fix(introspection): resolve ofType.fields returning null

execution/engine/federation_integration_test.go

@@ -507,6 +507,68 @@ func TestFederationIntegrationTest(t *testing.T) {
 		expected := `{"data":{"cat":{"name":"Pepper"},"me":{"id":"1234","username":"Me","realName":"User Usington","reviews":[{"body":"A highly effective form of birth control."},{"body":"Fedoras are one of the most fashionable hats around and can look great with a variety of outfits."}],"history":[{},{"rating":5},{}]}}}`
 		assert.Equal(t, compact(expected), string(resp))
 	})
+
+	// Regression test for https://github.com/wundergraph/cosmo/issues/991
+	t.Run("introspection ofType returns fields", func(t *testing.T) {
+		t.Parallel()
+		ctx, cancel := context.WithCancel(context.Background())
+		t.Cleanup(cancel)
+		resp := gqlClient.Query(ctx, setup.GatewayServer.URL, testQueryPath("queries/introspection_oftype_fields.query"), nil, t)
+		expected := `{"data":{"__type":{"name":"User","kind":"OBJECT","fields":[{"name":"id","type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"ID","fields":null}}},{"name":"username","type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","fields":null}}},{"name":"history","type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"LIST","name":null,"fields":null}}},{"name":"realName","type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","fields":null}}},{"name":"reviews","type":{"kind":"LIST","name":null,"ofType":{"kind":"OBJECT","name":"Review","fields":[{"name":"body"},{"name":"author"},{"name":"product"},{"name":"attachments"},{"name":"comment"}]}}}]}}}`
+		assert.Equal(t, compact(expected), string(resp))
+	})
+
+	// Complex nested introspection proving enrichment works end-to-end:
+	// - Aliases (review:, attachment:)
+	// - Deep ofType nesting: ofType.fields.type.ofType with full type resolution
+	// - Union possibleTypes with fields (Attachment → Question, Rating, Video)
+	// - Cycle prevention: User.reviews and Product.reviews both reference Review,
+	//   which is already being serialized → "fields": null proves truncation
+	t.Run("introspection nested enriched types", func(t *testing.T) {
+		t.Parallel()
+		ctx, cancel := context.WithCancel(context.Background())
+		t.Cleanup(cancel)
+		resp := gqlClient.Query(ctx, setup.GatewayServer.URL, testQueryPath("queries/introspection_nested_enriched.query"), nil, t)
+		expected := `{"data":{
+			"review":{
+				"kind":"OBJECT","name":"Review",
+				"fields":[
+					{"name":"body","type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","fields":null,"possibleTypes":[]}}},
+					{"name":"author","type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"User",
+						"fields":[
+							{"name":"id","type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"ID","fields":null}}},
+							{"name":"username","type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","fields":null}}},
+							{"name":"history","type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"LIST","name":null,"fields":null}}},
+							{"name":"realName","type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","fields":null}}},
+							{"name":"reviews","type":{"kind":"LIST","name":null,"ofType":{"kind":"OBJECT","name":"Review","fields":null}}}
+						],
+						"possibleTypes":[]
+					}}},
+					{"name":"product","type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"OBJECT","name":"Product",
+						"fields":[
+							{"name":"upc","type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","fields":null}}},
+							{"name":"name","type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"String","fields":null}}},
+							{"name":"price","type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"Int","fields":null}}},
+							{"name":"inStock","type":{"kind":"NON_NULL","name":null,"ofType":{"kind":"SCALAR","name":"Int","fields":null}}},
+							{"name":"reviews","type":{"kind":"LIST","name":null,"ofType":{"kind":"OBJECT","name":"Review","fields":null}}}
+						],
+						"possibleTypes":[]
+					}}},
+					{"name":"attachments","type":{"kind":"LIST","name":null,"ofType":{"kind":"UNION","name":"Attachment","fields":null,"possibleTypes":[{"kind":"OBJECT","name":"Question"},{"kind":"OBJECT","name":"Rating"},{"kind":"OBJECT","name":"Video"}]}}},
+					{"name":"comment","type":{"kind":"INTERFACE","name":"Comment","ofType":null}}
+				]
+			},
+			"attachment":{
+				"kind":"UNION","name":"Attachment",
+				"possibleTypes":[
+					{"kind":"OBJECT","name":"Question","fields":[{"name":"upc"},{"name":"body"},{"name":"subject"}]},
+					{"kind":"OBJECT","name":"Rating","fields":[{"name":"upc"},{"name":"body"},{"name":"score"}]},
+					{"kind":"OBJECT","name":"Video","fields":[{"name":"upc"},{"name":"size"},{"name":"subject"}]}
+				]
+			}
+		}}`
+		assert.Equal(t, compact(expected), string(resp))
+	})
 }
 
