diff --git a/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs b/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs
index 46bd8de8..2a3353de 100644
--- a/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs
+++ b/src/GameCult.Caching.MessagePack/CultDocumentMessagePackSerialization.cs
@@ -83,7 +83,9 @@ public static class CultDocumentMessagePackSerialization
     {
         if (registry == null) throw new ArgumentNullException(nameof(registry));
         if (value != null) registry.GetRequired(type);
-        return MessagePackSerializer.Serialize(type, value, OptionsFor(type.Assembly));
+        var payload = MessagePackSerializer.Serialize(type, value, OptionsFor(type.Assembly));
+        CultPayloadDepth.Check(payload, type, PayloadDirection.Write);
+        return payload;
     }
 
     public static object DeserializeUntyped(Type type, byte[] payload)
@@ -95,6 +97,7 @@ public static class CultDocumentMessagePackSerialization
     {
         if (registry == null) throw new ArgumentNullException(nameof(registry));
         registry.GetRequired(type);
+        CultPayloadDepth.Check(payload, type, PayloadDirection.Read);
         return MessagePackSerializer.Deserialize(type, payload, OptionsFor(type.Assembly))
             ?? throw new InvalidOperationException($"MessagePack returned null for Cult document type {type.FullName}.");
     }
diff --git a/src/GameCult.Caching.MessagePack/CultMessagePackSecurity.cs b/src/GameCult.Caching.MessagePack/CultMessagePackSecurity.cs
index 64b237d5..01130724 100644
--- a/src/GameCult.Caching.MessagePack/CultMessagePackSecurity.cs
+++ b/src/GameCult.Caching.MessagePack/CultMessagePackSecurity.cs
@@ -7,7 +7,15 @@ namespace GameCult.Caching.MessagePack;
 // UntrustedData, plus a collision-resistant comparer for CultRecordRef<T> dictionary keys over the key string.
 public sealed class CultMessagePackSecurity : MessagePackSecurity
 {
-    public static readonly CultMessagePackSecurity Instance = new(UntrustedData);
+    /// <summary>
+    /// The deepest a document payload may nest, on read and on write, in every CultCache runtime: every MessagePack array and
+    /// map counts, read by a formatter or skipped past it, and the outermost array is depth 1. A payload of this depth is
+    /// accepted and one container deeper is refused. MessagePack's own object-graph counter is set to the same value as a
+    /// backstop; the rule itself is <c>CultPayloadDepth.Check</c>, run by the document serialization entry points.
+    /// </summary>
+    public const int MaximumPayloadDepth = 256;
+
+    public static readonly CultMessagePackSecurity Instance = new(UntrustedData.WithMaximumObjectGraphDepth(MaximumPayloadDepth));
 
     private CultMessagePackSecurity(MessagePackSecurity copyFrom) : base(copyFrom) { }
 
diff --git a/src/GameCult.Caching.MessagePack/CultPayloadDepth.cs b/src/GameCult.Caching.MessagePack/CultPayloadDepth.cs
new file mode 100644
index 00000000..258663e8
--- /dev/null
+++ b/src/GameCult.Caching.MessagePack/CultPayloadDepth.cs
@@ -0,0 +1,94 @@
+using System;
+using System.Buffers.Binary;
+using MessagePack;
+
+namespace GameCult.Caching.MessagePack;
+
+internal enum PayloadDirection { Read, Write }
+
+/// <summary>
+/// The one payload nesting count. Walks the MessagePack headers of a document payload iteratively, counting every array and
+/// map (read by a formatter or skipped past it; the outermost array is depth 1) and refusing the first container past
+/// <see cref="CultMessagePackSecurity.MaximumPayloadDepth"/>. Scalars are skipped by length, nothing is allocated per
+/// element, and the count stack is bounded by the limit. A refusal names the type, the depth reached and the limit,
+/// never payload bytes. A payload that ends early is the deserializer's to refuse.
+/// </summary>
+internal static class CultPayloadDepth
+{
+    public static void Check(ReadOnlySpan<byte> bytes, Type type, PayloadDirection direction)
+    {
+        const int limit = CultMessagePackSecurity.MaximumPayloadDepth;
+        Span<long> remaining = stackalloc long[limit];
+        var depth = 0;
+        long position = 0;
+        while (position < bytes.Length)
+        {
+            var code = bytes[(int)position];
+            long children = 0;
+            long header = 0;
+            long skip = 0;
+            var container = false;
+            switch (code)
+            {
+                case <= 0x7f or >= 0xe0 or 0xc0 or 0xc1 or 0xc2 or 0xc3: skip = 1; break;
+                case <= 0x8f: container = true; header = 1; children = 2L * (code & 0x0f); break;
+                case <= 0x9f: container = true; header = 1; children = code & 0x0f; break;
+                case <= 0xbf: skip = 1 + (code & 0x1f); break;
+                case 0xc4 or 0xd9: if (!Length(bytes, position, 1, out var l8)) return; skip = 2 + l8; break;
+                case 0xc5 or 0xda: if (!Length(bytes, position, 2, out var l16)) return; skip = 3 + l16; break;
+                case 0xc6 or 0xdb: if (!Length(bytes, position, 4, out var l32)) return; skip = 5 + l32; break;
+                case 0xc7: if (!Length(bytes, position, 1, out var e8)) return; skip = 3 + e8; break;
+                case 0xc8: if (!Length(bytes, position, 2, out var e16)) return; skip = 4 + e16; break;
+                case 0xc9: if (!Length(bytes, position, 4, out var e32)) return; skip = 6 + e32; break;
+                case 0xca or 0xce or 0xd2: skip = 5; break;
+                case 0xcb or 0xcf or 0xd3: skip = 9; break;
+                case 0xcc or 0xd0: skip = 2; break;
+                case 0xcd or 0xd1 or 0xd4: skip = 3; break;
+                case 0xd5: skip = 4; break;
+                case 0xd6: skip = 6; break;
+                case 0xd7: skip = 10; break;
+                case 0xd8: skip = 18; break;
+                case 0xdc: if (!Length(bytes, position, 2, out var a16)) return; container = true; header = 3; children = a16; break;
+                case 0xdd: if (!Length(bytes, position, 4, out var a32)) return; container = true; header = 5; children = a32; break;
+                case 0xde: if (!Length(bytes, position, 2, out var m16)) return; container = true; header = 3; children = 2 * m16; break;
+                default: if (!Length(bytes, position, 4, out var m32)) return; container = true; header = 5; children = 2 * m32; break;
+            }
+
+            if (container)
+            {
+                if (depth == limit)
+                    throw new MessagePackSerializationException(
+                        $"{type.FullName} payload nests {limit + 1} MessagePack containers deep; the limit is {limit} (refused on {(direction == PayloadDirection.Read ? "read" : "write")}).");
+                position += header;
+                if (children > 0)
+                {
+                    remaining[depth++] = children;
+                    continue;
+                }
+            }
+            else
+            {
+                position += skip;
+            }
+
+            // One element is complete; close every container it was the last element of.
+            while (depth > 0 && --remaining[depth - 1] == 0) depth--;
+            if (depth == 0) return;
+        }
+    }
+
+    // The big-endian length that follows the code byte, when the payload is long enough to hold it.
+    private static bool Length(ReadOnlySpan<byte> bytes, long position, int width, out long length)
+    {
+        length = 0;
+        if (position + 1 + width > bytes.Length) return false;
+        var field = bytes.Slice((int)position + 1, width);
+        length = width switch
+        {
+            1 => field[0],
+            2 => BinaryPrimitives.ReadUInt16BigEndian(field),
+            _ => BinaryPrimitives.ReadUInt32BigEndian(field)
+        };
+        return true;
+    }
+}
diff --git a/src/GameCult.Caching.MessagePack/CultUnionGuard.cs b/src/GameCult.Caching.MessagePack/CultUnionGuard.cs
index af7a63c2..2b60c80b 100644
--- a/src/GameCult.Caching.MessagePack/CultUnionGuard.cs
+++ b/src/GameCult.Caching.MessagePack/CultUnionGuard.cs
@@ -46,6 +46,8 @@ internal sealed class CultUnionGuard(IFormatterResolver inner) : IFormatterResol
                     throw new MessagePackSerializationException($"Union {typeof(T).FullName} has an arm key outside Int32 (slot 0 of the arm); declared keys: {declared}.");
                 if (!keys.Contains(key))
                     throw new MessagePackSerializationException($"Union {typeof(T).FullName} has no arm for key {key} (slot 0 of the arm); declared keys: {declared}.");
+                if (peek.TryReadNil())
+                    throw new MessagePackSerializationException($"Union {typeof(T).FullName} arm {key} has nil slots (slot 1 of the arm); declared keys: {declared}.");
             }
 
