diff --git a/packages/core/content-type-builder/admin/src/components/DataManagerProvider/tests/reducer_add_custom_field_attribute_action.test.js b/packages/core/content-type-builder/admin/src/components/DataManagerProvider/tests/reducer_add_custom_field_attribute_action.test.js
new file mode 100644
index 0000000000..d351ea5a3d
--- /dev/null
+++ b/packages/core/content-type-builder/admin/src/components/DataManagerProvider/tests/reducer_add_custom_field_attribute_action.test.js
@@ -0,0 +1,59 @@
+import cloneDeep from 'lodash/cloneDeep';
+import reducer, { initialState } from '../reducer';
+import { ADD_CUSTOM_FIELD_ATTRIBUTE } from '../constants';
+
+describe('CTB | components | DataManagerProvider | reducer | ADD_CUSTOM_FIELD_ATTRIBUTE', () => {
+  it('adds a custom field to a contentType', () => {
+    const contentType = {
+      uid: 'api::test.test',
+      schema: {
+        name: 'test',
+        description: '',
+        connection: 'default',
+        collectionName: '',
+        attributes: [{ name: 'test', type: 'string' }],
+      },
+    };
+
+    const action = {
+      type: ADD_CUSTOM_FIELD_ATTRIBUTE,
+      attributeToSet: {
+        type: 'string',
+        name: 'color',
+        options: { format: 'hex' },
+      },
+      forTarget: 'contentType',
+      targetUid: 'api::test.test',
+      initialAttribute: {},
+    };
+
+    const state = {
+      ...initialState,
+      modifiedData: {
+        components: {},
+        contentType,
+      },
+    };
+
+    const updatedContentType = cloneDeep(contentType);
+    updatedContentType.schema.attributes.push({
+      name: 'color',
+      // The type is converted to customField on the server,
+      // The underlying data type is expected here
+      type: 'string',
+      options: {
+        format: 'hex',
+      },
+    });
+
+    const expected = {
+      ...initialState,
+      modifiedData: {
+        components: {},
+        contentType: updatedContentType,
+      },
+    };
+
+    expect(reducer(state, action)).toEqual(expected);
+  });
+});