diff --git a/CHANGELOG.md b/CHANGELOG.md
index c30ddcb..41143c6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- **DP086 assemble-contract copy**: message/description now name sync product `T` and async `Task<T>` / `ValueTask<T>` as valid `[BuilderAssemble]` returns, and call out void, bare `Task`/`ValueTask`, duplicate assemble, and multiple `CancellationToken` as illegal ([#325](https://github.com/Skymly/DesignPatterns/issues/325); Relates to [#324](https://github.com/Skymly/DesignPatterns/issues/324)).
 - **F3 Work Graph MVP complete**: ROADMAP Top-3 marked `[x]` after Samples [DesignPatterns.Samples#25](https://github.com/Skymly/DesignPatterns.Samples/issues/25) / [PR#28](https://github.com/Skymly/DesignPatterns.Samples/pull/28); Design Doc references the request-prep sample; Spec [#308](https://github.com/Skymly/DesignPatterns/issues/308) closed (Phase 2+ Dop/trace/DI/unregistered Analyzer deferred).
 - **Peer-presence Unregistered\* base**: extract `UnregisteredPayloadPeerAnalyzerBase` for DP024 / DP044 / DP072; `UnregisteredHandlerAnalyzer`, `UnregisteredEventHandlerAnalyzer`, and `UnregisteredCommandHandlerAnalyzer` become thin adapters supplying attribute-peer and declared-peer extraction. Strategy/Factory contract-peer base unchanged. Diagnostic behaviour frozen (including no `IStreamCommandHandler` peer-presence).
 - **DI registration map consumer seam**: `CaptiveDependencyAnalyzer` and `SingletonLifecycleAnalyzer` obtain the map only via `DiRegistrationMap.Build(Compilation)` at compilation analysis time; incremental `DiRegistrationMapBuilder` wiring is a private implementation detail of `Build`. Diagnostic behaviour for DP062/066/068–071 is unchanged. `Build` skips generated syntax trees (`.g.cs` / auto-generated headers) so visibility matches the prior `RegisterSyntaxNodeAction` + `GeneratedCodeAnalysisFlags.None` path.
diff --git a/DesignPatterns.Diagnostics/DesignPatternsDiagnosticDescriptors.cs b/DesignPatterns.Diagnostics/DesignPatternsDiagnosticDescriptors.cs
index 0c87942..f2ef286 100644
--- a/DesignPatterns.Diagnostics/DesignPatternsDiagnosticDescriptors.cs
+++ b/DesignPatterns.Diagnostics/DesignPatternsDiagnosticDescriptors.cs
@@ -736,8 +736,8 @@ public static class DesignPatternsDiagnosticDescriptors
     public static DiagnosticDescriptor GenerateBuilderAssembleContractMismatch { get; } = Create(
         DiagnosticIds.GenerateBuilderAssembleContractMismatch,
         "GenerateBuilder assemble method contract is invalid",
-        "Method '{0}' on holder '{1}' is marked [BuilderAssemble] but does not satisfy the assemble contract (exactly one assemble method returning a non-void product type). Fix the signature or remove duplicate [BuilderAssemble] attributes.",
-        "The assemble method is the sole product factory for the generated builder and must return the product type.",
+        "Method '{0}' on holder '{1}' is marked [BuilderAssemble] but does not satisfy the assemble contract (exactly one assemble method returning sync product T, or async Task<T> / ValueTask<T>; not void, bare Task/ValueTask, or multiple CancellationToken parameters). Fix the signature or remove duplicate [BuilderAssemble] attributes.",
+        "The assemble method is the sole product factory for the generated builder. Return sync product type T, or async Task<T>/ValueTask<T>. Reject void, bare Task/ValueTask, duplicate [BuilderAssemble], and more than one CancellationToken.",
         DiagnosticSeverity.Error,
         GeneratorCategory);
 
diff --git a/tests/DesignPatterns.SourceGenerators.Tests/Generators/GenerateBuilderGeneratorTests.ReportsDp086WhenAssembleReturnsVoid.verified.txt b/tests/DesignPatterns.SourceGenerators.Tests/Generators/GenerateBuilderGeneratorTests.ReportsDp086WhenAssembleReturnsVoid.verified.txt
index ad29f01..9c07385 100644
--- a/tests/DesignPatterns.SourceGenerators.Tests/Generators/GenerateBuilderGeneratorTests.ReportsDp086WhenAssembleReturnsVoid.verified.txt
+++ b/tests/DesignPatterns.SourceGenerators.Tests/Generators/GenerateBuilderGeneratorTests.ReportsDp086WhenAssembleReturnsVoid.verified.txt
@@ -1,6 +1,6 @@
 [
   {
     Id: DP086,
-    Message: Method 'Assemble' on holder 'VoidAssembleSchema' is marked [BuilderAssemble] but does not satisfy the assemble contract (exactly one assemble method returning a non-void product type). Fix the signature or remove duplicate [BuilderAssemble] attributes.
