diff --git a/src/SIL.Harmony.Tests/ChangeConverterTests.cs b/src/SIL.Harmony.Tests/ChangeConverterTests.cs
index 039252b..9f413e8 100644
--- a/src/SIL.Harmony.Tests/ChangeConverterTests.cs
+++ b/src/SIL.Harmony.Tests/ChangeConverterTests.cs
@@ -1,6 +1,7 @@
 using System.Text.Json;
 using Microsoft.Extensions.DependencyInjection;
 using SIL.Harmony.Changes;
+using SIL.Harmony.Config;
 using SIL.Harmony.Sample;
 using SIL.Harmony.Sample.Changes;
 
@@ -8,9 +9,10 @@ namespace SIL.Harmony.Tests;
 
 public class ChangeConverterTests
 {
-    private static JsonSerializerOptions SampleOptions() =>
+    private static JsonSerializerOptions SampleOptions(UnknownChangeHandling handling = UnknownChangeHandling.Fallback) =>
         new ServiceCollection()
             .AddCrdtDataSample(":memory:")
+            .Configure<HarmonyConfig>(c => c.UnknownChangeHandling = handling)
             .BuildServiceProvider()
             .GetRequiredService<JsonSerializerOptions>();
 
@@ -45,6 +47,20 @@ public void Unknown_type_deserializes_to_OpaqueChange()
         opaque.RawJson.GetProperty("Priority").GetInt32().Should().Be(7);
         opaque.SupportsNewEntity().Should().BeFalse();
         opaque.SupportsApplyChange().Should().BeFalse();
+        opaque.EntityType.Should().BeNull();
+    }
+
+    [Fact]
+    public void Unknown_type_throws_when_fallback_disabled()
+    {
+        var options = SampleOptions(UnknownChangeHandling.Throw);
+        var json = """
+            {"$type":"SetWordPriorityChange","EntityId":"aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee","Priority":7}
+            """;
+
+        var act = () => JsonSerializer.Deserialize<IChange>(json, options);
+
+        act.Should().Throw<JsonException>().WithMessage("*SetWordPriorityChange*");
     }
 
     [Fact]
diff --git a/src/SIL.Harmony.Tests/DataModelPerformanceTests.cs b/src/SIL.Harmony.Tests/DataModelPerformanceTests.cs
index 5eca65c..8a76489 100644
--- a/src/SIL.Harmony.Tests/DataModelPerformanceTests.cs
+++ b/src/SIL.Harmony.Tests/DataModelPerformanceTests.cs
@@ -17,12 +17,13 @@ namespace SIL.Harmony.Tests;
 [Trait("Category", "Performance")]
 public class DataModelPerformanceTests(ITestOutputHelper output)
 {
-    [Fact]
-    public void AddingChangePerformance()
-    {
+    [Fact(
 #if DEBUG
-        Assert.Fail("This test is disabled in debug builds, not reliable");
+        Skip = "This test is disabled in debug builds, not reliable"
 #endif
+        )]
+    public void AddingChangePerformance()
+    {
         var summary =
             BenchmarkRunner.Run<DataModelPerformanceBenchmarks>(
                 ManualConfig.CreateEmpty()
diff --git a/src/SIL.Harmony/Changes/Change.cs b/src/SIL.Harmony/Changes/Change.cs
index c816403..68ecc5b 100644
--- a/src/SIL.Harmony/Changes/Change.cs
+++ b/src/SIL.Harmony/Changes/Change.cs
@@ -15,7 +15,7 @@ public interface IChange
     Guid EntityId { get; set; }
 
     [JsonIgnore]
-    Type EntityType { get; }
+    Type? EntityType { get; }
 
     ValueTask ApplyChange(IObjectBase entity, IChangeContext context);
     ValueTask<IObjectBase> NewEntity(Commit commit, IChangeContext context);
diff --git a/src/SIL.Harmony/Changes/OpaqueChange.cs b/src/SIL.Harmony/Changes/OpaqueChange.cs
index 2a33314..c09a44b 100644
--- a/src/SIL.Harmony/Changes/OpaqueChange.cs
+++ b/src/SIL.Harmony/Changes/OpaqueChange.cs
@@ -13,8 +13,7 @@ public sealed class OpaqueChange : IChange
 
     public Guid EntityId { get; set; }
 
-    public Type EntityType =>
-        throw new NotSupportedException($"Opaque change '{TypeName}' has no known entity type.");
+    public Type? EntityType => null;
 
     public ValueTask ApplyChange(IObjectBase entity, IChangeContext context) => default;
 
diff --git a/src/SIL.Harmony/Changes/PeekThenConcreteChangeConverter.cs b/src/SIL.Harmony/Changes/PeekThenConcreteChangeConverter.cs
index b129b07..0c13bed 100644
--- a/src/SIL.Harmony/Changes/PeekThenConcreteChangeConverter.cs
+++ b/src/SIL.Harmony/Changes/PeekThenConcreteChangeConverter.cs
@@ -2,6 +2,7 @@
 using System.Text.Json;
 using System.Text.Json.Serialization;
 using System.Text.Json.Serialization.Metadata;
+using SIL.Harmony.Config;
 
 namespace SIL.Harmony.Changes;
 
@@ -9,17 +10,20 @@ namespace SIL.Harmony.Changes;
 /// Owns <see cref="IChange"/> discrimination. Requires <c>$type</c> as the first JSON property
 /// (matching synthetic write order from <see cref="Config.HarmonyConfig"/>).
 /// Known discriminators deserialize via cached concrete <see cref="JsonTypeInfo"/>;
-/// unknown → <see cref="OpaqueChange"/> preserving the raw payload.
+/// an unknown discriminator either throws or falls back to <see cref="OpaqueChange"/>
+/// (preserving the raw payload), depending on <see cref="UnknownChangeHandling"/>.
 /// </summary>
 internal sealed class PeekThenConcreteChangeConverter : JsonConverter<IChange>
 {
     private readonly KnownType[] _known;
     private readonly byte[] _discriminatorPropertyUtf8;
+    private readonly UnknownChangeHandling _unknownChangeHandling;
 
