diff --git a/Server/src/main/java/prompto/graphql/GraphQLServlet.java b/Server/src/main/java/prompto/graphql/GraphQLServlet.java
index e0838bb1..8dd458f9 100644
--- a/Server/src/main/java/prompto/graphql/GraphQLServlet.java
+++ b/Server/src/main/java/prompto/graphql/GraphQLServlet.java
@@ -21,6 +21,8 @@
 import graphql.ExecutionResult;
 import graphql.GraphQL;
 import graphql.schema.GraphQLSchema;
+import graphql.schema.idl.SchemaPrinter;
+import graphql.schema.idl.SchemaPrinter.Options;
 import prompto.code.ICodeStore;
 import prompto.declaration.IDeclaration;
 import prompto.server.CleverServlet;
@@ -92,16 +94,21 @@ private ExecutionResult executeRequest(GraphQLRequest request) {
 		return gql.execute(input);
 	}
 	
-	GraphQLSchema getGraphQLSchema() {
+	GraphQLSchema getSchema() {
 		if(schema==null) synchronized(this) {
 			schema = new GraphQLSchemaBuilder().build();
 		}
 		return schema;
 	}
 
+	public String getSDL() {
+		var options = Options.defaultOptions().includeDirectives(false);
+		return new SchemaPrinter(options).print(getSchema());
+	}
+
 	GraphQL getGraphQL() {
 		if(graphQL==null) synchronized(this) {
-			graphQL = GraphQL.newGraphQL(getGraphQLSchema()).build();
+			graphQL = GraphQL.newGraphQL(getSchema()).build();
 		}
 		return graphQL;
 	}
diff --git a/Server/src/main/java/prompto/graphql/GraphQLType.java b/Server/src/main/java/prompto/graphql/GraphQLType.java
index f581653a..85ec8392 100644
--- a/Server/src/main/java/prompto/graphql/GraphQLType.java
+++ b/Server/src/main/java/prompto/graphql/GraphQLType.java
@@ -148,12 +148,15 @@ private GraphQLOutputType _buildOutputCategoryType(CategoryDeclaration decl, Gra
 			private GraphQLUnionType _buildOutputUnionCategoryType(CategoryDeclaration decl, List<CategoryType> derived, Builder registry) {
 				var builder = GraphQLUnionType.newUnionType()
 						.name(decl.getName() + "Union");
-				builder.possibleType(_buildOutputSimpleCategoryType(decl, registry));
-				derived.forEach(type -> {
+				if(!decl.isAbstract())
+					builder = builder.possibleType(_buildOutputSimpleCategoryType(decl, registry));
+				for(var type : derived) {
 					var outputType = build(type, registry).outputType();
 					if(outputType instanceof GraphQLObjectType)
-						builder.possibleType((GraphQLObjectType)outputType);
-				});
+						builder = builder.possibleType((GraphQLObjectType)outputType);
+					else if(outputType instanceof GraphQLUnionType)
+						builder = builder.possibleTypes(((GraphQLUnionType)outputType).getTypes().toArray(new GraphQLObjectType[0]));
+				};
 				GraphQLUnionType unionType = builder.build();
 				registry.typeResolver(unionType, env -> {
 					Object o = env.getObject();
diff --git a/Server/src/test/java/prompto/graphql/TestGraphQLServlet.java b/Server/src/test/java/prompto/graphql/TestGraphQLServlet.java
index 4270b6c9..0ff34ba0 100644
--- a/Server/src/test/java/prompto/graphql/TestGraphQLServlet.java
+++ b/Server/src/test/java/prompto/graphql/TestGraphQLServlet.java
@@ -52,6 +52,8 @@ private <T> T linkResourceAndRunQuery(String resourceName, Dialect dialect, Stri
 		Standalone.synchronizeSchema(ICodeStore.getInstance(), DataStore.getInstance());
 		GraphQLServlet.reset();
 		GraphQLServlet servlet = GraphQLServlet.instance.get();
+		@SuppressWarnings("unused")
+		var sdl = servlet.getSDL();
 		GraphQL graphQL = servlet.getGraphQL();
 		ExecutionResult result = graphQL.execute(query);
 		assertNotNull(result);
@@ -101,20 +103,20 @@ public void returnsInstances() throws Exception {
 
 	@Test
 	public void returnsAbstractChild() throws Exception {
-		Map<String, Object> s = linkResourceAndRunQuery("instance", Dialect.O, "{ returnsAbstractChild { __typename ... on Child1 { name, persons { firstName } } ... on Child2 { name, persons { firstName } } } }");
+		Map<String, Object> s = linkResourceAndRunQuery("instance", Dialect.O, "{ returnsAbstractChild { __typename ... on Child { name, persons { firstName } } ... on GrandChild { name, persons { firstName } } } }");
 		Map<String, Object> i = (Map<String, Object>) s.get("returnsAbstractChild");
-		assertEquals("Child1", i.get("name"));
+		assertEquals("Child", i.get("name"));
 	}
 
 	@Test
 	public void returnsAbstractChildren() throws Exception {
-		Map<String, Object> s = linkResourceAndRunQuery("instance", Dialect.O, "{ returnsAbstractChildren { __typename ... on Parent { name } ... on Child1 { name, persons { firstName } } ... on Child2 { name, persons { firstName } } } }");
+		Map<String, Object> s = linkResourceAndRunQuery("instance", Dialect.O, "{ returnsAbstractChildren { __typename ... on Parent { name } ... on Child { name, persons { firstName } } ... on GrandChild { name, persons { firstName } } } }");
 		List<Map<String, Object>> l = (List<Map<String, Object>>) s.get("returnsAbstractChildren");
 		assertEquals(3, l.size());
 		Map<String, Object> i = l.get(0);
-		assertEquals("Child1", i.get("name"));
+		assertEquals("Child", i.get("name"));
 		i = l.get(1);
-		assertEquals("Child2", i.get("name"));
+		assertEquals("GrandChild", i.get("name"));
 		i = l.get(2);
 		assertEquals("Parent", i.get("name"));
 	}
diff --git a/Server/src/test/resources/graphql-tests/instance.poc b/Server/src/test/resources/graphql-tests/instance.poc
index 0005dd41..ee903a34 100644
--- a/Server/src/test/resources/graphql-tests/instance.poc
+++ b/Server/src/test/resources/graphql-tests/instance.poc
@@ -16,15 +16,15 @@ Person[] method returnsInstances() {
 
 attribute persons: Person[];
 category Parent(name, persons);
-category Child1 extends Parent;
-category Child2 extends Parent;
+category Child extends Parent;
+category GrandChild extends Child;
 
 @GraphQLQuery
 Parent method returnsAbstractChild() {
-	return (Parent)Child1(name = "Child1", persons = [Person(firstName = "Eric")]);
+	return (Parent)Child(name = "Child", persons = [Person(firstName = "Eric")]);
 }
 
 @GraphQLQuery
 Parent[] method returnsAbstractChildren() {
-	return [Child1(name = "Child1", persons= [Person(firstName = "Eric")]), Child2(name = "Child2"), Parent(name = "Parent")];
+	return [Child(name = "Child", persons= [Person(firstName = "Eric")]), GrandChild(name = "GrandChild"), Parent(name = "Parent")];
 }
\ No newline at end of file