             return inner.Deserialize(ref reader, options);
diff --git a/src/GameCult.Caching/Contracts/cultcache-persistence-format.md b/src/GameCult.Caching/Contracts/cultcache-persistence-format.md
index 1205d904..57b5d7fb 100644
--- a/src/GameCult.Caching/Contracts/cultcache-persistence-format.md
+++ b/src/GameCult.Caching/Contracts/cultcache-persistence-format.md
@@ -217,6 +217,13 @@ Under `cultcache.store.v1` a record has exactly these four slots. A reader refus
 header allows, naming the version or the record's key and schema id. It never skips a
 slot it does not understand.
 
+A record's payload nests at most 256 MessagePack containers. Every array and map
+counts, whether a reader decodes it or skips past it, and the outermost array is
+depth 1: a payload of depth 256 is accepted and one of depth 257 is refused. Every
+runtime enforces the same bound on read and on write, and the refusal names the type,
+the depth and the limit, never payload bytes. The vectors are in
+`tests/vectors/union-parity` (`bounds-outcomes.msgpack`).
+
 ## Variant Records
 
 A variant record is a record whose document is a base record's document with some
diff --git a/tests/GameCult.Caching.Tests/PayloadDepthTests.cs b/tests/GameCult.Caching.Tests/PayloadDepthTests.cs
new file mode 100644
index 00000000..91aa9a6f
--- /dev/null
+++ b/tests/GameCult.Caching.Tests/PayloadDepthTests.cs
@@ -0,0 +1,157 @@
+#nullable enable
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using GameCult.Caching.MessagePack;
+using MessagePack;
+using NUnit.Framework;
+
+namespace GameCult.Caching.Tests
+{
+    // The payload walk at its own seam: every MessagePack header and scalar width must be skipped by its length, so that a
+    // container after it is counted at its true depth. Each case is a two-slot document [probe, chain] whose chain brings
+    // the whole payload to exactly the limit (accepted) or one past it (refused); a header or length read wrongly loses the
+    // chain and flips one of the two. The probes' filler bytes look like container headers on purpose.
+    public class PayloadDepthTests
+    {
+        private const int Limit = CultMessagePackSecurity.MaximumPayloadDepth;
+
+        private static byte[] Cat(params byte[][] parts) => parts.SelectMany(p => p).ToArray();
+
+        private static byte[] Bytes(params int[] values) => values.Select(v => (byte)v).ToArray();
+
+        private static byte[] Filler(int n) => Enumerable.Range(0, n).Select(i => (byte)(i % 2 == 0 ? 0x91 : 0xdd)).ToArray();
+
+        // `depth` containers nested in one another, the innermost empty; a fixarray header nests by one element.
+        private static byte[] Chain(int depth, byte[]? leaf = null) =>
+            Cat(Enumerable.Repeat((byte)0x91, depth - 1).ToArray(), leaf ?? Bytes(0x90));
+
+        private static byte[] Doc(byte[] probe, byte[] second) => Cat(Bytes(0x92), probe, second);
+
+        private static byte[] Len16(int n) => Bytes(n >> 8, n & 0xff);
+
+        private static byte[] Len32(int n) => Bytes(n >> 24, (n >> 16) & 0xff, (n >> 8) & 0xff, n & 0xff);
+
+        private static readonly (string Name, byte[] Bytes)[] Scalars =
+        {
+            ("positive fixint", Bytes(0x7f)), ("negative fixint", Bytes(0xe0)), ("max negative fixint", Bytes(0xff)),
+            ("nil", Bytes(0xc0)), ("false", Bytes(0xc2)), ("true", Bytes(0xc3)),
+            ("uint8", Bytes(0xcc, 0x91)), ("uint16", Bytes(0xcd, 0x91, 0x91)), ("uint32", Bytes(0xce, 0x91, 0x91, 0x91, 0x91)),
+            ("uint64", Cat(Bytes(0xcf), Filler(8))), ("int8", Bytes(0xd0, 0x91)), ("int16", Bytes(0xd1, 0x91, 0x91)),
+            ("int32", Bytes(0xd2, 0x91, 0x91, 0x91, 0x91)), ("int64", Cat(Bytes(0xd3), Filler(8))),
+            ("float32", Cat(Bytes(0xca), Filler(4))), ("float64", Cat(Bytes(0xcb), Filler(8))),
+            ("empty fixstr", Bytes(0xa0)), ("fixstr", Cat(Bytes(0xa3), Filler(3))), ("longest fixstr", Cat(Bytes(0xbf), Filler(31))),
+            ("str8", Cat(Bytes(0xd9, 5), Filler(5))), ("empty str8", Bytes(0xd9, 0)),
+            ("str16", Cat(Bytes(0xda), Len16(256), Filler(256))), ("str32", Cat(Bytes(0xdb), Len32(65536), Filler(65536))),
+            ("bin8", Cat(Bytes(0xc4, 3), Filler(3))), ("bin16", Cat(Bytes(0xc5), Len16(256), Filler(256))),
+            ("bin32", Cat(Bytes(0xc6), Len32(65536), Filler(65536))),
+            ("fixext1", Cat(Bytes(0xd4, 1), Filler(1))), ("fixext2", Cat(Bytes(0xd5, 1), Filler(2))),
+            ("fixext4", Cat(Bytes(0xd6, 1), Filler(4))), ("fixext8", Cat(Bytes(0xd7, 1), Filler(8))),
+            ("fixext16", Cat(Bytes(0xd8, 1), Filler(16))),
+            ("ext8", Cat(Bytes(0xc7, 3, 1), Filler(3))), ("ext16", Cat(Bytes(0xc8), Len16(256), Bytes(1), Filler(256))),
+            ("ext32", Cat(Bytes(0xc9), Len32(65536), Bytes(1), Filler(65536))),
+        };
+
+        // Containers holding scalar children, so that the child count (a map's is twice its pair count) is what keeps the walk in step.
+        private static readonly (string Name, byte[] Bytes)[] Containers =
+        {
+            ("fixarray", Cat(Bytes(0x9f), Enumerable.Repeat((byte)1, 15).ToArray())),
+            ("array16", Cat(Bytes(0xdc), Len16(3), Bytes(1, 1, 1))),
+            ("array32", Cat(Bytes(0xdd), Len32(2), Bytes(1, 1))),
+            ("fixmap", Cat(Bytes(0x82), Bytes(1, 1, 1, 1))),
+            ("largest fixmap", Cat(Bytes(0x8f), Enumerable.Repeat((byte)1, 30).ToArray())),
+            ("map16", Cat(Bytes(0xde), Len16(2), Bytes(1, 1, 1, 1))),
+            ("map32", Cat(Bytes(0xdf), Len32(1), Bytes(1, 1))),
+            ("empty fixarray", Bytes(0x90)), ("empty array16", Bytes(0xdc, 0, 0)), ("empty array32", Bytes(0xdd, 0, 0, 0, 0)),