+    Message: Method 'Assemble' on holder 'VoidAssembleSchema' is marked [BuilderAssemble] but does not satisfy the assemble contract (exactly one assemble method returning sync product T, or async Task<T> / ValueTask<T>; not void, bare Task/ValueTask, or multiple CancellationToken parameters). Fix the signature or remove duplicate [BuilderAssemble] attributes.
   }
 ]
\ No newline at end of file
diff --git a/tests/DesignPatterns.SourceGenerators.Tests/StepBuilderDiagnosticDescriptorsTests.cs b/tests/DesignPatterns.SourceGenerators.Tests/StepBuilderDiagnosticDescriptorsTests.cs
index 87f699f..41a2ab2 100644
--- a/tests/DesignPatterns.SourceGenerators.Tests/StepBuilderDiagnosticDescriptorsTests.cs
+++ b/tests/DesignPatterns.SourceGenerators.Tests/StepBuilderDiagnosticDescriptorsTests.cs
@@ -145,13 +145,20 @@ public void Invalid_holder_descriptor_is_generator_error_with_actionable_message
     public void Assemble_contract_mismatch_descriptor_is_generator_error_with_actionable_message()
     {
         var descriptor = DesignPatternsDiagnosticDescriptors.GenerateBuilderAssembleContractMismatch;
+        var message = descriptor.MessageFormat.ToString();
+        var description = descriptor.Description.ToString();
 
         Assert.Equal(DiagnosticIds.GenerateBuilderAssembleContractMismatch, descriptor.Id);
         Assert.Equal(DiagnosticSeverity.Error, descriptor.DefaultSeverity);
         Assert.Equal("DesignPatterns.Generators", descriptor.Category);
         Assert.Equal(DiagnosticHelpLinks.For(descriptor.Id), descriptor.HelpLinkUri);
-        Assert.False(string.IsNullOrWhiteSpace(descriptor.Description.ToString()));
-        Assert.Contains("[BuilderAssemble]", descriptor.MessageFormat.ToString(), StringComparison.Ordinal);
-        Assert.Contains("{0}", descriptor.MessageFormat.ToString(), StringComparison.Ordinal);
+        Assert.False(string.IsNullOrWhiteSpace(description));
+        Assert.Contains("[BuilderAssemble]", message, StringComparison.Ordinal);
+        Assert.Contains("{0}", message, StringComparison.Ordinal);
+        Assert.Contains("{1}", message, StringComparison.Ordinal);
+        Assert.Contains("Task<T>", message, StringComparison.Ordinal);
+        Assert.Contains("ValueTask<T>", message, StringComparison.Ordinal);
+        Assert.Contains("Task<T>", description, StringComparison.Ordinal);
+        Assert.Contains("ValueTask<T>", description, StringComparison.Ordinal);
     }
 }