diff --git a/packages/mermaid/src/config.spec.ts b/packages/mermaid/src/config.spec.ts
index 457cb8244..6a9eb204c 100644
--- a/packages/mermaid/src/config.spec.ts
+++ b/packages/mermaid/src/config.spec.ts
@@ -1,4 +1,6 @@
 import * as configApi from './config.js';
+import isObject from 'lodash-es/isObject.js';
+import type { MermaidConfig } from './config.type.js';
 
 describe('when working with site config', function () {
   beforeEach(() => {
@@ -69,4 +71,43 @@ describe('when working with site config', function () {
     const config_4 = configApi.getConfig();
     expect(config_4.altFontFamily).toBeUndefined();
   });
+
+  it('test new default config', async function () {
+    const { default: oldDefault } = (await import('./oldDefaultConfig.js')) as {
+      default: Required<MermaidConfig>;
+    };
+    // gitGraph used to not have this option (aka it was `undefined`)
+    oldDefault.gitGraph.useMaxWidth = false;
+
+    // class diagrams used to not have these options set (aka they were `undefined`)
+    oldDefault.class.htmlLabels = false;
+
+    const { default: newDefault } = await import('./defaultConfig.js');
+
+    // check subsets of the objects, to improve vitest error messages
+    // we can't just use `expect(newDefault).to.deep.equal(oldDefault);`
+    // because the functions in the config won't be the same
+    expect(new Set(Object.keys(newDefault))).to.deep.equal(new Set(Object.keys(oldDefault)));
+
+    // @ts-ignore: Expect that all the keys in newDefault are valid MermaidConfig keys
+    Object.keys(newDefault).forEach((key: keyof MermaidConfig) => {
+      // recurse through object, since we want to treat functions differently
+      if (!Array.isArray(newDefault[key]) && isObject(newDefault[key])) {
+        expect(new Set(Object.keys(newDefault[key]))).to.deep.equal(
+          new Set(Object.keys(oldDefault[key]))
+        );
+        for (const key2 in newDefault[key]) {
+          if (typeof newDefault[key][key2] === 'function') {
+            expect(newDefault[key][key2].toString()).to.deep.equal(
+              oldDefault[key][key2].toString()
+            );
+          } else {
+            expect(newDefault[key]).to.have.deep.property(key2, oldDefault[key][key2]);
+          }
+        }
+      } else {
+        expect(newDefault[key]).to.deep.equal(oldDefault[key]);
+      }
+    });
+  });
 });