+            ("empty fixmap", Bytes(0x80)), ("empty map16", Bytes(0xde, 0, 0)), ("empty map32", Bytes(0xdf, 0, 0, 0, 0)),
+        };
+
+        // A single-child carrier of each container kind, for nesting through every header.
+        private static readonly (string Name, Func<byte[], byte[]> Wrap)[] Carriers =
+        {
+            ("fixarray", inner => Cat(Bytes(0x91), inner)),
+            ("array16", inner => Cat(Bytes(0xdc, 0, 1), inner)),
+            ("array32", inner => Cat(Bytes(0xdd, 0, 0, 0, 1), inner)),
+            ("fixmap", inner => Cat(Bytes(0x81, 1), inner)),
+            ("map16", inner => Cat(Bytes(0xde, 0, 1, 1), inner)),
+            ("map32", inner => Cat(Bytes(0xdf, 0, 0, 0, 1, 1), inner)),
+        };
+
+        private static void Accepts(byte[] payload, string what) =>
+            Assert.That(() => CultPayloadDepth.Check(payload, typeof(object), PayloadDirection.Read), Throws.Nothing, what);
+
+        private static void Refuses(byte[] payload, string what) =>
+            Assert.That(() => CultPayloadDepth.Check(payload, typeof(object), PayloadDirection.Read),
+                Throws.TypeOf<MessagePackSerializationException>().With.Message.Contains($"the limit is {Limit}"), what);
+
+        [Test]
+        public void EveryScalarIsSkippedByItsLength()
+        {
+            foreach (var (name, scalar) in Scalars)
+            {
+                Accepts(Doc(scalar, Chain(Limit - 1)), $"{name} then a chain of depth {Limit}");
+                Refuses(Doc(scalar, Chain(Limit)), $"{name} then a chain of depth {Limit + 1}");
+            }
+        }
+
+        [Test]
+        public void EveryContainerKnowsItsChildCount()
+        {
+            foreach (var (name, container) in Containers)
+            {
+                Accepts(Doc(container, Chain(Limit - 1)), $"{name} then a chain of depth {Limit}");
+                Refuses(Doc(container, Chain(Limit)), $"{name} then a chain of depth {Limit + 1}");
+            }
+        }
+
+        [Test]
+        public void NestingThroughEveryHeaderIsCountedAndAnEmptyLeafCounts()
+        {
+            foreach (var (name, wrap) in Carriers)
+            {
+                Accepts(wrap(Chain(Limit - 1)), $"{name} carrying a chain, depth {Limit}");
+                Refuses(wrap(Chain(Limit)), $"{name} carrying a chain, depth {Limit + 1}");
+            }
+
+            // The deepest container is empty and the payload ends with its header: it still counts.
+            foreach (var (name, leaf) in Containers.Where(c => c.Name.StartsWith("empty")))
+            {
+                Accepts(Chain(Limit, leaf), $"{name} as the leaf at depth {Limit}");
+                Refuses(Chain(Limit + 1, leaf), $"{name} as the leaf at depth {Limit + 1}");
+            }
+        }
+
+        [Test]
+        public void ARefusalNamesTheTypeTheDepthTheLimitAndTheDirection()
+        {
+            var deep = Chain(Limit + 5, Cat(Bytes(0xa5), System.Text.Encoding.ASCII.GetBytes("CANRY")));
+            var read = Assert.Throws<MessagePackSerializationException>(() => CultPayloadDepth.Check(deep, typeof(PayloadDepthTests), PayloadDirection.Read))!;
+            var write = Assert.Throws<MessagePackSerializationException>(() => CultPayloadDepth.Check(deep, typeof(PayloadDepthTests), PayloadDirection.Write))!;
+            Assert.That(read.Message, Is.EqualTo($"{typeof(PayloadDepthTests).FullName} payload nests {Limit + 1} MessagePack containers deep; the limit is {Limit} (refused on read)."));
+            Assert.That(write.Message, Is.EqualTo($"{typeof(PayloadDepthTests).FullName} payload nests {Limit + 1} MessagePack containers deep; the limit is {Limit} (refused on write)."));
+            Assert.That(read.Message + write.Message, Does.Not.Contain("CANRY"));
+        }
+
+        [Test]
+        public void ATruncatedPayloadIsTheDeserializersToRefuse()
+        {
+            foreach (var truncated in new[] { Bytes(), Bytes(0xdc), Bytes(0xdc, 0), Bytes(0xdd, 0, 0, 0), Bytes(0xd9), Bytes(0xda, 1), Bytes(0xc6, 0, 0, 1) })
+                Accepts(truncated, $"{Convert.ToHexString(truncated)} ends inside a header");
+
+            // A header that fits exactly is read, not treated as cut short.
+            Refuses(Chain(Limit + 1, Bytes(0xdc, 0, 0)), "an array16 header ending the payload is read");
+            Accepts(Cat(Bytes(0x91, 0xd9, 200), Filler(10)), "a string cut short is not walked past its end");
+
+            // A container header cut short at the limit is not yet a container: the deserializer refuses it, not the walk.
+            foreach (var cut in new[] { Bytes(0xdc, 0), Bytes(0xdd, 0, 0, 0), Bytes(0xde, 0), Bytes(0xdf, 0, 0, 0) })
+                Accepts(Chain(Limit + 1, cut), $"{Convert.ToHexString(cut)} cut short at depth {Limit + 1}");
+        }
+
+        [Test]
+        public void OnlyTheFirstValueIsThePayload()
+        {
+            Accepts(Cat(Bytes(0x90), Chain(Limit + 10)), "bytes after the first value are not walked");
+            Accepts(Cat(Bytes(0x01), Chain(Limit + 10)), "nor after a scalar");
+        }
+    }
+}
diff --git a/tests/GameCult.Caching.Tests/UnionParityVectorTests.cs b/tests/GameCult.Caching.Tests/UnionParityVectorTests.cs
index a710a96b..2561b028 100644
--- a/tests/GameCult.Caching.Tests/UnionParityVectorTests.cs
+++ b/tests/GameCult.Caching.Tests/UnionParityVectorTests.cs
@@ -5,6 +5,7 @@ using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Security.Cryptography;
+using System.Threading;
 using GameCult.Caching;
 using GameCult.Caching.MessagePack;
 using MessagePack;
@@ -66,10 +67,17 @@ namespace GameCult.Caching.Tests
 
