The error:

My frontmatter for the cover image is defined via Astro's content.config.ts schema:

    cover: z
      .union([
        z.string(),
        z.object({
          src: z.string(),
          title: z.string().optional(),
        }),
      ])
      .optional(),

which means both of the following are valid:

cover: some-filename.jpg

cover:
  src: some-filename.jpg
  title: a title for the file

In my frontmatter.json, I see no way to configure an "either or" for the field type of the cover:

  "frontMatter.taxonomy.contentTypes": [
    {
      "name": "default",
      ...
      "fields": [
        ...
        {
          "title": "Content preview",
          "name": "cover",
          "type": "image",
          "isPreviewImage": true
        }
      ]
    }
  ]

With this frontmatter.json, the second cover format is invalid (which is okay by me) and throws an error because an expected string does not have a .startsWith method. The field needs to be typecast somehow before applying filters and transformations.

Is there a way to make Frontmatter understand two possible field formats, or have it ignore the field (not validate it)?

Even if not, it should not error loudly with a popup; instead, it should "simply" complain about incorrect frontmatter during the validation of the frontmatter when the file is opened/saved.