(json:with-decoder-simple-clos-semantics
  (let ((json:*json-symbols-package* nil))
    (let ((x (json:decode-json-from-string
               "{\"foo1\": [1, 2, 3], \"bar1\": true,
                 \"baz1\": \"!\"}")))
      (with-slots (foo1 bar1 baz1) x
        (values x foo1 bar1 baz1)))))

The slot FOO1 is unbound in the object #<#<JSON:FLUID-CLASS NIL
                                           {1004085F33}>
                                         {1003CAFB43}>.
   [Condition of type UNBOUND-SLOT]