[bug]: Custom Field requires type: json in schema to save data

[deivydarwoft]

What version of @strapi/sdk-plugin are you using?

• Npm version: 10.8.2
• Node.js version: 20.18.3
• React version: 18.3.1
• Strapi version: 5.23.3
• @strapi/sdk-plugin version: 5.3.2
• Browser: Chrome

What's Wrong?

When using a custom field from any plugin in a content type schema, if the field is defined as:

"myCustomField": {
  "type": "customField",
  "customField": "plugin::<plugin-name>.<custom-field-name>"
}

Strapi does not save the data unless the field type is changed to "json":

"myCustomField": {
  "type": "json",
  "customField": "plugin::<plugin-name>.<custom-field-name>"
}

This is not documented and causes confusion, since the admin panel recognizes the custom field, but the data is not persisted unless the type is set to json.

To Reproduce

1. Create a content type with a custom field using a local plugin built with the Strapi SDK (e.g. <plugin-name>), where the custom field is registered as type json in the plugin registration.

2. Add the custom field from the Content-Type Builder to any collection type.

3. In the schema, define the field as:

   "myCustomField": {
     "type": "customField",
     "customField": "plugin::<plugin-name>.<custom-field-name>"
   }

4. Go to the admin panel and try to save data using the custom field.

5. Observe that the data is not saved/persisted.

6. Change the field type to "json":

   "myCustomField": {
     "type": "json",
     "customField": "plugin::<plugin-name>.<custom-field-name>"
   }

7. Now, data is saved correctly.

Expected Behaviour

Actual Behaviour

• The admin panel recognizes the custom field and allows input, but Strapi does not persist the data unless the type is set to json.
• This behaviour is not documented in the official docs, which show examples using type: "customField".

Expected Behaviour

• Strapi should either:
  • Handle custom fields that store arrays/objects without requiring manual change to "json", or
  • Document this requirement clearly for custom field plugin authors and users.

Technical Comparison with Official Documentation

• Official docs show custom fields defined as:

"color": {
  "type": "customField",
  "customField": "plugin::color-picker.color",
  "options": {
    "format": "hex"
  }
}

• In practice, for custom fields that store complex data (arrays/objects), the type must be set to json for persistence:

"customFieldExample": {
  "type": "json",
  "customField": "plugin::<plugin-name>.<custom-field-name>"
}

• If type: "customField" is used, the data is not saved.

---

Custom Field Registration

File: <plugin-path>/admin/src/index.js

app.customFields.register({
  name: '<custom-field-name>',
  pluginId: '<plugin-name>',
  type: 'json', // Required for metadata persistence
  intlLabel: {
    id: '<plugin-name>.<custom-field-name>.label',
    defaultMessage: '<custom-field-name>',
  },
  intlDescription: {
    id: '<plugin-name>.<custom-field-name>.description',
    defaultMessage: 'Select any asset',
  },
  components: {
    Input: async () =>
      import('./components/Input/Input').then((module) => ({
        default: module.Input,
      })),
  },
  options: {
    // declare options here
  },
});

• The custom field must use type: 'json' for Strapi to persist complex data (arrays/objects).
• If type: 'customField' is used, data is not saved.