Validation error when saving entry from front end form, if Title Format is {id}

If you are submitting entries on the frontend via entries/save-entry, you'll end up with validation errors (title cannot be blank).

One hack around this is setting the slug/id to __id__ or something, and then replacing via events:

        Event::on(
            Content::class,
            Content::EVENT_BEFORE_SAVE_CONTENT,
            function (Event $event) {
                if ($event->element->title === '__id__') {
                    $event->element->title = $event->element->id;
                }
            }
        );

        Event::on(
            Element_SiteSettings::class,
            Element_SiteSettings::EVENT_BEFORE_INSERT,
            function (Event $event) {
                if ($event->sender->slug === '__id__') {
                    $event->sender->slug = $event->sender->elementId;
                }
            }
        );

With something like this in your form:

      {{ hiddenInput('title', '__id__') }}
      {{ hiddenInput('slug', '__id__') }}

Originally posted by @timkelty in https://github.com/craftcms/cms/issues/6110#issuecomment-663180551

Start by tracing the frontend entries/save-entry entry point and the validation path used when the title format is {id}; the issue also shows the related before-save event hooks and form inputs. Done means a frontend submission using {id} for the title and slug saves without a “title cannot be blank” validation error, without requiring the event-based workaround.