I sent a suggestion but auto formatting, you can add a {headers:{"Range":"bytes=0-499"}} to the fetch options to grab just the front matter only

Thanks for letting me know! The original implementation used Range requests (the reason why https://observablehq.com/@mootari/toolbox#readChunks has readChunks), but those weren't supported by Observable at the time. Seems support might have been added when https://github.com/observablehq/feedback/issues/283 was resolved.

Had to disable Range requests again, because they break preflight requests in Firefox and Safari.

:'(