         // ---- the fixture: a recursive union, a unit arm, a union inside a record inside a union, a list of unions ----
 
-        // Keys 0, 1, 2 and 5: 3 and 4 are retired, so the key map is not sequential.
-        [Union(0, typeof(Box)), Union(1, typeof(Text)), Union(2, typeof(Use)), Union(5, typeof(Image))]
+        // Keys 0, 1, 2, 5 and 6: 3 and 4 are retired, so the key map is not sequential. Wrap is the +2 nesting step beside
+        // Box's +3, so any container depth from 4 up is reachable exactly.
+        [Union(0, typeof(Box)), Union(1, typeof(Text)), Union(2, typeof(Use)), Union(5, typeof(Image)), Union(6, typeof(Wrap))]
         public abstract class Node { }
 
+        [MessagePackObject]
+        public sealed class Wrap : Node
+        {
+            [Key(0)] public Node? Inner;
+        }
+
         [MessagePackObject]
         public sealed class Box : Node
         {
@@ -109,13 +117,17 @@ namespace GameCult.Caching.Tests
             [Key(1)] public Node? Body;
         }
 
-        [Union(0, typeof(Pending)), Union(1, typeof(Completed)), Union(2, typeof(Denied))]
+        // Floor and Ceiling are unit arms at the ends of the Int32 key domain.
+        [Union(0, typeof(Pending)), Union(1, typeof(Completed)), Union(2, typeof(Denied)),
+         Union(int.MinValue, typeof(Floor)), Union(int.MaxValue, typeof(Ceiling))]
         public abstract class Outcome { }
 
         [MessagePackObject] public sealed class Pending : Outcome { [Key(0)] public string Message = string.Empty; }
         [MessagePackObject] public sealed class Completed : Outcome { [Key(0)] public int SourceVersion; }
         // A unit arm: written [2, []].
         [MessagePackObject] public sealed class Denied : Outcome { }
+        [MessagePackObject] public sealed class Floor : Outcome { }
+        [MessagePackObject] public sealed class Ceiling : Outcome { }
 
         // Item names Tool and Part under keys 1 and 2; CraftedItem, the inner union both derive from, names them under 0 and 1.
         [Union(0, typeof(Gem)), Union(1, typeof(Tool)), Union(2, typeof(Part))]
@@ -141,11 +153,13 @@ namespace GameCult.Caching.Tests
             [Key(6)] public Node? Missing;
         }
 
-        private static UnionSurface Constructed() => new()
+        private static UnionSurface Constructed() => Constructed(null);
+
+        private static UnionSurface Constructed(Node? root) => new()
         {
             Id = "surface",
             Version = 3,
-            Root = new Box
+            Root = root ?? new Box
             {
                 Id = "root",
                 Children =
@@ -183,6 +197,9 @@ namespace GameCult.Caching.Tests
             Pending p => $"Pending({p.Message})",
             Completed c => $"Completed({c.SourceVersion})",
             Denied => "Denied",
+            Floor => "Floor",
+            Ceiling => "Ceiling",
+            Wrap w => $"Wrap({Describe(w.Inner)})",
             Gem g => $"Gem({g.Name})",
             Tool t => $"Tool({t.Name},{t.Uses})",
             Part p => $"Part({p.Name},{p.Count})",
@@ -234,7 +251,7 @@ namespace GameCult.Caching.Tests
         private static Emit ImageArm(string id, string uri, string alt) => ArmOf(5, Str(id), Str(uri), Nil, Str(alt));
 
         // The constructed value's payload, with a part of the structure replaceable to build a refusal or an extra-slot case.
-        private static byte[] Surface(Emit? root = null, Emit? outcomes = null, Emit? crafted = null, Extra? extra = null)
+        private static byte[] Surface(Emit? root = null, Emit? outcomes = null, Emit? crafted = null, Extra? extra = null, Emit? trailing = null)
         {
             extra ??= new Extra();
             root ??= BoxArm("root", Slots(
@@ -245,10 +262,8 @@ namespace GameCult.Caching.Tests
                 ImageArm("i2", "uri://b", "")), extra.RootArm);
             outcomes ??= Slots(ArmOf(0, Str("wait")), ArmOf(1, Num(7)), ArmOf(2));
             crafted ??= ArmOf(1, Str("gear"), Num(4));
-            return Raw(Plus(extra.Record,
-                Str("surface"), Num(3), root, outcomes,
-                ArmOf(1, Str("hammer"), Num(12)),
-                crafted, Nil));
+            var slots = new[] { Str("surface"), Num(3), root, outcomes, ArmOf(1, Str("hammer"), Num(12)), crafted, Nil };
+            return Raw(Slots(trailing != null ? slots.Append(trailing).ToArray() : extra.Record ? slots.Append(ExtraSlot).ToArray() : slots));
         }
 
         private static byte[] SerializeValue(UnionSurface value) => CultDocumentMessagePackSerialization.SerializeUntyped(value, typeof(UnionSurface), Registry);
@@ -258,8 +273,8 @@ namespace GameCult.Caching.Tests
 
         // ---- the refusal and extra-slot vectors, each with its expected outcome as data ----
 
