The json output for the subvolume related commands is currently enabled only in the experimental build. The format hasn't been finalized, at least list does not print complete subvolume information and it still depends on the parameters, the way multiple objects are enumerated in the json is also not unified.

To see the difference run:

$ btrfs subvolume list /mnt
{
  "__header": {
    "version": "1"
  },
  "subvolume-list": [
    {
      "id": 19112,
      "generation": 7730702,
      "top_level": 5,
      "path": "test"
    },
    {
      "id": 19113,
      "generation": 7730702,
      "top_level": 5,
      "path": "test-snap"
    }
  ]
}

and

btrfs --format=json subvol list -pcguqR /mnt
{
  "__header": {
    "version": "1"
  },
  "subvolume-list": [
    {
      "id": 19112,
      "generation": 7730702,
      "cgeneration": 7730701,
      "parent": 5,
      "top_level": 5,
      "parent_uuid": null,
      "received_uuid": null,
      "uuid": "dc16dd1b-825f-3245-94a8-557672d6cf85",
      "path": "test"
    },
    {
      "id": 19113,
      "generation": 7730702,
      "cgeneration": 7730702,
      "parent": 5,
      "top_level": 5,
      "parent_uuid": "dc16dd1b-825f-3245-94a8-557672d6cf85",
      "received_uuid": null,
      "uuid": "91708196-fd07-2242-8b8e-af9375474a09",
      "path": "test-snap"
    }
  ]
}

This is still missing the send and receive related transaction ids, times, and maybe other inforamtion that's available and does not have to be conditional like that.

Ohh, I did not realise that, thanks!

Perhaps a note in the error message or in Documentation/dev/dev-json.rst could be added? It was not clear to me at all as an outsider that there were such things as experimental builds (even though

Either way, it is entirely reasonable that the feature is not on by default if the format is not yet stabilised, even though I really wish it were. The horrendous greps I'm doing to at least attempt to handle newlines in filenames and such haunt me in my dreams. :)

Sorry for the false report. Unless you wish to use the issue to track the stabilisation or docs update, feel free to close this.