Quick Peek adds native previews for Markdown, JSON, YAML, and CSV, including tree and table views, search, and local-only handling. Its basic reader is free, with a $9.99 one-time Pro option for editing.
It requires macOS 26.2 or later. The best test is simple: select a JSON or agent-instructions file in Finder, press Space, and see whether the preview is good enough to avoid opening a separate editor. Action: check compatibility and test with a disposable file.