-        // file, outcome ("refused" | "read-only"), union (logical name), arm key (-1 for none), detail.
-        private sealed record Case(string File, string Outcome, string Union, int Key, string Detail, byte[] Payload);
+        // file, outcome ("refused" | "read-only" | "accepted"), union (logical name), arm key (null for none), detail.
+        private sealed record Case(string File, string Outcome, string Union, int? Key, string Detail, byte[] Payload);
 
         private static IReadOnlyList<Case> Cases() => new[]
         {
@@ -273,9 +288,9 @@ namespace GameCult.Caching.Tests
                 Surface(crafted: ArmOf(2, Str("gear"), Num(4)))),
             new Case("arm-arity.msgpack", "refused", "Node", 1, "arity",
                 Surface(root: (ref MessagePackWriter w) => { w.WriteArrayHeader(1); w.Write(1); })),
-            new Case("extra-slot-record.msgpack", "read-only", "UnionSurface", -1, "record",
+            new Case("extra-slot-record.msgpack", "read-only", "UnionSurface", null, "record",
                 Surface(extra: new Extra(Record: true))),
-            new Case("extra-slot-nested-record.msgpack", "read-only", "Composition", -1, "nested-record",
+            new Case("extra-slot-nested-record.msgpack", "read-only", "Composition", null, "nested-record",
                 Surface(extra: new Extra(Composition: true))),
             new Case("extra-slot-arm.msgpack", "read-only", "Node", 0, "arm",
                 Surface(extra: new Extra(RootArm: true))),
@@ -291,7 +306,7 @@ namespace GameCult.Caching.Tests
             {
                 w.WriteArrayHeader(5);
                 w.Write(c.File); w.Write(c.Outcome); w.Write(c.Union);
-                if (c.Key < 0) w.WriteNil(); else w.Write(c.Key);
+                if (c.Key is { } key) w.Write(key); else w.WriteNil();
                 w.Write(c.Detail);
             }
         });
@@ -301,6 +316,7 @@ namespace GameCult.Caching.Tests
             "Node" => typeof(Node).FullName!,
             "Outcome" => typeof(Outcome).FullName!,
             "CraftedItem" => typeof(CraftedItem).FullName!,
+            "UnionSurface" => typeof(UnionSurface).FullName!,
             _ => throw new ArgumentOutOfRangeException(nameof(logical), logical, null)
         };
 
@@ -429,12 +445,288 @@ namespace GameCult.Caching.Tests
             }
         }
 
