diff --git a/examples/fastify/.meshrc.yml b/examples/fastify/.meshrc.yml
index 4b528585e71c1..f1547a053f2de 100644
--- a/examples/fastify/.meshrc.yml
+++ b/examples/fastify/.meshrc.yml
@@ -1,3 +1,4 @@
+merger: bare
 sources:
   - name: Swapi
     handler:
@@ -5,3 +6,7 @@ sources:
         source: ./swagger/pets.json
         endpoint: >-
           http://localhost:4001
+    transforms:
+      - extend:
+          typeDefs: ./src/extensions/typeDefs.graphql
+          resolvers: ./src/extensions/resolvers
diff --git a/examples/fastify/src/extensions/resolvers.ts b/examples/fastify/src/extensions/resolvers.ts
new file mode 100644
index 0000000000000..36ece5dbebffa
--- /dev/null
+++ b/examples/fastify/src/extensions/resolvers.ts
@@ -0,0 +1,37 @@
+/* eslint-disable @typescript-eslint/naming-convention */
+import type { Resolvers, MeshContext } from '../../.mesh';
+
+const resolvers: Resolvers = {
+  NewPetResponseUnion: {
+    __resolveType: (val: any) => {
+      return (val as any).foo
+        ? 'NewPetResponse'
+        : 'Error';
+    },
+  },
+  Query: {
+    newPet: async (root, args, context: MeshContext, info): Promise<any> => {
+      const {
+        petId,
+        extraId
+      } = args;
+
+      const data = (await context.Swapi.Query.pet_by_petId({
+        root,
+        args: {
+          petId
+        },
+        context,
+        info,
+      })) as any;
+
+      console.log('Data from pet_by_petId', JSON.stringify(data, null, 4));
+
+      return {
+        foo: JSON.stringify(data)
+      };
+    },
+  },
+};
+
+export default resolvers;
diff --git a/examples/fastify/src/extensions/typeDefs.graphql b/examples/fastify/src/extensions/typeDefs.graphql
new file mode 100644
index 0000000000000..49bfe131702a2
--- /dev/null
+++ b/examples/fastify/src/extensions/typeDefs.graphql
@@ -0,0 +1,12 @@
+type NewPetResponse {
+  foo: String
+}
+
+union NewPetResponseUnion = Error | NewPetResponse
+
+extend type Query {
+  newPet(
+    petId: String!
+    extraId: String!
+  ): NewPetResponseUnion
+}
diff --git a/examples/fastify/src/upstream.ts b/examples/fastify/src/upstream.ts
index bb39f660756c8..dfe476f0dff96 100644
--- a/examples/fastify/src/upstream.ts
+++ b/examples/fastify/src/upstream.ts
@@ -6,12 +6,8 @@ upstream.route({
   method: ['GET', 'POST'],
   url: '/pet/:petId',
   async handler(request, reply) {
-    const { petId } = request.params as { petId: string };
-
-    if (petId === 'pet200') {
-      return reply.status(200).send({ name: 'Bob' });
-    }
-
-    return reply.status(500).send({ error: `Error` });
+    return reply.status(200).send({
+      "id": "0fc9111f-570d-4ebe-a72e-ff4eb274bc65",
+    });
   },
 });
diff --git a/examples/fastify/swagger/pets.json b/examples/fastify/swagger/pets.json
index f07d6d5dbe758..2fae8322c6900 100644
--- a/examples/fastify/swagger/pets.json
+++ b/examples/fastify/swagger/pets.json
@@ -1,46 +1,66 @@
 {
-  "openapi": "3.0.3",
+  "openapi": "3.0.1",
   "paths": {
-    "/pet/{petId}": {
-      "get": {
-        "parameters": [
-          {
-            "name": "petId",
-            "in": "path",
-            "description": "ID of pet to return",
-            "required": true,
-            "schema": {
-              "type": "string"
-            }
-          }
-        ],
-        "responses": {
-          "200": {
-            "description": "successful operation",
-            "content": {
-              "application/json": {
-                "schema": {
-                  "$ref": "#/components/schemas/Pet"
-                }
+      "/pet/{petId}": {
+          "get": {
+              "parameters": [
+                    {
+                        "name": "petId",
+                        "in": "path",
+                        "required": true,
+                        "schema": {
+                            "type": "string"
+                        }
+                    }
+              ],
+              "responses": {
+                  "200": {
+                      "description": "Returns the Pet",
+                      "content": {
+                          "application/json": {
+                              "schema": {
+                                  "$ref": "#/components/schemas/Pet"
+                              }
+                          }
+                      }
+                  },
+                  "500": {
+                      "description": "Server error",
+                      "content": {
+                          "application/json": {
+                              "schema": {
+                                  "$ref": "#/components/schemas/Error"
+                              }
+                          }
+                      }
+                  }
               }
-            }
           }
-        }
       }
-    }
   },
   "components": {
-    "schemas": {
-      "Pet": {
-        "required": ["name"],
-        "type": "object",
-        "properties": {
-          "name": {
-            "type": "string",
-            "example": "doggie"
-          }
+      "schemas": {
+        "Pet": {
+            "required": [
+                "id"
+            ],
+            "type": "object",
+            "properties": {
+                "id": {
+                    "type": "string"
+                }
+            },
+            "additionalProperties": false
+        },
+        "Error": {
+            "type": "object",
+            "properties": {
+                "errors": {
+                  "type": "string"
+                }
+            },
+            "additionalProperties": false
         }
       }
-    }
   }
 }
diff --git a/examples/fastify/tests/fastify.spec.ts b/examples/fastify/tests/fastify.spec.ts
index e26a787382026..67ff208f0df20 100644
--- a/examples/fastify/tests/fastify.spec.ts
+++ b/examples/fastify/tests/fastify.spec.ts
@@ -26,16 +26,29 @@ describe('fastify', () => {
       body: JSON.stringify({
         query: /* GraphQL */ `
           {
-            pet_by_petId(petId: "pet200") {
-              name
-            }
+            pet_by_petId(petId: "0fc9111f-570d-4ebe-a72e-ff4eb274bc65"){
+              __typename
+              ... on Pet {
+                  id
+              }
+              ... on Error {
+                  errors
+              }
+          }
           }
         `,
       }),
     });
 
