diff --git a/tests/GameCult.Caching.Tests/ElementIdWalkTests.cs b/tests/GameCult.Caching.Tests/ElementIdWalkTests.cs
new file mode 100644
index 00000000..cd6b3be3
--- /dev/null
+++ b/tests/GameCult.Caching.Tests/ElementIdWalkTests.cs
@@ -0,0 +1,144 @@
+#nullable enable
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Runtime.Serialization;
+using GameCult.Caching;
+using MessagePack;
+using NUnit.Framework;
+
+namespace GameCult.Caching.Tests
+{
+    // The id walk's own contracts, one mutant-sized fact at a time: the order of the id path, a union that names itself, a list of
+    // values never counting as an id, and the attribute predicates that decide whether a type is walked or written whole.
+    public class ElementIdWalkTests
+    {
+        // The codec asks MessagePack about the id path in order and refuses at the first type it cannot allow, so the order is what
+        // an error names first: the document, then the collection types from the outermost in, then the element and its arms.
+        [Test]
+        public void TheIdPathListsCollectionsFromTheOutermostIn()
+        {
+            Assert.That(CultElementIds.Inspect(typeof(MsMapMap)).IdPath, Is.EqualTo(new[]
+            {
+                typeof(MsMapMap), typeof(Dictionary<string, Dictionary<string, List<ElementIdEngineTests.EIded>>>),
+                typeof(Dictionary<string, List<ElementIdEngineTests.EIded>>), typeof(List<ElementIdEngineTests.EIded>), typeof(ElementIdEngineTests.EIded)
+            }), "maps nested in maps");
+            Assert.That(CultElementIds.Inspect(typeof(MsListDeep)).IdPath, Is.EqualTo(new[]
+            {
+                typeof(MsListDeep), typeof(List<List<List<ElementIdEngineTests.EIded>>>), typeof(List<List<ElementIdEngineTests.EIded>>),
+                typeof(List<ElementIdEngineTests.EIded>), typeof(ElementIdEngineTests.EIded)
+            }), "lists nested in lists");
+        }
+
+        // ConcreteTypes stops at a type it has already expanded, so a union that lists its own base is finished, not a stack overflow.
+        [Test]
+        public void AUnionThatNamesItselfIsWalkedOnce()
+        {
+            var survey = CultElementIds.Inspect(typeof(MsSelfHolder));
+            Assert.That(survey.Problems, Is.Empty);
+            Assert.That(survey.IdPath, Is.EquivalentTo(new[] { typeof(MsSelfHolder), typeof(List<MsSelfBase>), typeof(MsSelfBase), typeof(MsSelfArm) }));
+        }
+
+        // Value lists hold no ids however they are nested, and a root that is itself a value list holds none either.
+        [Test]
+        public void ListsOfValuesHoldNoIds()
+        {
+            var values = new MsValues { Numbers = { 1 }, Tags = { "a" }, Grid = { new List<int> { 1 } }, Map = { ["k"] = new List<int> { 2 } } };
+            Assert.That(CultElementIds.Holds(typeof(MsValues), values), Is.False);
+            Assert.That(CultElementIds.Holds(typeof(List<int>), new List<int> { 1, 2 }), Is.False);
+            Assert.That(CultElementIds.Holds(typeof(List<List<int>>), new List<List<int>> { new() { 1 } }), Is.False);
+            Assert.That(CultElementIds.Holds(typeof(List<string>), new List<string> { "a" }), Is.False);
+        }
+
+        // A nullable struct is walked as the struct it wraps; a nullable value is not walked at all.
+        [Test]
+        public void ANullableKeyedStructWalksAsItsOwner()
+        {
+            Assert.That(CultElementIds.WalksAsOwner(typeof(MsKeyedStruct)), Is.True);
+            Assert.That(CultElementIds.WalksAsOwner(typeof(MsKeyedStruct?)), Is.True);
+            Assert.That(CultElementIds.WalksAsOwner(typeof(int?)), Is.False);
+            Assert.That(CultElementIds.WalksAsOwner(typeof(int)), Is.False);
+        }
+
+        // [Key] is read through an override, so an element whose id property overrides a keyed base member is still an id with a key.
+        [Test]
+        public void AKeyOnTheBaseOfAnOverrideCountsForTheOverride()
+        {
+            Assert.That(CultElementIds.Inspect(typeof(MsOverrideHolder)).Problems, Is.Empty);
+            Assert.That(CultElementIds.Inspect(typeof(MsOverrideHolder)).IdPath, Does.Contain(typeof(MsOverrideEl)));
+        }
+
+        // A type is walked by key when it is [MessagePackObject] or [Union], read through its bases for the former; it is one whole
+        // value when it declares its own formatter, is [DataContract] without being [MessagePackObject], or is keyed by name.
