Yea, I'm on my local terminal I can do

zed permission check tenant:96317cea-7e74-4b8d-956c-7ba8998c314b cash_bids_read subject:7ac9049b-29db-47df-a493-fcf0048bad94 --caveat-context='{"current_tenant":"96317cea-7e74-4b8d-956c-7ba8998c314b"}'

without trouble but in the Plaground I get

invalid caveat context JSON: invalid character '\'' looking for beginning of value