diff --git a/src/ZB.MOM.WW.MxGateway.Tests/Contracts/ClientBehaviorFixtureTests.cs b/src/ZB.MOM.WW.MxGateway.Tests/Contracts/ClientBehaviorFixtureTests.cs
index 282f327..b8a6855 100644
--- a/src/ZB.MOM.WW.MxGateway.Tests/Contracts/ClientBehaviorFixtureTests.cs
+++ b/src/ZB.MOM.WW.MxGateway.Tests/Contracts/ClientBehaviorFixtureTests.cs
@@ -72,18 +72,33 @@ public sealed class ClientBehaviorFixtureTests
 
         foreach (JsonElement fixture in fixtures)
         {
+            string fixtureId = GetFixtureId(fixture);
             MxCommandReply reply = ParseFixture<MxCommandReply>(
                 fixture,
                 MxCommandReply.Parser);
 
+            // Universal invariants: every command-reply fixture parses to a concrete
+            // command kind and a concrete protocol status, regardless of what MXAccess
+            // reply detail (if any) it carries.
             Assert.NotEqual(MxCommandKind.Unspecified, reply.Kind);
             Assert.NotEqual(ProtocolStatusCode.Unspecified, reply.ProtocolStatus.Code);
-            Assert.True(reply.HasHresult, $"Fixture '{GetFixtureId(fixture)}' must carry an HRESULT.");
+
+            // The malformed-reply and credential-redaction fixtures
+            // (command-reply.authenticate-user.*) deliberately omit hresult, statuses,
+            // and/or return_value to exercise the "absent detail" contract paths — see
+            // docs/ClientBehaviorFixtures.md. The strict MXAccess-reply-detail
+            // invariants below apply only to fixtures that carry that detail.
+            if (fixtureId.StartsWith("command-reply.authenticate-user.", StringComparison.Ordinal))
+            {
+                continue;
+            }
+
+            Assert.True(reply.HasHresult, $"Fixture '{fixtureId}' must carry an HRESULT.");
             Assert.NotEmpty(reply.Statuses);
             Assert.NotEqual(MxDataType.Unspecified, reply.ReturnValue.DataType);
             Assert.True(
                 reply.ReturnValue.KindCase != MxValue.KindOneofCase.None || reply.ReturnValue.IsNull,
-                $"Fixture '{GetFixtureId(fixture)}' must carry a typed value, raw value, or explicit null.");
+                $"Fixture '{fixtureId}' must carry a typed value, raw value, or explicit null.");
         }
 
         MxCommandReply failedWrite = ParseFixture<MxCommandReply>(