public class CoreManagerTests
+{
+    [Theory]
+    [InlineData(ECoreType.sing_box)]
+    [InlineData(ECoreType.mihomo)]
+    [InlineData(ECoreType.Xray)]
+    public void ShouldRunAsSudo_TunLaunchOnNonWindows_RequiresElevation(ECoreType coreType)
+    {
+        CoreManager.ShouldRunAsSudo(isTunLaunch: true, coreType, isNonWindows: true).Should().BeTrue();
+    }
+
+    [Fact]
+    public void ShouldRunAsSudo_NonTunLaunch_ShouldNotElevate()
+    {
+        // Regression guard for the macOS TUN failure: the elevation decision must follow
+        // the context snapshot that generated the config. A launch whose snapshot has TUN
+        // disabled must never elevate, and a launch whose snapshot has TUN enabled must
+        // elevate regardless of later changes to the live config.
+        CoreManager.ShouldRunAsSudo(isTunLaunch: false, ECoreType.sing_box, isNonWindows: true).Should().BeFalse();
+        CoreManager.ShouldRunAsSudo(isTunLaunch: false, ECoreType.Xray, isNonWindows: true).Should().BeFalse();
+    }
+
+    [Fact]
+    public void ShouldRunAsSudo_OnWindows_ShouldNotElevate()
+    {
+        CoreManager.ShouldRunAsSudo(isTunLaunch: true, ECoreType.sing_box, isNonWindows: false).Should().BeFalse();
+    }
+
+    [Theory]
+    [InlineData(ECoreType.v2fly)]
+    [InlineData(ECoreType.hysteria)]
+    [InlineData(null)]
+    public void ShouldRunAsSudo_UnsupportedCoreType_ShouldNotElevate(ECoreType? coreType)
+    {
+        CoreManager.ShouldRunAsSudo(isTunLaunch: true, coreType, isNonWindows: true).Should().BeFalse();
+    }
+}
diff --git a/v2rayN/ServiceLib/Manager/CoreManager.cs b/v2rayN/ServiceLib/Manager/CoreManager.cs
index 7610673c..04ac4f3c 100644
--- a/v2rayN/ServiceLib/Manager/CoreManager.cs
+++ b/v2rayN/ServiceLib/Manager/CoreManager.cs
@@ -183,7 +183,7 @@ public class CoreManager
         var coreInfo = CoreInfoManager.Instance.GetCoreInfo(coreType);
 
         var displayLog = node.ConfigType != EConfigType.Custom || node.DisplayLog;
-        var proc = await RunProcess(coreInfo, Global.CoreConfigFileName, displayLog, true);
+        var proc = await RunProcess(coreInfo, Global.CoreConfigFileName, displayLog, true, context.IsTunEnabled);
         if (proc is null)
         {
             return;
@@ -201,7 +201,7 @@ public class CoreManager
             if (result.Success)
             {
                 var coreInfo = CoreInfoManager.Instance.GetCoreInfo(preCoreType);
-                var proc = await RunProcess(coreInfo, Global.CorePreConfigFileName, true, true);
+                var proc = await RunProcess(coreInfo, Global.CorePreConfigFileName, true, true, preContext.IsTunEnabled);
                 if (proc is null)
                 {
                     return;
@@ -289,7 +289,20 @@ public class CoreManager
 
     #region Process
 
-    private async Task<ProcessService?> RunProcess(CoreInfo? coreInfo, string configPath, bool displayLog, bool mayNeedSudo)
+    /// <summary>
+    ///     Decides whether a core launch must be elevated on non-Windows platforms.
+    ///     The TUN state comes from the immutable <see cref="CoreConfigContext" /> snapshot that
+    ///     generated the config, never from the live mutable config: the generated config and the
+    ///     launch mode must always agree, even if TUN is toggled while a reload is in flight.
+    /// </summary>
+    public static bool ShouldRunAsSudo(bool isTunLaunch, ECoreType? coreType, bool isNonWindows)
+    {
+        return isTunLaunch
+            && coreType is ECoreType.sing_box or ECoreType.mihomo or ECoreType.Xray
+            && isNonWindows;
+    }
+
+    private async Task<ProcessService?> RunProcess(CoreInfo? coreInfo, string configPath, bool displayLog, bool mayNeedSudo, bool isTunLaunch = false)
     {
         var fileName = CoreInfoManager.Instance.GetCoreExecFile(coreInfo, out var msg);
         if (fileName.IsNullOrEmpty())
@@ -301,9 +314,7 @@ public class CoreManager
         try
         {
             if (mayNeedSudo
-                && _config.TunModeItem.EnableTun
-                && (coreInfo.CoreType is ECoreType.sing_box or ECoreType.mihomo or ECoreType.Xray)
-                && Utils.IsNonWindows())
+                && ShouldRunAsSudo(isTunLaunch, coreInfo.CoreType, Utils.IsNonWindows()))
             {
                 _linuxSudo = true;
                 await CoreAdminManager.Instance.Init(_config, _updateFunc);