diff --git a/index.js b/index.js
index 876af5c..241bcf7 100644
--- a/index.js
+++ b/index.js
@@ -84,7 +84,6 @@ export function intercept(interceptFn) {
 // resets content an initialize basic types ()
 export function resetContent() {
   initBasetypes();
-  workingCopiesByType = {};
   slugs = [];
   interceptor = () => { };
   versionsMeta = {};
@@ -96,6 +95,8 @@ export function resetContent() {
 // Removes all base types (needed for a few very generic tests)
 export function clearBaseTypes() {
   types = {};
+  workingCopiesByType = {};
+  contentByType = {};
 }
 
 export function addWorkingCopy(type, id, content) {
@@ -135,20 +136,17 @@ export function removeSlug(slug) {
     && p.value === slug.value
     && p.path === slug.path));
 }
-export function addType(type) {
+export function addType(type, allowRedefine = false) {
   if (!type.properties) {
     type.properties = {};
   }
-  if (!types[type.name]) {
-    types[type.name] = mapType(type);
-
-  }
-  if (!contentByType[type.name]) {
-    contentByType[type.name] = {};
-  }
-  if (!workingCopiesByType[type.name]) {
-    workingCopiesByType[type.name] = {};
+  if (types[type.name] && !allowRedefine) {
+    throw new Error(`Type ${type.name} is already defined`);
   }
+
+  types[type.name] = mapType(type);
+  contentByType[type.name] = {};
+  workingCopiesByType[type.name] = {};
 }
 
 export function peekContent(type, id) {
@@ -214,8 +212,7 @@ function interceptable(routeInterceptor, route) {
 }
 
 function initBasetypes() {
-  contentByType = { channel: {}, "publishing-group": {} };
-  types = {};
+  clearBaseTypes();
   addType({
     name: "channel",
     properties: { attributes: { type: "object", properties: { name: { type: "string" } } } },
@@ -227,6 +224,7 @@ function initBasetypes() {
   addType({
     name: "article",
     properties: { attributes: { type: "object", properties: { name: { type: "string" }, headline: { type: "string" } } } },
+    canHaveSlugs: true,
   });
 }
 
diff --git a/package-lock.json b/package-lock.json
index 1a406a1..8955c35 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "@bonniernews/fake-tool-api",
-  "version": "1.2.0",
+  "version": "1.3.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "@bonniernews/fake-tool-api",
-      "version": "1.2.0",
+      "version": "1.3.0",
       "license": "MIT",
       "dependencies": {
         "nock": "^13.5.3",
@@ -16,7 +16,7 @@
         "@bonniernews/eslint-config": "^1.1.0",
         "chai": "^5.1.0",
         "eslint": "^8.57.1",
-        "mocha": "^10.3.0",
+        "mocha": "^11.0.0",
         "node-fetch": "^3.3.2"
       }
     },
@@ -2968,10 +2968,11 @@
       }
     },
     "node_modules/mocha": {
-      "version": "10.7.3",
-      "resolved": "https://registry.npmjs.org/mocha/-/mocha-10.7.3.tgz",
-      "integrity": "sha512-uQWxAu44wwiACGqjbPYmjo7Lg8sFrS3dQe7PP2FQI+woptP4vZXSMcfMyFL/e1yFEeEpV4RtyTpZROOKmxis+A==",
+      "version": "11.0.0",
+      "resolved": "https://registry.npmjs.org/mocha/-/mocha-11.0.0.tgz",
+      "integrity": "sha512-9VQaK0N4YQ2F89Vy4wTIEyTm/Ggcv1PejfVeI82wOw0vBO6BjFyBGHCiNbl+wyHmgWDyFmHb2Yw1QlLaWzaEoA==",
       "dev": true,
+      "license": "MIT",
       "dependencies": {
         "ansi-colors": "^4.1.3",
         "browser-stdout": "^1.3.1",
@@ -2999,7 +3000,7 @@
         "mocha": "bin/mocha.js"
       },
       "engines": {
-        "node": ">= 14.0.0"
+        "node": "^18.18.0 || ^20.9.0 || >=21.1.0"
       }
     },
     "node_modules/mocha/node_modules/minimatch": {
diff --git a/package.json b/package.json
index 3e6ce93..25a21b1 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@bonniernews/fake-tool-api",
-  "version": "1.2.0",
+  "version": "1.3.0",
   "type": "module",
   "description": "Mocked Bonnier News tool api, for use in automated tests",
   "main": "index.js",
@@ -25,7 +25,7 @@
     "@bonniernews/eslint-config": "^1.1.0",
     "chai": "^5.1.0",
     "eslint": "^8.57.1",
-    "mocha": "^10.3.0",
+    "mocha": "^11.0.0",
     "node-fetch": "^3.3.2"
   },
   "dependencies": {
diff --git a/test/index.spec.js b/test/index.spec.js
index b5d3a77..f773122 100644
--- a/test/index.spec.js
+++ b/test/index.spec.js
@@ -12,7 +12,7 @@ describe("Fake tool api", () => {
     fakeToolApi.init(baseUrl, (msg) => {
       events.push(JSON.parse(msg.data));
     });
-    fakeToolApi.addType({ name: "article" });
+    fakeToolApi.addType({ name: "article" }, true);
     events.length = 0;
   });
   const id = randomUUID();
@@ -85,6 +85,29 @@ describe("Fake tool api", () => {
 
   });
 
+  describe("#addType", () => {
+    beforeEach(() => {
+      fakeToolApi.clearBaseTypes();
+    });
+
+    it("should let user know if a type has already been defined", () => {
+      fakeToolApi.addType({ name: "test" });
+      expect(() => fakeToolApi.addType({ name: "test" })).to.throw("Type test is already defined");
+    });
+
+    it("should allow type redefinitions if explicitly set", async () => {
+      fakeToolApi.addType({ name: "test" });
+      fakeToolApi.addType({
+        name: "test",
+        properties: { attributes: { type: "object", properties: { cool: { type: "string" } } } },
+      }, true);
+
+      const res = await fetch(`${baseUrl}/types`);
+      const types = await res.json();
+      expect(types[0].properties.attributes.properties).to.have.property("cool");
+    });
+  });
+
   describe("PUT /content", () => {
 
     it("should reject non-uuid ids", async () => {