diff --git a/tests/vectors/union-parity/.gitattributes b/tests/vectors/union-parity/.gitattributes
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+*.msgpack binary
+*.cc binary
diff --git a/tests/vectors/union-parity/README.md b/tests/vectors/union-parity/README.md
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+# union-parity vectors
+
+Written only by the C# reference, `tests/GameCult.Caching.Tests/UnionParityVectorTests.cs`, with
+`CULTCACHE_WRITE_VECTORS=1`. Every other run reads these files and compares them byte for byte; a changed
+vector is a reviewed commit. Other runtimes decode them and are held to the same outcomes.
+
+Document `tests.union_surface` (schema `tests.union_surface.v1`), a positional array of
+`[Id, Version, Root, Outcomes, Part, Crafted, Missing]`. A union value is `nil` or `[armKey, [armSlots...]]`.
+
+| 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, []]` |
+| 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 |
+
+Composition is a record, not an arm: `[Name, Body: Node]`.
+
+## Files
+
+- `union-surface.msgpack`: the payload. Value: Id `surface`, Version 3; Root is a Box `root` holding Text `t1`, a Use
+  `u1` and Image `i2`; the Use's Composition `comp` holds Box `inner` (Text `deep`, Image `img`), and its Slots are
+  Text `s1`, nil, Box `b2` (empty); Outcomes are Pending, Completed, Denied; Part is Tool `hammer`; Crafted (a
+  CraftedItem field) is Part `gear`; Missing is nil. A union inside a record inside a union, a unit arm, a list of
+  unions with a nil element, an inner union keyed apart from its outer. Written by C#.
+- `union-surface.cc`: a single-file store written by the C# store holding that one record under key `surface`. Its
+  payload equals `union-surface.msgpack`; rewriting the loaded store is byte-identical.
+- `outcomes.msgpack`: the expected outcome of every file below, as data. An array of entries
+  `[file, outcome, union, key, detail]`: `outcome` is `refused` or `read-only`; `union` is the union (or record)
+  named; `key` is the arm key or nil; `detail` is as in the tables below.
+
+## Refusals (outcome `refused`)
+
+A refusal names the union and the arm key and never echoes the payload. Each payload is `union-surface.msgpack`
+with one part replaced.
+
+| File | Replaced part | detail | Must name |
+|---|---|---|---|
+| `unknown-arm-field.msgpack` | Root is an arm with the retired key 3 | unknown-arm | Node, key 3 |
+| `unknown-arm-list.msgpack` | an Outcomes element has key 7 | unknown-arm | Outcome, key 7 |
+| `unknown-arm-nested.msgpack` | key 9 three Boxes deep in Children | unknown-arm | Node, key 9 |
+| `inner-union-outer-key.msgpack` | the Crafted field carries key 2, declared only by Item | unknown-arm | CraftedItem, key 2 |
+| `arm-arity.msgpack` | Root is written `[1]`, an arm with no slots array | arity | a refusal of the arm as malformed (C#: MessagePack's "Invalid Union data") |
+
+## Extra trailing slot (outcome `read-only`)
+
+Each payload is `union-surface.msgpack` with one slot appended past the reader's last declared slot of one
+record (ruling `cultlib-gaps:ruling:extra-trailing-slots`: the record is read, read-only at any depth, every write
+of it refused). A reader decodes the value of `union-surface.msgpack` from each; `detail` says where the slot is.
+
+| File | detail | Record with the extra slot |
+|---|---|---|
+| `extra-slot-record.msgpack` | record | the document (slot 7) |
+| `extra-slot-nested-record.msgpack` | nested-record | Composition `comp` |
+| `extra-slot-arm.msgpack` | arm | the Root Box arm |
+| `extra-slot-list-element.msgpack` | list-element | Text `s1`, an element of Use.Slots |
+
+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.
diff --git a/tests/vectors/union-parity/arm-arity.msgpack b/tests/vectors/union-parity/arm-arity.msgpack
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diff --git a/tests/vectors/union-parity/union-surface.cc b/tests/vectors/union-parity/union-surface.cc
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diff --git a/tests/vectors/union-parity/unknown-arm-field.msgpack b/tests/vectors/union-parity/unknown-arm-field.msgpack
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