Why can't users of hybrid themes create new template parts via the Site Editor UI?

[mrleemon]

Currently, in the Site Editor’s "Add Pattern" component (packages/edit-site/src/components/add-new-pattern/index.js), there is a specific check that prevents the "Add Template Part" option from appearing unless the theme is a full block-based theme:

if ( isBlockBasedTheme && canCreateTemplatePart ) {
    controls.push( {
        icon: symbolFilled,
        onClick: () => setShowTemplatePartModal( true ),
        title: addNewTemplatePartLabel,
    } );
}

What is the primary reason for keeping the isBlockBasedTheme check here, specifically for themes that have opted into add_theme_support( 'block-template-parts' )?

Is this check intended to prevent UI clutter for hybrid theme users, or is there a deeper technical dependency on the Site Editor infrastructure that hybrid themes lack?

[rajpratham1]

Users of hybrid themes can’t create new template parts in the Site Editor because hybrid themes are not fully block-based themes.

The Site Editor’s full template editing features (like creating new template parts) are designed mainly for block themes that use theme.json and the full site editing structure.

Hybrid themes mix classic PHP templates with some block features. Since they still rely on traditional theme files, WordPress limits certain Site Editor capabilities — including creating new template parts from the UI.

If you need full template part creation from the Site Editor, you’ll need to use a fully block-based theme.