-        // Runs whatever the switch reads when the test starts: it sets the switch itself, and writes only a probe file.
+        // ---- the bounds vectors: arm keys at the ends of the Int32 domain, nil arm slots, payload nesting ----
+
+        private const int Limit = CultMessagePackSecurity.MaximumPayloadDepth;
+
+        // The outermost array is depth 1, a Wrap arm ([6, [inner]]) adds 2, a Box with no children 3, a Text leaf 2.
+        // The at-limit and past-limit payloads are the same chain of Wrap arms; the leaf is what makes the depth odd or even.
+        private const int AtLimitWraps = (Limit - 1 - 3) / 2;
+        private const int PastLimitWraps = (Limit + 1 - 1 - 2) / 2;
+
+        private static Emit RawKey(Emit key, params Emit[] slots) => (ref MessagePackWriter w) =>
+        {
+            w.WriteArrayHeader(2); key(ref w);
+            Slots(slots)(ref w);
+        };
+
+        private static Emit WithOutcome(Emit element) => Slots(ArmOf(0, Str("wait")), ArmOf(1, Num(7)), ArmOf(2), element);
+
+        private static Emit WrapChain(int wraps, Emit leaf) => wraps == 0 ? leaf : ArmOf(6, WrapChain(wraps - 1, leaf));
+
+        private static Node WrapChainValue(int wraps, Node leaf)
+        {
+            for (var i = 0; i < wraps; i++) leaf = new Wrap { Inner = leaf };
+            return leaf;
+        }
+
+        // n arrays nested inside one another around a single string.
+        private static Emit Nested(int n) => n == 1 ? Slots(Str(Canary)) : Slots(Nested(n - 1));
+
+        private static Emit AtLimitLeaf => BoxArm("leaf", Slots());
+        private static Emit PastLimitLeaf => TextArm("leaf", Canary);
+
+        private static IReadOnlyList<Case> BoundsCases() => new[]
+        {
+            new Case("arm-key-int32-min.msgpack", "accepted", "Outcome", int.MinValue, "arm-key-int32-min.msgpack",
+                Surface(outcomes: WithOutcome(ArmOf(int.MinValue)))),
+            new Case("arm-key-int32-max.msgpack", "accepted", "Outcome", int.MaxValue, "arm-key-int32-max.msgpack",
+                Surface(outcomes: WithOutcome(ArmOf(int.MaxValue)))),
+            new Case("arm-key-wide-coding.msgpack", "accepted", "Outcome", int.MaxValue, "arm-key-int32-max.msgpack",
+                Surface(outcomes: WithOutcome(RawKey((ref MessagePackWriter w) => w.WriteUInt64(int.MaxValue))))),
+            new Case("arm-key-above-int32.msgpack", "refused", "Outcome", null, "key-out-of-range",
+                Surface(outcomes: WithOutcome(RawKey((ref MessagePackWriter w) => w.Write(0x80000000u))))),
+            new Case("arm-key-below-int32.msgpack", "refused", "Outcome", null, "key-out-of-range",
+                Surface(outcomes: WithOutcome(RawKey((ref MessagePackWriter w) => w.Write(-2147483649L))))),
+            new Case("arm-key-float32.msgpack", "refused", "Outcome", null, "key-not-integer",
+                Surface(outcomes: WithOutcome(RawKey((ref MessagePackWriter w) => w.Write(1.0f), Num(7))))),
+            new Case("arm-key-float64-negzero.msgpack", "refused", "Outcome", null, "key-not-integer",
+                Surface(outcomes: WithOutcome(RawKey((ref MessagePackWriter w) => w.Write(-0.0), Str("wait"))))),
+            new Case("nil-arm-slots.msgpack", "refused", "Outcome", 1, "nil-slots",
+                Surface(outcomes: WithOutcome((ref MessagePackWriter w) => { w.WriteArrayHeader(2); w.Write(1); w.WriteNil(); }))),
+            new Case("depth-at-limit.msgpack", "accepted", "UnionSurface", null, "depth-at-limit.msgpack",
+                Surface(root: WrapChain(AtLimitWraps, AtLimitLeaf))),
+            new Case("depth-past-limit.msgpack", "refused", "UnionSurface", null, "depth",
+                Surface(root: WrapChain(PastLimitWraps, PastLimitLeaf))),
+            new Case("depth-skipped-at-limit.msgpack", "read-only", "UnionSurface", null, "depth",
+                Surface(trailing: Nested(Limit - 1))),
+            new Case("depth-skipped-past-limit.msgpack", "refused", "UnionSurface", null, "depth",
+                Surface(trailing: Nested(Limit))),
+        };
+
+        private static byte[] BoundsOutcomesFile() => Raw((ref MessagePackWriter w) =>
+        {
+            var cases = BoundsCases();
+            w.WriteArrayHeader(cases.Count + 1);
+            w.WriteArrayHeader(2); w.Write("limit"); w.Write(Limit);
+            foreach (var c in cases)
+            {
+                w.WriteArrayHeader(5);
+                w.Write(c.File); w.Write(c.Outcome); w.Write(c.Union);
+                if (c.Key is { } key) w.Write(key); else w.WriteNil();
+                w.Write(c.Detail);
+            }
+        });
+
+        // The container depth of a payload, counted with the reader's typed reads and an explicit stack: not the rule's code.
+        private static int DepthOf(byte[] payload)
+        {
+            var reader = new MessagePackReader(payload);
+            var open = new Stack<long>();
+            var deepest = 0;
+            while (true)
+            {
+                switch (reader.NextMessagePackType)
+                {
+                    case MessagePackType.Array:
+                    case MessagePackType.Map:
+                        var isArray = reader.NextMessagePackType == MessagePackType.Array;
+                        long children = isArray ? reader.ReadArrayHeader() : 2L * reader.ReadMapHeader();
+                        deepest = Math.Max(deepest, open.Count + 1);
+                        if (children > 0) { open.Push(children); continue; }
+                        break;
+                    case MessagePackType.Nil: reader.ReadNil(); break;
+                    case MessagePackType.Boolean: reader.ReadBoolean(); break;
+                    case MessagePackType.Integer: if (reader.NextCode == MessagePackCode.UInt64) reader.ReadUInt64(); else reader.ReadInt64(); break;
+                    case MessagePackType.Float: reader.ReadDouble(); break;
+                    case MessagePackType.String: reader.ReadString(); break;
+                    case MessagePackType.Binary: reader.ReadBytes(); break;
+                    case MessagePackType.Extension: reader.ReadExtensionFormat(); break;
+                    default: throw new InvalidOperationException(reader.NextMessagePackType.ToString());
+                }
+
+                while (open.Count > 0)
+                {
+                    var left = open.Pop() - 1;
+                    if (left > 0) { open.Push(left); break; }
+                }
+
+                if (open.Count == 0) return deepest;
+            }
+        }
+
+        private static Exception Refusal(Action action)
+        {
+            try { action(); } catch (Exception exception) { return exception; }
+            throw new AssertionException("the payload was accepted");
+        }
+
+        [Test]
+        public void BoundsOutcomesFromCommittedFiles()
+        {
+            foreach (var c in BoundsCases()) Check(c.File, c.Payload);
+            Check("bounds-outcomes.msgpack", BoundsOutcomesFile());
+
+            // From here only the committed files speak.
+            var golden = Describe(Constructed());
+            var reader = new MessagePackReader(Committed("bounds-outcomes.msgpack"));
+            var count = reader.ReadArrayHeader();
+            Assert.That(count, Is.EqualTo(BoundsCases().Count + 1));
+            Assert.That(reader.ReadArrayHeader(), Is.EqualTo(2));
+            Assert.That(reader.ReadString(), Is.EqualTo("limit"));
+            Assert.That(reader.ReadInt32(), Is.EqualTo(CultMessagePackSecurity.MaximumPayloadDepth), "the limit entry states the reference's limit");
+            for (var i = 1; i < count; i++)
+            {
+                Assert.That(reader.ReadArrayHeader(), Is.EqualTo(5));
+                var file = reader.ReadString()!;
+                var outcome = reader.ReadString()!;
+                var union = reader.ReadString()!;
+                int? key = reader.TryReadNil() ? null : reader.ReadInt32();
+                var detail = reader.ReadString()!;
+                var payload = Committed(file);
+
+                switch (outcome)
+                {
+                    case "accepted":
+                        Assert.That(Convert.ToHexString(SerializeValue(Decode(payload))), Is.EqualTo(Convert.ToHexString(Committed(detail))), $"{file} re-encodes to {detail}");
+                        break;
+                    case "read-only":
+                        Assert.That(Describe(Decode(payload)), Is.EqualTo(golden), $"{file}: the extra slot is read past");
+                        break;
+                    case "refused":
+                        var message = Chain(Refusal(() => Decode(payload)));
