If you’ve ever pressed Spacebar in Finder and gotten the wrong Quick Look preview — plain text instead of Markdown, someone else’s renderer, or a blank stare from macOS — you already know the pain. PluginKit registrations are fiddly, conflicting extensions are common, and the fix usually involves a scavenger hunt through Terminal.

Quick Look Doctor is a free macOS utility that turns that scavenger hunt into a proper diagnosis-and-repair flow. Drop a file, see who’s claiming it, prefer the handler you want, and ignore the rest — with confirmation and Undo.

Why it exists

Marked QL is a Mac App Store Quick Look preview for Markdown. Once it shipped, support mail started looking familiar: “Spacebar shows monospace text,” “Nova/Folder Quick Look/Highland is winning,” “I followed the troubleshooting page and I’m still stuck.”

A lot of those tickets boil down to the same macOS reality: multiple preview extensions can claim the same UTI, there’s no nice per-type preference UI, and pluginkit is the blunt instrument underneath. Doctor was built so people (and I) don’t have to re-run that dance by hand every time, and to avoid sending people to Terminal when I don’t have to.

What it automates

The Marked QL troubleshooting guide walks through registration, conflicts, and diagnostics. Doctor automates the parts that hurt most:

  • File-first diagnosis — drop any file (not just Markdown) and see which preview extensions claim its type
  • Active handler probe — see which extension is actually serving the Spacebar preview when possible
  • Prefer / ignore — pick a handler, confirm which peers will be ignored, apply, and Undo if you change your mind
  • Extensions tab — manage the full preview-plugin inventory (Use / Ignore / Default)
  • Copy report — put a plain-text diagnosis on the clipboard for support tickets

It still won’t invent a per-UTI preference API that Apple never shipped. Ignoring an extension is all-or-nothing for that plugin. Doctor is honest about that up front — no silent peer-killing, no “we own Markdown forever” marketing.

Marked QL and Doctor

They’re related but separate:

  • Marked QL — App Store preview app for rich Markdown in Quick Look
  • Quick Look Doctor — free, notarized utility for any Quick Look handler conflict

If Marked QL’s in-app Diagnose flow points you at a mess of competing extensions, Doctor is the “fix it without Terminal” path. In the upcoming Marked QL v1.0.5, Help → Get Quick Look Doctor… opens the download page; if Doctor is already installed, you get Open Quick Look Doctor… instead.

Grab it

Download is free:

Download Quick Look Doctor

Unzip, drop it in /Applications, launch once, and point it at a file that’s misbehaving. If you’re filing a support request, Copy report first — that clipboard dump saves everyone a round trip.