Not yet cleaned up and not simplified reproducer:

(define (call-command-with-output-error-to-string cmd)
    (let* ((err-cons (pipe))
           (port (with-error-to-port
                  (cdr err-cons)
                  (lambda () (open-output-pipe cmd))))
           (_ (setvbuf (car err-cons) 'block (* 1024 1024 16)))
           (_ (scm->json bril-program port))
           (result "hi" ;; (read-delimited "" port)
                   ))
      (close-port (cdr err-cons))
      (values
       result
       (read-delimited "" (car err-cons)))))