diff --git a/src/GameCult.Networking/CultNetTransport.cs b/src/GameCult.Networking/CultNetTransport.cs
index a6102de3..ac2c2952 100644
--- a/src/GameCult.Networking/CultNetTransport.cs
+++ b/src/GameCult.Networking/CultNetTransport.cs
@@ -1490,15 +1490,30 @@ namespace GameCult.Networking
 
         internal const string UnsendablePacketReason = "packet could not be sent";
 
+        /// <summary>
+        /// The fixed name of a failed send's cause: EMSGSIZE, EINVAL or EAFNOSUPPORT for a failure
+        /// that can never pass (<see cref="IsPermanentSendError"/>), UNKNOWN for anything else, as
+        /// every runtime's goodbye names it. Never the error's own message, which is the platform's
+        /// wording.
+        /// </summary>
+        internal static string SendErrorCode(SocketException error) => error.SocketErrorCode switch
+        {
+            SocketError.MessageSize => "EMSGSIZE",
+            SocketError.InvalidArgument => "EINVAL",
+            SocketError.AddressFamilyNotSupported => "EAFNOSUPPORT",
+            _ => "UNKNOWN",
+        };
+
         /// <summary>
         /// Ends a session that owes its peer a packet that can never be sent as built. Resending it
         /// would fail forever, and dropping it would leave a reliable sequence the peer waits on for
-        /// good, so the session cannot be kept. The goodbye's reason names the error.
+        /// good, so the session cannot be kept. The goodbye's reason names the failure by its fixed
+        /// name (<see cref="SendErrorCode"/>).
         /// </summary>
         internal CultNetRudpPacket EndUnsendable(SocketException error)
         {
             ResetPeerState();
-            return CreateDisconnect(Encoding.UTF8.GetBytes(UnsendablePacketReason + ": " + error.Message));
+            return CreateDisconnect(Encoding.UTF8.GetBytes(UnsendablePacketReason + ": " + SendErrorCode(error)));
         }
 
         internal uint NextSequence => _nextSequence;
diff --git a/tests/GameCult.Networking.Tests/CultNetRudpSendFailureTests.cs b/tests/GameCult.Networking.Tests/CultNetRudpSendFailureTests.cs
index 81d255f7..3decfa46 100644
--- a/tests/GameCult.Networking.Tests/CultNetRudpSendFailureTests.cs
+++ b/tests/GameCult.Networking.Tests/CultNetRudpSendFailureTests.cs
@@ -137,6 +137,18 @@ namespace GameCult.Networking.Tests
         // caller-directed send throws it and queues nothing; inside a poll it ends that peer's
         // session and never the poll.
 
+        [Test]
+        public void An_unsendable_goodbye_names_the_failure_by_a_fixed_code_never_its_message()
+        {
+            var session = new CultNetRudpSession(new CultNetRudpSessionOptions { ConnectionId = 1 });
+            Assert.That(
+                Encoding.UTF8.GetString(session.EndUnsendable(new SocketException((int)SocketError.MessageSize)).Payload!),
+                Is.EqualTo("packet could not be sent: EMSGSIZE"));
+            Assert.That(CultNetRudpSession.SendErrorCode(new SocketException((int)SocketError.InvalidArgument)), Is.EqualTo("EINVAL"));
+            Assert.That(CultNetRudpSession.SendErrorCode(new SocketException((int)SocketError.AddressFamilyNotSupported)), Is.EqualTo("EAFNOSUPPORT"));
+            Assert.That(CultNetRudpSession.SendErrorCode(new SocketException((int)SocketError.ConnectionRefused)), Is.EqualTo("UNKNOWN"));
+        }
+
         private static bool IsUnsendableReason(byte[]? reason)
         {
             var text = Encoding.UTF8.GetString(reason ?? Array.Empty<byte>());