Fix IList<T> misclassified as interface on mutation error types

src/HotChocolate/Core/src/Types.Mutations/MutationConventionTypeInterceptor.cs

@@ -713,13 +713,22 @@ private void EnsureTypeReferenceRegistered(
             return;
         }
 
+        // Unwrap arrays and collection-interface wrappers (IList<T>, ICollection<T>,
+        // IEnumerable<T>, IReadOnlyList<T>, ...). The wrapper itself is not a named
+        // schema type, and CLR collection interfaces carry TypeAttributes.Interface
+        // which would otherwise misclassify them as a GraphQL interface here.
+        while (runtimeTypeReference.Type is { IsArrayOrList: true, ElementType: { } element })
+        {
+            runtimeTypeReference = runtimeTypeReference.WithType(element);
+        }
+
         if (_typeRegistry.TryGetTypeRef(runtimeTypeReference, out var existingTypeReference)
             && _typeRegistry.IsRegistered(existingTypeReference))
         {
             return;
         }
 
-        if (!_context.TryInferSchemaType(typeReference, out var schemaTypeReferences))
+        if (!_context.TryInferSchemaType(runtimeTypeReference, out var schemaTypeReferences))
         {
             return;
         }

src/HotChocolate/Core/test/Types.Mutations.Tests/AnnotationBasedMutations.cs

@@ -747,6 +747,24 @@ public async Task Union_Result_2_Error_Name_Collision()
         exception.Message.MatchSnapshot();
     }
 
+    [Fact]
+    public async Task Error_Type_With_IList_Property_Should_Be_Treated_As_List()
+    {
+        // arrange & act
+        var schema =
+            await new ServiceCollection()
+                .AddGraphQL()
+                .AddMutationType<MutationWithIListError>()
+                .AddMutationConventions()
+                .ModifyOptions(o => o.StrictValidation = false)
+                .BuildSchemaAsync();
+
+        // assert
+        var errorType = schema.Types.GetType<ObjectType>("ErrorWithCodes");
+        Assert.Equal("[Int!]!", errorType.Fields["codes"].Type.ToString());
+        Assert.False(schema.Types.TryGetType<InterfaceType>("IListOfInt32", out _));
+    }
+
     [Fact]
     public async Task Union_Result_3()
     {
@@ -1746,6 +1764,17 @@ public FooError(string message)
         public string Message { get; }
     }
 
+    public class MutationWithIListError
+    {
+        public FieldResult<string, ErrorWithCodes> DoSomething(int[] codes)
+            => new ErrorWithCodes(codes);
+    }
+
+    public record ErrorWithCodes(IList<int> Codes)
+    {
+        public string Message => $"Codes {string.Join(", ", Codes)} not found.";
+    }
+
     public class MutationWithPayloadOverride
     {
         [Error<CustomException>]