+                        Assert.That(message, Does.Not.Contain(Canary), $"{file}: a refusal never echoes the payload");
+                        Assert.That(message, Does.Contain(UnionName(union)), file);
+                        switch (detail)
+                        {
+                            case "key-out-of-range": Assert.That(message, Does.Contain("outside Int32").And.Not.Contain("has no arm for key"), file); break;
+                            case "key-not-integer": Assert.That(message, Does.Contain("non-integer"), file); break;
+                            case "nil-slots": Assert.That(message, Does.Contain("nil slots").And.Contain($"arm {key} "), file); break;
+                            case "depth": Assert.That(message, Does.Contain($"nests {Limit + 1} MessagePack containers deep").And.Contain($"the limit is {Limit}").And.Contain("refused on read"), file); break;
+                            default: Assert.Fail($"{file}: unknown detail {detail}"); break;
+                        }
+
+                        break;
+                    default:
+                        Assert.Fail($"{file}: unknown outcome {outcome}");
+                        break;
+                }
+            }
+        }
+
+        [Test]
+        public void ArmKeyDomainVectors()
+        {
+            var min = Committed("arm-key-int32-min.msgpack");
+            var max = Committed("arm-key-int32-max.msgpack");
+            var wide = Committed("arm-key-wide-coding.msgpack");
+            Assert.That(Convert.ToHexString(min), Does.Contain("D280000000"), "Floor is written as an int32 key");
+            Assert.That(Convert.ToHexString(max), Does.Contain("CE7FFFFFFF"), "Ceiling is written as a uint32 key");
+            Assert.That(Convert.ToHexString(wide), Does.Contain("CF000000007FFFFFFF"), "the wide coding is a uint64 key");
+            Assert.That(Decode(min).Outcomes, Has.Count.EqualTo(4));
+            Assert.That(Decode(min).Outcomes[3], Is.TypeOf<Floor>());
+            Assert.That(Decode(max).Outcomes[3], Is.TypeOf<Ceiling>());
+            Assert.That(Decode(wide).Outcomes[3], Is.TypeOf<Ceiling>(), "an unsigned 64-bit coding of an in-range key is the same key");
+            Assert.That(Convert.ToHexString(SerializeValue(Decode(wide))), Is.EqualTo(Convert.ToHexString(max)), "and re-encodes as the canonical coding");
+            Assert.That(Convert.ToHexString(SerializeValue(Decode(min))), Is.EqualTo(Convert.ToHexString(min)));
+
+            Assert.That(Chain(Refusal(() => Decode(Committed("arm-key-above-int32.msgpack")))), Does.Contain("outside Int32").And.Contain(UnionName("Outcome")));
+            Assert.That(Chain(Refusal(() => Decode(Committed("arm-key-below-int32.msgpack")))), Does.Contain("outside Int32").And.Contain(UnionName("Outcome")));
+            Assert.That(Chain(Refusal(() => Decode(Committed("arm-key-float32.msgpack")))), Does.Contain("non-integer").And.Contain(UnionName("Outcome")));
+            Assert.That(Chain(Refusal(() => Decode(Committed("arm-key-float64-negzero.msgpack")))), Does.Contain("non-integer").And.Contain(UnionName("Outcome")));
+        }
+
+        [Test]
+        public void NilArmSlotsRefused()
+        {
+            var message = Chain(Refusal(() => Decode(Committed("nil-arm-slots.msgpack"))));
+            Assert.That(message, Does.Contain(UnionName("Outcome")).And.Contain("arm 1 has nil slots"));
+        }
+
+        [Test]
+        public void DepthAtLimitAcceptedBothWays()
+        {
+            var payload = Committed("depth-at-limit.msgpack");
+            Assert.That(DepthOf(payload), Is.EqualTo(Limit), "the vector nests exactly the limit, counted independently");
+            Assert.That(Convert.ToHexString(SerializeValue(Decode(payload))), Is.EqualTo(Convert.ToHexString(payload)), "read, then written again");
+            var built = Constructed(WrapChainValue(AtLimitWraps, new Box { Id = "leaf" }));
+            Assert.That(Convert.ToHexString(SerializeValue(built)), Is.EqualTo(Convert.ToHexString(payload)), "the constructed value writes the vector");
+        }
+
+        [Test]
+        public void DepthPastLimitRefusedBothWays()
+        {
+            var payload = Committed("depth-past-limit.msgpack");
+            Assert.That(DepthOf(payload), Is.EqualTo(Limit + 1), "one container past the limit, counted independently");
+            var read = Chain(Refusal(() => Decode(payload)));
+            Assert.That(read, Does.Contain(typeof(UnionSurface).FullName).And.Contain($"the limit is {Limit}").And.Contain("refused on read").And.Not.Contain(Canary));
+
+            byte[]? written = null;
+            var write = Chain(Refusal(() => written = SerializeValue(Constructed(WrapChainValue(PastLimitWraps, new Text { Id = "leaf", Content = Canary })))));
+            Assert.That(written, Is.Null, "nothing is returned");
+            Assert.That(write, Does.Contain(typeof(UnionSurface).FullName).And.Contain($"the limit is {Limit}").And.Contain("refused on write").And.Not.Contain(Canary));
+
+            // A single-file store holding the payload fails to load through the same entry point.
+            var work = Path.Combine(Path.GetTempPath(), $"cultlib-unionparity-{Guid.NewGuid():N}");
+            Directory.CreateDirectory(work);
+            try
+            {
+                var snapshot = CultDocumentMessagePackSerialization.DeserializeSnapshot(Committed("union-surface.cc"));
+                snapshot.Records[0].Payload = payload;
+                var scratch = Path.Combine(work, "deep.cc");
+                File.WriteAllBytes(scratch, CultDocumentMessagePackSerialization.SerializeSnapshot(snapshot));
+                var load = Chain(Refusal(() => CultCacheMessagePack.Create(scratch, new CultCacheOpenOptions { Registry = Registry }).Dispose()));
+                Assert.That(load, Does.Contain("surface").And.Contain($"the limit is {Limit}").And.Not.Contain(Canary));
+            }
+            finally
+            {
+                Directory.Delete(work, recursive: true);
+            }
+        }
+
+        [Test]
+        public void SkippedContainersCount()
+        {
+            var past = Committed("depth-skipped-past-limit.msgpack");
+            var atLimit = Committed("depth-skipped-at-limit.msgpack");
+            Assert.That(DepthOf(Committed("union-surface.msgpack")), Is.LessThan(20), "what the formatters read is shallow");
+            Assert.That(DepthOf(atLimit), Is.EqualTo(Limit));
+            Assert.That(DepthOf(past), Is.EqualTo(Limit + 1));
+
+            Assert.That(Describe(Decode(atLimit)), Is.EqualTo(Describe(Constructed())), "skipped content at the limit is read past");
+            var message = Chain(Refusal(() => Decode(past)));
+            Assert.That(message, Does.Contain(typeof(UnionSurface).FullName).And.Contain($"the limit is {Limit}").And.Not.Contain(Canary));
+
+            // MessagePack alone counts only what its formatters read: it accepts the payload the walk refuses.
+            Assert.That(() => MessagePackSerializer.Deserialize(typeof(UnionSurface), past, CultDocumentMessagePackSerialization.OptionsFor(typeof(UnionSurface).Assembly)),
+                Throws.Nothing, "MessagePack's own counter never sees skipped containers");
+        }
+
+        [Test]
+        public void WalkIsIterative()
+        {
+            // 100000 nested arrays in an extra trailing slot, on a thread with a 256 KiB stack.
+            var deep = Surface(trailing: (ref MessagePackWriter w) =>
+            {
+                w.WriteRaw(Enumerable.Repeat((byte)0x91, 100_000).ToArray());
+                w.WriteArrayHeader(0);
+            });
+            Exception? thrown = null;
+            var thread = new Thread(() => { thrown = Refusal(() => Decode(deep)); }, 256 * 1024);
+            thread.Start();
+            thread.Join();
+            Assert.That(thrown, Is.Not.Null);
+            Assert.That(Chain(thrown!), Does.Contain($"the limit is {Limit}"));
+        }
+
+        // Runs whatever the switch reads when the test starts: it sets the switch itself, runs every test body that calls Check
+        // under each off value, and writes only a probe file. The fingerprint is each file's bytes and write time, so a rewrite
+        // with identical bytes shows.
         [Test, NonParallelizable]
         public void VectorsNotRewrittenByDefault()
         {
             string Fingerprint() => string.Join("|", Directory.EnumerateFiles(VectorDirectory()).OrderBy(f => f, StringComparer.Ordinal)
-                .Select(f => Path.GetFileName(f) + "=" + Convert.ToHexString(SHA256.HashData(File.ReadAllBytes(f)))));
+                .Select(f => Path.GetFileName(f) + "=" + Convert.ToHexString(SHA256.HashData(File.ReadAllBytes(f))) + "@" + File.GetLastWriteTimeUtc(f).Ticks));
 
