diff --git a/packages/cultnet-py/tests/test_cultnet.py b/packages/cultnet-py/tests/test_cultnet.py
index 5ece34be..0c8bcbf3 100644
--- a/packages/cultnet-py/tests/test_cultnet.py
+++ b/packages/cultnet-py/tests/test_cultnet.py
@@ -13,6 +13,7 @@ from datetime import UTC, datetime
 from pathlib import Path
 from typing import Callable
 from uuid import uuid4
+import msgpack
 from cultcache_py import CultCache, define_database_entry_type
 from cultnet_py.benchmark import run_benchmark
 from cultnet_py.compare_csharp import (
@@ -1600,25 +1601,13 @@ class CultNetTests(unittest.TestCase):
             with self.subTest(field=field, bad=bad), self.assertRaisesRegex(ValueError, field):
                 CultNetDatabaseChange.from_wire({**base, field: bad})
 
-    def test_a_two_change_commit_from_the_csharp_server_parses(self) -> None:
-        # CultNetDatabaseServer.DeliverCommit (CultNetDatabaseServer.cs:791-797) tags every change of a commit with its
-        # commitId and commitCount = the number of changes delivered, so a swap of two records arrives with commitCount 2.
-        # Shaped as the C# [MessagePackObject] writes it: every key present, nil where unset.
-        swap = [
-            {
-                "schemaVersion": "cultnet.database_change_raw.v0",
-                "messageId": f"change-{index}",
-                "subscriptionId": "sub-1",
-                "changeKind": "updated",
-                "document": None,
-                "recordKey": key,
-                "schemaId": "schema-note",
-                "commitId": "9f1c0f0e4b7a4c1f9d2e",
-                "commitCount": 2,
-            }
-            for index, key in enumerate(("note:a", "note:b"))
-        ]
-        changes = [CultNetDatabaseChange.from_wire(message) for message in swap]
+    def test_a_two_change_commit_written_by_csharp_parses(self) -> None:
+        # tests/vectors/database-change-commit/two-change-commit.msgpack: the two change messages of one commit, written by the
+        # C# serializer (CultNetReplicaCommitTests), so a rename or retyping of commitId/commitCount fails here.
+        vector = Path(__file__).resolve().parents[3] / "tests" / "vectors" / "database-change-commit" / "two-change-commit.msgpack"
+        messages = list(msgpack.Unpacker(io.BytesIO(vector.read_bytes()), raw=False))
+        self.assertEqual(len(messages), 2)
+        changes = [CultNetDatabaseChange.from_wire(message) for message in messages]
         self.assertEqual([change.commit_count for change in changes], [2, 2])
         self.assertEqual({change.commit_id for change in changes}, {"9f1c0f0e4b7a4c1f9d2e"})
         self.assertEqual([change.to_wire()["commitCount"] for change in changes], [2, 2])
diff --git a/tests/GameCult.Networking.Tests/CultNetReplicaCommitTests.cs b/tests/GameCult.Networking.Tests/CultNetReplicaCommitTests.cs
index 6059d7ba..b38592ec 100644
--- a/tests/GameCult.Networking.Tests/CultNetReplicaCommitTests.cs
+++ b/tests/GameCult.Networking.Tests/CultNetReplicaCommitTests.cs
@@ -225,6 +225,39 @@ namespace GameCult.Networking.Tests
             Assert.That(run.Faults, Has.Count.EqualTo(1));
         }
 
+        [Test]
+        public void ATwoChangeCommitIsWrittenAsTheVectorOtherRuntimesDecode()
+        {
+            // tests/vectors/database-change-commit/two-change-commit.msgpack: the two change messages of one commit, as the C#
+            // serializer writes them, concatenated. Written with CULTCACHE_WRITE_VECTORS=1; every other run compares bytes.
+            var commit = Enumerable.Range(0, 2).Select(index => new CultNetDatabaseChangeRawMessage
+            {
+                MessageId = $"change-{index}",
+                SubscriptionId = "sub-1",
+                ChangeKind = "removed",
+                RecordKey = index == 0 ? "note:a" : "note:b",
+                SchemaId = "schema-note",
+                CommitId = "9f1c0f0e4b7a4c1f9d2e",
+                CommitCount = 2
+            });
+            var bytes = commit.SelectMany(message => CultNetSchemaMessageSerialization.Serialize(message)).ToArray();
+
+            var directory = TestContext.CurrentContext.TestDirectory;
+            while (directory != null && !System.IO.Directory.Exists(System.IO.Path.Combine(directory, "tests", "vectors")))
+                directory = System.IO.Path.GetDirectoryName(directory);
+            Assert.That(directory, Is.Not.Null, "tests/vectors is above the test directory");
+            var path = System.IO.Path.Combine(directory!, "tests", "vectors", "database-change-commit", "two-change-commit.msgpack");
+            if (Environment.GetEnvironmentVariable("CULTCACHE_WRITE_VECTORS") == "1")
+            {
+                System.IO.Directory.CreateDirectory(System.IO.Path.GetDirectoryName(path)!);
+                System.IO.File.WriteAllBytes(path, bytes);
+                return;
+            }
+
+            Assert.That(System.IO.File.Exists(path), Is.True, "the vector is committed");
+            Assert.That(Convert.ToHexString(bytes), Is.EqualTo(Convert.ToHexString(System.IO.File.ReadAllBytes(path))));
+        }
+
         [Test]
         public void AnOlderPeerReadsTheChangeAndIgnoresTheCommitFields()
         {
diff --git a/tests/vectors/database-change-commit/README.md b/tests/vectors/database-change-commit/README.md
new file mode 100644
index 00000000..39a78fa1
--- /dev/null
+++ b/tests/vectors/database-change-commit/README.md
@@ -0,0 +1,10 @@
+# database-change-commit vectors
+
+Written only by the C# reference, `ATwoChangeCommitIsWrittenAsTheVectorOtherRuntimesDecode` in
+`tests/GameCult.Networking.Tests/CultNetReplicaCommitTests.cs`, with `CULTCACHE_WRITE_VECTORS=1`. Every other run
+compares the bytes.
+
+- `two-change-commit.msgpack`: two `cultnet.database_change_raw.v0` messages (`removed`, `note:a` and `note:b`)
+  concatenated, each as the C# `[MessagePackObject]` writes it (every key present, nil where unset), both carrying
+  `commitId` `9f1c0f0e4b7a4c1f9d2e` and `commitCount` 2. Read it as a msgpack stream. Python walks it in
+  `packages/cultnet-py/tests/test_cultnet.py`.
diff --git a/tests/vectors/database-change-commit/two-change-commit.msgpack b/tests/vectors/database-change-commit/two-change-commit.msgpack
new file mode 100644
index 00000000..055dc4ae
--- /dev/null
+++ b/tests/vectors/database-change-commit/two-change-commit.msgpack
@@ -0,0 +1 @@
+‰­schemaVersion¾cultnet.database_change_raw.v0©messageId¨change-0®subscriptionId¥sub-1ªchangeKind§removed¨documentÀ©recordKey¦note:a¨schemaId«schema-note¨commitId´9f1c0f0e4b7a4c1f9d2e«commitCount‰­schemaVersion¾cultnet.database_change_raw.v0©messageId¨change-1®subscriptionId¥sub-1ªchangeKind§removed¨documentÀ©recordKey¦note:b¨schemaId«schema-note¨commitId´9f1c0f0e4b7a4c1f9d2e«commitCount
\ No newline at end of file