The --json flag is a global option on the top-level nrfutil command. Place it anywhere after nrfutil:
nrfutil --json <command> [SUBCOMMAND] [OPTIONS]
The flag works with built-in commands, such as nrfutil list and nrfutil search, and with installable commands, such as nrfutil device and nrfutil sdk-manager.
Related global output flags include --json-pretty for formatted JSON output and --skip-overhead to output only the data payload from info messages.
Note
For details about the JSON message format and examples of its usage in scripts, see Managing command output for scripting.