+        [Test]
+        public void KeyedTypesWalkAndWholeTypesDoNot()
+        {
+            Assert.That(CultElementIds.WalksAsOwner(typeof(MsMpBase)), Is.True, "[MessagePackObject]");
+            Assert.That(CultElementIds.WalksAsOwner(typeof(MsUBase)), Is.True, "[Union]");
+            Assert.That(CultElementIds.WalksAsOwner(typeof(MsMpDerived)), Is.True, "[MessagePackObject] inherited");
+            Assert.That(CultElementIds.WalksAsOwner(typeof(MsPlain)), Is.False, "[Key] members without [MessagePackObject] or [Union]");
+            Assert.That(CultElementIds.WalksAsOwner(typeof(MsUArm)), Is.True, "an arm that is itself [MessagePackObject]");
+
+            Assert.That(CultElementIds.WalksAsOwner(typeof(MsDc)), Is.False, "[DataContract] alone");
+            Assert.That(CultElementIds.WalksAsOwner(typeof(MsDcMp)), Is.True, "[DataContract] beside [MessagePackObject]");
+            Assert.That(CultElementIds.WalksAsOwner(typeof(MsDcOverMpBase)), Is.True, "[DataContract] over an inherited [MessagePackObject]");
+
+            Assert.That(CultElementIds.WalksAsOwner(typeof(MsPropName)), Is.False, "KeyAsPropertyName");
+            Assert.That(CultElementIds.WalksAsOwner(typeof(MsPropNameDerived)), Is.False, "KeyAsPropertyName inherited");
+            Assert.That(CultElementIds.WalksAsOwner(typeof(MsStringKey)), Is.False, "a string [Key]");
+
+            Assert.That(CultElementIds.WalksAsOwner(typeof(MsFormattedBase)), Is.False, "[MessagePackFormatter]");
+            Assert.That(CultElementIds.WalksAsOwner(typeof(MsFormattedDerived)), Is.False, "[MessagePackFormatter] inherited");
+        }
+
+        [MessagePackObject] public class MsMapMap { [Key(0)] public Dictionary<string, Dictionary<string, List<ElementIdEngineTests.EIded>>> M { get; set; } = new(); }
+        [MessagePackObject] public class MsListDeep { [Key(0)] public List<List<List<ElementIdEngineTests.EIded>>> G { get; set; } = new(); }
+
+        [Union(0, typeof(MsSelfBase)), Union(1, typeof(MsSelfArm))] public abstract class MsSelfBase { }
+        [MessagePackObject] public class MsSelfArm : MsSelfBase { [Key(0)] [CultElementId] public string Id { get; set; } = ""; }
+        [MessagePackObject] public class MsSelfHolder { [Key(0)] public List<MsSelfBase> Items { get; set; } = new(); }
+
+        [MessagePackObject]
+        public class MsValues
+        {
+            [Key(0)] public List<int> Numbers { get; set; } = new();
+            [Key(1)] public List<string> Tags { get; set; } = new();
+            [Key(2)] public List<List<int>> Grid { get; set; } = new();
+            [Key(3)] public Dictionary<string, List<int>> Map { get; set; } = new();
+        }
+
+        [MessagePackObject] public struct MsKeyedStruct { [Key(0)] public int X; }
+
+        [MessagePackObject]
+        public class MsOverrideBase
+        {
+            [Key(0)] [CultElementId] public virtual string Id { get; set; } = "";
+        }
+
+        [MessagePackObject]
+        public class MsOverrideEl : MsOverrideBase
+        {
+            public override string Id { get; set; } = "";
+        }
+
+        [MessagePackObject] public class MsOverrideHolder { [Key(0)] public List<MsOverrideEl> Items { get; set; } = new(); }
+
+        [MessagePackObject] public class MsMpBase { [Key(0)] public int A { get; set; } }
+        public class MsMpDerived : MsMpBase { }
+        public class MsPlain { [Key(0)] public int A { get; set; } }
+        [Union(0, typeof(MsUArm))] public abstract class MsUBase { }
+        [MessagePackObject] public class MsUArm : MsUBase { [Key(0)] public int A { get; set; } }
+
+        [DataContract] public class MsDc { [DataMember] public int A { get; set; } }
+        [DataContract] [MessagePackObject] public class MsDcMp { [Key(0)] public int A { get; set; } }
+        [DataContract] public class MsDcOverMpBase : MsMpBase { }
+
+        [MessagePackObject(keyAsPropertyName: true)] public class MsPropName { public int A { get; set; } }
+        public class MsPropNameDerived : MsPropName { }
+        [MessagePackObject] public class MsStringKey { [Key("a")] public int A { get; set; } }
+
+        public sealed class MsNoFormatter { }
+        [MessagePackFormatter(typeof(MsNoFormatter))] [MessagePackObject] public class MsFormattedBase { [Key(0)] public int A { get; set; } }
+        [MessagePackObject] public class MsFormattedDerived : MsFormattedBase { [Key(1)] public int B { get; set; } }
+    }
+}