function cmd::test::unit_json_output() {
  test::section "Unit: JSON output"

  command::_load_mixins 2>/dev/null || true

  # json::envelope produces valid structure
  local result
  result=$(echo '{"peers":[]}' | json::envelope "list" "0")
  cmd::test::assert "envelope ok field"      "$(echo "$result" | grep -o '"ok":true')"  '"ok":true'
  cmd::test::assert "envelope command field" "$(echo "$result" | grep -o '"command":"list"')" '"command":"list"'
  cmd::test::assert "envelope meta field"    "$(echo "$result" | grep -o '"meta":')"    '"meta":'
  cmd::test::assert "envelope count field"   "$(echo "$result" | grep -o '"count":0')"  '"count":0'

  # command::mixin registration
  load_command list
  cmd::test::assert_true "json_output mixin registered" "declare -f command::mixin::json_output::register >/dev/null 2>&1"
  cmd::test::assert_true "command::json accessor exists" "declare -f command::json >/dev/null 2>&1"

  # json::error_envelope
  local err_result
  err_result=$(json::error_envelope "inspect" "Peer not found")