Code:

(define (json-rpc-dispatch request)
  (define (dispatch j)
    (let* ((method-pair (assoc 'method j))
           (params-pair (assoc 'params j)))
      (unless (and method-pair params-pair)
        (raise (make-json-rpc-invalid-request-error)))
      (let ((method-name (cdr method-pair)))
        (unless (and (not (pair? method-name))
                     (string? method-name))
          (raise (make-json-rpc-invalid-request-error)))
        (let* ((method (hash-table-ref/default
                        (json-rpc-handler-table)
                        method-name
                        #f)))
          (unless method
            (raise (make-json-rpc-method-not-found-error method-name)))
          (if params-pair
              (let ((params (extract-params params-pair)))
                (method params))
              (method))))))
  (define id (assoc 'id request))
  (let ((result (dispatch request)))
    (cond ((not result) ;; notification
           #f)
          (id (make-response (cdr id)
                             result
                             #f))
          (else (make-response 'null
                               #f
                               (make-json-rpc-invalid-request-error))))))

Scrutinizer dump:

[debug|37]                                      walk: ##core#variable (method-name868) (loc: (dispatch862 json-rpc.lolevel#json-rpc-dispatch), dest: #f, flow: (78 71))
[debug|37]                                      walked ##core#variable -> ((list-of pair)) flow: (78 71)
[debug|37]                                        call: ((procedure scheme#string? (*) boolean) (list-of pair)), te: ()
[debug|37]                                      match (any) * <-> (list-of pair)
[debug|37]                                         match1: * <-> (list-of pair)
[debug|37]                                      match (any) * <-> (list-of pair) -> #t
[debug|37]                                      match (all) string <-> (list-of pair)
[debug|37]                                         match1: string <-> (list-of pair)
[debug|37]                                      match (all) string <-> (list-of pair) -> #f
[debug|37]                                      match (all) (not string) <-> (list-of pair)
[debug|37]                                         match1: (not string) <-> (list-of pair)
[debug|37]                                         match1: string <-> (list-of pair)
[debug|37]                                      match (all) (not string) <-> (list-of pair) -> #t

Note: Predicate is always false
  In file `json-rpc/lolevel-impl.scm:138',
  In module `json-rpc.lolevel',
  In procedure `json-rpc-dispatch',
  In procedure `dispatch',
  In procedure call:

    (scheme#string? method-name)

  The predicate will always return false.

  Procedure `string?' from module `scheme' is a predicate for:

    string

  The given argument has this type:

    (list-of pair)
[debug|37]                                        specialized: `string' for ((not string))
[debug|37]                                      simplify: false -> false
[debug|37]                                        result-types: (false)
[debug|37]                                      walk: let (tmp1459) (loc: (dispatch862 json-rpc.lolevel#json-rpc-dispatch), dest: #f, flow: (78 71))
[debug|38]                                       walk: ##core#the/result ((list-of pair)) (loc: (dispatch862 json-rpc.lolevel#json-rpc-dispatch), dest: tmp1459, flow: (78 71))
[debug|38]                                       walked ##core#the/result -> ((list-of pair)) flow: (78 71)
[debug|38]                                       walk: quote (#f) (loc: (dispatch862 json-rpc.lolevel#json-rpc-dispatch), dest: #f, flow: (78 71))
[debug|38]                                       walked quote -> (false) flow: (78 71)
[debug|37]                                      walked let -> (false) flow: (78 71)
[debug|37]                                      simplify: false -> false
[debug|36]                                     walked ##core#call -> (false) flow: (78 71)
[debug|36]                                     walk: quote (#f) (loc: (dispatch862 json-rpc.lolevel#json-rpc-dispatch), dest: #f, flow: (79 71))
[debug|36]                                     walked quote -> (false) flow: (79 71)
[debug|36]                                     merge branch results: (false) + (false)
[debug|36]                                     simplify: (or false false) -> false
[debug|35]                                    walked if -> (false) flow: (71)

Note: Test is always false
  In module `json-rpc.lolevel',
  In procedure `json-rpc-dispatch',
  In procedure `dispatch',
  In conditional expression:

    (if (if (scheme#not (scheme#pair? method-name))
          (scheme#string? method-name)
          #f)
      (##core#undefined)
      (scheme.base#raise (json-rpc.lolevel#make-json-rpc-invalid-request-error)))