             const string probe = "gate-probe.msgpack";
             var path = VectorPath(probe);
@@ -452,6 +744,9 @@ namespace GameCult.Caching.Tests
                     Assert.Throws<AssertionException>(() => Check(probe, new byte[] { 0x90 }), $"bytes that differ from the vector fail the run, switch {off ?? "unset"}");
                     Assert.That(File.ReadAllBytes(path), Is.EqualTo(new byte[] { 0x91 }), $"switch {off ?? "unset"} rewrites nothing");
                     File.Delete(path);
+
+                    foreach (var body in new Action[] { UnionSurfaceVectorMatches, UnionRefusalVectorsRefused, BoundsOutcomesFromCommittedFiles })
+                        body();
                 }
 
                 Environment.SetEnvironmentVariable(WriteVariable, "1");
diff --git a/tests/vectors/union-parity/README.md b/tests/vectors/union-parity/README.md
index 277c15ad..29101484 100644
--- a/tests/vectors/union-parity/README.md
+++ b/tests/vectors/union-parity/README.md
@@ -9,8 +9,8 @@ Document `tests.union_surface` (schema `tests.union_surface.v1`), a positional a
 
 | Union | Arm keys | Arm slots |
 |---|---|---|
-| Node | 0 Box, 1 Text, 2 Use, 5 Image (3 and 4 retired) | Box `[Id, Children: Node list]`; Text `[Id, Content]`; Use `[Id, Composition, Slots: Node list]`; Image `[Id, Uri, nil (retired 2), Alt]` |
-| Outcome | 0 Pending, 1 Completed, 2 Denied | Pending `[Message]`; Completed `[SourceVersion]`; Denied is a unit arm, `[2, []]` |
+| Node | 0 Box, 1 Text, 2 Use, 5 Image, 6 Wrap (3 and 4 retired) | Box `[Id, Children: Node list]`; Text `[Id, Content]`; Use `[Id, Composition, Slots: Node list]`; Image `[Id, Uri, nil (retired 2), Alt]`; Wrap `[Inner: Node]` |
+| Outcome | 0 Pending, 1 Completed, 2 Denied, -2147483648 Floor, 2147483647 Ceiling | Pending `[Message]`; Completed `[SourceVersion]`; Denied, Floor and Ceiling are unit arms, `[2, []]` |
 | Item | 0 Gem, 1 Tool, 2 Part | Gem `[Name]`; Tool `[Name, Uses]`; Part `[Name, Count]` |
 | CraftedItem | 0 Tool, 1 Part | the same arms as Item under its own keys; a key only Item declares is unknown to it |
 
@@ -58,3 +58,38 @@ of it refused). A reader decodes the value of `union-surface.msgpack` from each;
 
 The C# reference reads these today and its slot-tolerance cut is what makes them read-only; this cut asserts only
 that the value is read.
+
+## Bounds (`bounds-outcomes.msgpack`)
+
+A second outcomes file, so runtimes that read only `outcomes.msgpack` stay green. Its first entry is
+`["limit", 256]`, the payload nesting limit; every other entry is `[file, outcome, union, key, detail]` with
+`outcome` `accepted`, `refused` or `read-only`. For `accepted`, `detail` names the file the value's re-encoding
+must equal. Each payload is `union-surface.msgpack` with one part replaced.
+
+Arm keys are Int32 whatever their MessagePack width; a float is never a key.
+
+| File | Replaced part | outcome, detail | Must |
+|---|---|---|---|
+| `arm-key-int32-min.msgpack` | a fourth Outcomes element Floor, key `d2 80000000` | accepted, itself | re-encode byte-identical |
+| `arm-key-int32-max.msgpack` | a fourth element Ceiling, key `ce 7fffffff` | accepted, itself | re-encode byte-identical |
+| `arm-key-wide-coding.msgpack` | Ceiling's key as uint64 `cf 000000007fffffff` | accepted, `arm-key-int32-max.msgpack` | re-encode to the canonical coding |
+| `arm-key-above-int32.msgpack` | a unit arm with key `ce 80000000` | refused, key-out-of-range | name Outcome, say outside Int32 |
+| `arm-key-below-int32.msgpack` | a unit arm with key `d3 ffffffff7fffffff` | refused, key-out-of-range | name Outcome, say outside Int32 |
+| `arm-key-float32.msgpack` | Completed with key float32 `ca 3f800000` | refused, key-not-integer | name Outcome, say non-integer |
+| `arm-key-float64-negzero.msgpack` | Pending with key float64 `cb 8000000000000000` | refused, key-not-integer | name Outcome, say non-integer |
+| `nil-arm-slots.msgpack` | a fourth element `[1, nil]` | refused, nil-slots | name Outcome and key 1; never read as a null element |
+
+Payload nesting: every array and map counts, read or skipped, and the outermost array is depth 1. Depth 256 is
+accepted and 257 refused, on read and on write, naming the type, the depth reached and the limit, never the payload.
+`depth-at-limit` and `depth-past-limit` are chains of Wrap arms ending in a leaf (a Box with no children, or a Text
+whose content is a canary string); `depth-skipped-*` is the base document with one extra trailing slot of plain nested
+arrays around a canary string, which no formatter reads.
+
+| File | Depth | outcome, detail | Must |
+|---|---|---|---|
+| `depth-at-limit.msgpack` | 256 | accepted, itself | decode, re-encode byte-identical; the constructed value writes it |
+| `depth-past-limit.msgpack` | 257 | refused, depth | refuse on read; refuse to write the constructed value and return nothing |
+| `depth-skipped-at-limit.msgpack` | 256 | read-only, depth | read the value of `union-surface.msgpack` |
+| `depth-skipped-past-limit.msgpack` | 257 | refused, depth | refuse on read though the typed depth is under 10 |
+
+A walk of the payload must not recurse: 100000 nested arrays in an extra slot are refused on a 256 KiB stack.
diff --git a/tests/vectors/union-parity/arm-key-above-int32.msgpack b/tests/vectors/union-parity/arm-key-above-int32.msgpack
new file mode 100644
index 00000000..94402175
Binary files /dev/null and b/tests/vectors/union-parity/arm-key-above-int32.msgpack differ
diff --git a/tests/vectors/union-parity/arm-key-below-int32.msgpack b/tests/vectors/union-parity/arm-key-below-int32.msgpack
new file mode 100644
index 00000000..28faad4a
Binary files /dev/null and b/tests/vectors/union-parity/arm-key-below-int32.msgpack differ
diff --git a/tests/vectors/union-parity/arm-key-float32.msgpack b/tests/vectors/union-parity/arm-key-float32.msgpack
new file mode 100644
index 00000000..e3d848d0
Binary files /dev/null and b/tests/vectors/union-parity/arm-key-float32.msgpack differ
diff --git a/tests/vectors/union-parity/arm-key-float64-negzero.msgpack b/tests/vectors/union-parity/arm-key-float64-negzero.msgpack
new file mode 100644
index 00000000..7abdbb56
Binary files /dev/null and b/tests/vectors/union-parity/arm-key-float64-negzero.msgpack differ
diff --git a/tests/vectors/union-parity/arm-key-int32-max.msgpack b/tests/vectors/union-parity/arm-key-int32-max.msgpack
new file mode 100644
index 00000000..022d4839
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