     const json = await response.json();
-    expect(json.data).toEqual({ pet_by_petId: { name: 'Bob' } });
+    expect(json).toEqual({
+      data: {
+        pet_by_petId: {
+          "__typename": "Pet",
+          "id": "0fc9111f-570d-4ebe-a72e-ff4eb274bc65",
+        },
+      }
+    });
   });
 
   it('should work too', async () => {
@@ -47,28 +60,28 @@ describe('fastify', () => {
       body: JSON.stringify({
         query: /* GraphQL */ `
           {
-            pet_by_petId(petId: "pet500") {
-              name
+            newPet(extraId: "dbd9e7c1-24f1-42e8-bd34-e7ce5bbafd7b", petId: "0fc9111f-570d-4ebe-a72e-ff4eb274bc65"){
+                __typename
+                ... on Error {
+                    errors
+                }
+                ... on NewPetResponse {
+                    foo
+                }
             }
           }
         `,
       }),
     });
 
-    const resJson = await response.json();
-
-    expect(resJson).toEqual({
-      data: { pet_by_petId: null },
-      errors: [
-        {
-          message: 'HTTP Error: 500, Could not invoke operation GET /pet/{args.petId}',
-          path: ['pet_by_petId'],
-          extensions: {
-            request: { url: 'http://localhost:4001/pet/pet500', method: 'GET' },
-            responseJson: { error: 'Error' },
-          },
-        },
-      ],
+    const json = await response.json();
+    expect(json).toEqual({
+      data: {
+        "newPet": {
+          "__typename": "NewPetResponse",
+          "foo": "{\"__typename\":\"Pet\", \"id\": \"0fc9111f-570d-4ebe-a72e-ff4eb274bc65\"}",
+        }
+      }
     });
   });
 });