diff --git a/CHANGELOG.md b/CHANGELOG.md
index b05e20b..0541f06 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -55,6 +55,7 @@ To have a consistent experience across all packages, some public interfaces have
 - **request.validation:** Return no problems when validating a `null` instance of a reference type instead of dereferencing it (`NullReferenceException`)
 - **request.validation:** Make `Severity` meaningful — `Validation.IsValid` now only fails on `Error` problems; `Warning`/`Info` no longer invalidate. Added `Validation.IsValidFor(Severity)` to set the failing-severity threshold
 - **request.dispatcher:** Throw `InvalidOperationException` listing candidates when multiple handlers match a request instead of silently resolving via `.First()`. Also throws when no handler is registered
+- **request.dispatcher:** Resolve handlers/behaviors in registration order across closed and open generics instead of the fixed closed-before-open reflection order
 
 ### Removed
 
diff --git a/src/request.dispatcher.tests/ScalarDispatcherTests.cs b/src/request.dispatcher.tests/ScalarDispatcherTests.cs
index 8c5ffb1..f274da9 100644
--- a/src/request.dispatcher.tests/ScalarDispatcherTests.cs
+++ b/src/request.dispatcher.tests/ScalarDispatcherTests.cs
@@ -259,6 +259,29 @@ internal sealed class ScalarDispatcherTests
 		}
 	}
 
+	[Test]
+	public async Task I_can_see_candidates_ordered_by_registration_not_closed_first()
+	{
+		var sc = new ServiceCollection();
+		sc.AddRequestDispatcher(builder => builder
+			.Add(typeof(InterfaceConstrainedScalarHandler<>))
+			.Add(typeof(InterfaceInheritedScalarHandler)));
+		var provider = sc.BuildServiceProvider();
+		var dispatcher = provider.GetRequiredService<IRequestDispatcher>();
+
+		var request = new InterfaceInheritedScalarRequest { Name = "Constrained" };
+
+		// The open generic handler is registered before the closed handler, so candidates must be
+		// listed in registration order (open first), not the old fixed closed-before-open order.
+		var ex = await Assert.That(async () => await dispatcher.DispatchAsync(request)).Throws<InvalidOperationException>();
+
+		var message = ex?.Message ?? string.Empty;
+		var openIndex = message.IndexOf("InterfaceConstrainedScalarHandler", StringComparison.Ordinal);
+		var closedIndex = message.IndexOf("InterfaceInheritedScalarHandler", StringComparison.Ordinal);
+
+		await Assert.That(openIndex >= 0 && closedIndex >= 0 && openIndex < closedIndex).IsTrue();
+	}
+
 	[Test]
 	public async Task I_can_see_it_throw_if_dispatcher_options_are_modified_after_build()
 	{
diff --git a/src/request.dispatcher/RequestDispatcherOptions.cs b/src/request.dispatcher/RequestDispatcherOptions.cs
index d18def9..6560f46 100644
--- a/src/request.dispatcher/RequestDispatcherOptions.cs
+++ b/src/request.dispatcher/RequestDispatcherOptions.cs
@@ -46,11 +46,16 @@ internal sealed class RequestDispatcherOptions
 
 		protected readonly Dictionary<Type, List<Type>> _closedTypeInfo = [];
 		protected readonly List<Type> _openTypeInfo = [];
+		protected readonly Dictionary<Type, int> _order = [];
 
 		protected TypeIndex(IEnumerable<Type> collection, Func<Type, bool> predicate)
 		{
+			var index = 0;
+
 			foreach (var type in collection)
 			{
+				_order[type] = index++;
+
 				if (type.IsGenericTypeDefinition)
 				{
 					if (type.GetInterfaces().Any(predicate))
@@ -102,11 +107,14 @@ internal sealed class RequestDispatcherOptions
 	{
 		protected override IReadOnlyList<Type> IsAssignableTo(Type @interface)
 		{
-			var result = new List<Type>();
+			var result = new List<(int Order, Type Type)>();
 
 			if (_closedTypeInfo.TryGetValue(@interface, out var list))
 			{
-				result.AddRange(list);
+				foreach (var type in list)
+				{
+					result.Add((_order[type], type));
+				}
 			}
 
 			var requestType = @interface.GetGenericArguments()[0];
@@ -121,7 +129,7 @@ internal sealed class RequestDispatcherOptions
 					var impl = type.MakeGenericType(requestType);
 					if (impl.IsAssignableTo(@interface))
 					{
-						result.Add(impl);
+						result.Add((_order[type], impl));
 					}
 				}
 				catch (ArgumentException)
@@ -137,7 +145,7 @@ internal sealed class RequestDispatcherOptions
 						var impl = type.MakeGenericType(requestType.GetGenericArguments());
 						if (impl.IsAssignableTo(@interface))
 						{
-							result.Add(impl);
+							result.Add((_order[type], impl));
 						}
 					}
 				}
@@ -146,7 +154,7 @@ internal sealed class RequestDispatcherOptions
 				}
 			}
 
-			return result;
+			return result.OrderBy(x => x.Order).Select(x => x.Type).ToList();
 		}
 	}
 
@@ -162,11 +170,14 @@ internal sealed class RequestDispatcherOptions
 	{
 		protected override IReadOnlyList<Type> IsAssignableTo(Type @interface)
 		{
-			var result = new List<Type>();
+			var result = new List<(int Order, Type Type)>();
 
 			if (_closedTypeInfo.TryGetValue(@interface, out var list))
 			{
-				result.AddRange(list);
+				foreach (var type in list)
+				{
+					result.Add((_order[type], type));
+				}
 			}
 
 			var requestType = @interface.GetGenericArguments()[0];
@@ -180,7 +191,7 @@ internal sealed class RequestDispatcherOptions
 					var impl = behaviour.MakeGenericType(requestType, responseType);
 					if (impl.IsAssignableTo(@interface))
 					{
-						result.Add(impl);
+						result.Add((_order[behaviour], impl));
 					}
 				}
 				catch (ArgumentException)
@@ -193,7 +204,7 @@ internal sealed class RequestDispatcherOptions
 					var impl = behaviour.MakeGenericType(requestType.GetGenericArguments());
 					if (impl.IsAssignableTo(@interface))
 					{
-						result.Add(impl);
+						result.Add((_order[behaviour], impl));
 					}
 				}
 				catch (ArgumentException)
@@ -201,7 +212,7 @@ internal sealed class RequestDispatcherOptions
 				}
 			}
 
-			return result;
+			return result.OrderBy(x => x.Order).Select(x => x.Type).ToList();
 		}
 	}
 }