 func compact(input string) string {

execution/federationtesting/testdata/queries/introspection_nested_enriched.query

@@ -0,0 +1,46 @@
+{
+  review: __type(name: "Review") {
+    kind
+    name
+    fields {
+      name
+      type {
+        kind
+        name
+        ofType {
+          kind
+          name
+          fields {
+            name
+            type {
+              kind
+              name
+              ofType {
+                kind
+                name
+                fields {
+                  name
+                }
+              }
+            }
+          }
+          possibleTypes {
+            kind
+            name
+          }
+        }
+      }
+    }
+  }
+  attachment: __type(name: "Attachment") {
+    kind
+    name
+    possibleTypes {
+      kind
+      name
+      fields {
+        name
+      }
+    }
+  }
+}

execution/federationtesting/testdata/queries/introspection_oftype_fields.query

@@ -0,0 +1,20 @@
+{
+  __type(name: "User") {
+    name
+    kind
+    fields {
+      name
+      type {
+        kind
+        name
+        ofType {
+          kind
+          name
+          fields {
+            name
+          }
+        }
+      }
+    }
+  }
+}

v2/pkg/engine/datasource/introspection_datasource/config_factory.go

@@ -23,6 +23,10 @@ func NewIntrospectionConfigFactory(schema *ast.Document) (*IntrospectionConfigFa
 		return nil, report
 	}
 
+	if err := data.Schema.BuildJSON(); err != nil {
+		return nil, err
+	}
+
 	return &IntrospectionConfigFactory{introspectionData: &data}, nil
 }
 

v2/pkg/engine/datasource/introspection_datasource/fixtures/comprehensive_config_input.golden

@@ -0,0 +1,75 @@
+{
+  "kind": "INPUT_OBJECT",
+  "name": "ConfigInput",
+  "description": "",
+  "inputFields": [
+    {
+      "name": "key",
+      "description": "",
+      "type": {
+        "kind": "NON_NULL",
+        "name": null,
+        "ofType": {
+          "kind": "SCALAR",
+          "name": "String",
+          "ofType": null,
+          "__typename": "__Type",
+          "description": "The 'String' scalar type represents textual data, represented as UTF-8 character sequences. The String type is most often used by GraphQL to represent free-form human-readable text.",
+          "inputFields": [],
+          "interfaces": [],
+          "possibleTypes": []
+        },
+        "__typename": "__Type"
+      },
+      "defaultValue": null,
+      "isDeprecated": false,
+      "deprecationReason": null,
+      "__typename": "__InputValue"
+    },
+    {
+      "name": "nested",
+      "description": "",
+      "type": {
+        "kind": "INPUT_OBJECT",
+        "name": "NestedInput",
+        "ofType": null,
+        "__typename": "__Type",
+        "description": "",
+        "inputFields": [
+          {
+            "name": "value",
+            "description": "",
+            "type": {
+              "kind": "NON_NULL",
+              "name": null,
+              "ofType": {
+                "kind": "SCALAR",
+                "name": "Int",
+                "ofType": null,
+                "__typename": "__Type",
+                "description": "The 'Int' scalar type represents non-fractional signed whole numeric values. Int can represent values between -(2^31) and 2^31 - 1.",
+                "inputFields": [],
+                "interfaces": [],
+                "possibleTypes": []
+              },
+              "__typename": "__Type"
+            },
+            "defaultValue": null,
+            "isDeprecated": false,
+            "deprecationReason": null,
+            "__typename": "__InputValue"
+          }
+        ],
+        "interfaces": [],
+        "possibleTypes": []
+      },
+      "defaultValue": null,
+      "isDeprecated": false,
+      "deprecationReason": null,
+      "__typename": "__InputValue"
+    }
+  ],
+  "interfaces": [],
+  "possibleTypes": [],
+  "__typename": "__Type"
+}