-    public PeekThenConcreteChangeConverter(IReadOnlyDictionary<string, Type> known)
+    public PeekThenConcreteChangeConverter(IReadOnlyDictionary<string, Type> known, UnknownChangeHandling unknownChangeHandling)
     {
         _discriminatorPropertyUtf8 = Encoding.UTF8.GetBytes(CrdtConstants.ChangeDiscriminatorProperty);
         _known = known.Select(kv => new KnownType(Encoding.UTF8.GetBytes(kv.Key), kv.Value)).ToArray();
+        _unknownChangeHandling = unknownChangeHandling;
     }
 
     public override IChange Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
@@ -42,6 +46,10 @@ public override IChange Read(ref Utf8JsonReader reader, Type typeToConvert, Json
 
         if (!TryFindKnown(ref reader, out var knownIndex, out var unknownTypeName))
         {
+            if (_unknownChangeHandling != UnknownChangeHandling.Fallback)
+                throw new JsonException(
+                    $"Unknown IChange \"{CrdtConstants.ChangeDiscriminatorProperty}\" discriminator '{unknownTypeName}'");
+
             reader = checkpoint;
             return ReadOpaque(ref reader, unknownTypeName!);
         }
diff --git a/src/SIL.Harmony/Config/HarmonyConfig.cs b/src/SIL.Harmony/Config/HarmonyConfig.cs
index 65556aa..1e34850 100644
--- a/src/SIL.Harmony/Config/HarmonyConfig.cs
+++ b/src/SIL.Harmony/Config/HarmonyConfig.cs
@@ -21,6 +21,12 @@ public class HarmonyConfig
     /// after adding any commit validate the commit history, not great for performance but good for testing.
     /// </summary>
     public bool AlwaysValidateCommits { get; set; } = true;
+    /// <summary>
+    /// Controls how an unknown <see cref="IChange"/> <c>$type</c> is handled during deserialization.
+    /// Defaults to <see cref="UnknownChangeHandling.Throw"/>; set to <see cref="UnknownChangeHandling.Fallback"/>
+    /// to preserve unknown changes as <see cref="OpaqueChange"/>.
+    /// </summary>
+    public UnknownChangeHandling UnknownChangeHandling { get; set; } = UnknownChangeHandling.Throw;
     public ChangeTypeListBuilder ChangeTypeListBuilder { get; } = new();
     public IReadOnlyList<RegisteredChangeType> ChangeTypes => ChangeTypeListBuilder.Types;
     public ObjectTypeListBuilder ObjectTypeListBuilder { get; } = new();
@@ -38,17 +44,25 @@ public HarmonyConfig()
 
     private JsonSerializerOptions CreateJsonSerializerOptions()
     {
-        var changeDiscriminators = _lazyChangeDiscriminatorMaps.Value;
 
-        var options = new JsonSerializerOptions(JsonSerializerDefaults.General)
-        {
-            TypeInfoResolver = MakeJsonTypeResolver()
-        };
-        options.Converters.Add(new PeekThenConcreteChangeConverter(changeDiscriminators.ByDiscriminator));
-        _jsonOptionsBuilder.ApplyTo(options);
+        var options = new JsonSerializerOptions(JsonSerializerDefaults.General);
+        ConfigureExternalJsonOptions(options);
         return options;
     }
 
+    /// <summary>
+    /// Configures <see cref="JsonSerializerOptions"/> for Harmony's serialization of <see cref="IChange"/> and <see cref="IObject"/> types.
+    /// Also applies any callbacks registered via <see cref="ConfigureJsonOptions(Action{JsonSerializerOptions})"/>.
+    /// </summary>
+    public void ConfigureExternalJsonOptions(JsonSerializerOptions options)
+    {
+        var changeDiscriminators = _lazyChangeDiscriminatorMaps.Value;
+        options.TypeInfoResolver = options.TypeInfoResolver?.WithAddedModifier(MakeJsonTypeModifier())
+            ?? MakeJsonTypeResolver();
+        options.Converters.Add(new PeekThenConcreteChangeConverter(changeDiscriminators.ByDiscriminator, UnknownChangeHandling));
+        _jsonOptionsBuilder.ApplyTo(options);
+    }
+
     /// <summary>
     /// Registers a callback to customize <see cref="JsonSerializerOptions"/> before they are frozen.
     /// Callbacks run after Harmony's type resolver and change converter are configured.
diff --git a/src/SIL.Harmony/Config/UnknownChangeHandling.cs b/src/SIL.Harmony/Config/UnknownChangeHandling.cs
new file mode 100644
index 0000000..925349c
--- /dev/null
+++ b/src/SIL.Harmony/Config/UnknownChangeHandling.cs
@@ -0,0 +1,13 @@
+namespace SIL.Harmony.Config;
+
+/// <summary>
+/// Controls how <c>PeekThenConcreteChangeConverter</c> handles an <see cref="Changes.IChange"/>
+/// whose <c>$type</c> is not registered on this client.
+/// </summary>
+public enum UnknownChangeHandling
+{
+    /// <summary>Throw a <see cref="System.Text.Json.JsonException"/> on an unknown $type (default).</summary>
+    Throw,
+    /// <summary>Fall back to <see cref="Changes.OpaqueChange"/>, preserving the raw JSON so it round-trips.</summary>
+    Fallback,
+}