diff --git a/guix/ci.scm b/guix/ci.scm
index dde93bbd53..cf39744567 100644
--- a/guix/ci.scm
+++ b/guix/ci.scm
@@ -20,9 +20,12 @@
 (define-module (guix ci)
   #:use-module (guix http-client)
   #:use-module (guix utils)
+  #:use-module ((guix build download)
+                #:select (resolve-uri-reference))
   #:use-module (json)
   #:use-module (srfi srfi-1)
   #:use-module (ice-9 match)
+  #:use-module (web uri)
   #:use-module (guix i18n)
   #:use-module (guix diagnostics)
   #:autoload   (guix channels) (channel)
@@ -146,16 +149,41 @@
   ;; Max number of builds requested in queries.
   1000)
 
+(define* (api-url base-url path #:rest query)
+  "Build a proper API url, taking into account BASE_URL's trailing slashes."
+
+  (define (build-query-string query)
+    (let lp ((query (or (reverse query) '())) (acc '()))
+      (match query
+        (() (string-concatenate acc))
+        (((_ #f) . rest) (lp rest acc))
+        (((name val) . rest)
+         (lp rest (cons*
+                   name "="
+                   (if (string? val) (uri-encode val) (number->string val))
+                   (if (null? acc) "" "&")
+                   acc))))))
+
+  (let* ((query-string (build-query-string query))
+         (base (string->uri base-url))
+         (ref (build-relative-ref #:path path #:query query-string)))
+    (resolve-uri-reference ref base)))
+
+
 (define (json-fetch url)
   (let* ((port (http-fetch url))
          (json (json->scm port)))
     (close-port port)
     json))
 
+(define* (json-api-fetch base-url path #:rest query)
+  (json-fetch (apply api-url base-url path query)))
+
+
 (define* (queued-builds url #:optional (limit %query-limit))
   "Return the list of queued derivations on URL."
-  (let ((queue (json-fetch (string-append url "/api/queue?nr="
-                                          (number->string limit)))))
+  (let ((queue
+         (json-api-fetch url "/api/queue" `("nr" ,limit))))
     (map json->build (vector->list queue))))
 
 (define* (latest-builds url #:optional (limit %query-limit)
@@ -163,28 +191,21 @@
   "Return the latest builds performed by the CI server at URL.  If EVALUATION
 is an integer, restrict to builds of EVALUATION.  If SYSTEM is true (a system
 string such as \"x86_64-linux\"), restrict to builds for SYSTEM."
-  (define* (option name value #:optional (->string identity))
-    (if value
-        (string-append "&" name "=" (->string value))
-        ""))
-
-  (let ((latest (json-fetch (string-append url "/api/latestbuilds?nr="
-                                           (number->string limit)
-                                           (option "evaluation" evaluation
-                                                   number->string)
-                                           (option "system" system)
-                                           (option "job" job)
-                                           (option "status" status
-                                                   number->string)))))
+  (let ((latest (json-api-fetch
+                 url "/api/latestbuilds"
+                 `("nr" ,limit)
+                 `("evaluation" ,evaluation)
+                 `("system" ,system)
+                 `("job" ,job)
+                 `("status" ,status))))
     ;; Note: Hydra does not provide a "derivation" field for entries in
     ;; 'latestbuilds', but Cuirass does.
     (map json->build (vector->list latest))))
 
 (define (evaluation url evaluation)
   "Return the given EVALUATION performed by the CI server at URL."
-  (let ((evaluation (json-fetch
-                     (string-append url "/api/evaluation?id="
-                                    (number->string evaluation)))))
+  (let ((evaluation
+         (json-api-fetch url "/api/evaluation" `("id" ,evaluation))))
     (json->evaluation evaluation)))
 
 (define* (latest-evaluations url
@@ -192,16 +213,10 @@ string such as \"x86_64-linux\"), restrict to builds for SYSTEM."
                              #:key spec)
   "Return the latest evaluations performed by the CI server at URL.  If SPEC
 is passed, only consider the evaluations for the given SPEC specification."
-  (let ((spec (if spec
-                  (format #f "&spec=~a" spec)
-                  "")))
-    (map json->evaluation
-         (vector->list
-          (json->scm
-           (http-fetch
-            (string-append url "/api/evaluations?nr="
-                           (number->string limit)
-                           spec)))))))
+  (map json->evaluation
+       (vector->list
+        (json-api-fetch
+         url "/api/evaluations" `("nr" ,limit) `("spec" ,spec)))))
 
 (define* (evaluations-for-commit url commit #:optional (limit %query-limit))
   "Return the evaluations among the latest LIMIT evaluations that have COMMIT
@@ -216,16 +231,14 @@ as one of their inputs."
   "Return the list of jobs of evaluation EVALUATION-ID."
   (map json->job
        (vector->list
-        (json->scm (http-fetch
-                    (string-append url "/api/jobs?evaluation="
-                                   (number->string evaluation-id)))))))
+        (json-api-fetch url "/api/jobs" `("evaluation" ,evaluation-id)))))
 
 (define (build url id)
   "Look up build ID at URL and return it.  Raise &http-get-error if it is not
 found (404)."
   (json->build
-   (http-fetch (string-append url "/build/"       ;note: no "/api" here
-                              (number->string id)))))
+   (http-fetch (api-url url (string-append "/build/"    ;note: no "/api" here
+                                           (number->string id))))))
 
 (define (job-build url job)
   "Return the build associated with JOB."
