diff --git a/guix/build/download-nar.scm b/guix/build/download-nar.scm
index f26ad28cd0..eb358d69d9 100644
--- a/guix/build/download-nar.scm
+++ b/guix/build/download-nar.scm
@@ -22,6 +22,7 @@
   #:autoload   (lzlib) (call-with-lzip-input-port)
   #:use-module (guix progress)
   #:use-module (web uri)
+  #:use-module (web response)
   #:use-module (srfi srfi-11)
   #:use-module (srfi srfi-26)
   #:use-module (ice-9 format)
@@ -69,7 +70,7 @@ item.  Return #t on success, #f otherwise."
       ((url rest ...)
        (format #t "Trying content-addressed mirror at ~a...~%"
                (uri-host (string->uri url)))
-       (let-values (((port size)
+       (let-values (((port response)
                      (catch #t
                        (lambda ()
                          (http-fetch (string->uri url)))
@@ -81,7 +82,7 @@ item.  Return #t on success, #f otherwise."
                          (values #f #f)))))
          (if (not port)
              (loop rest)
-             (begin
+             (let ((size (response-content-length response)))
                (if size
                    (format #t "Downloading from ~a (~,2h MiB)...~%" url
                            (/ size (expt 2 20.)))
diff --git a/guix/build/download.scm b/guix/build/download.scm
index 509dcc08c7..53a0997164 100644
--- a/guix/build/download.scm
+++ b/guix/build/download.scm
@@ -22,6 +22,7 @@
 (define-module (guix build download)
   #:use-module (web uri)
   #:use-module (web http)
+  #:use-module (web response)
   #:use-module ((web client) #:hide (open-socket-for-uri))
   #:use-module (web response)
   #:use-module (guix base64)
@@ -752,7 +753,7 @@ otherwise simply ignore them."
     (case (uri-scheme uri)
       ((http https)
        (false-if-exception*
-        (let-values (((port size)
+        (let-values (((port response)
                       (http-fetch uri
                                   #:verify-certificate? verify-certificate?
                                   #:timeout timeout)))
@@ -762,9 +763,13 @@ otherwise simply ignore them."
                           #:buffer-size %http-receive-buffer-size
                           #:reporter (if print-build-trace?
                                          (progress-reporter/trace
-                                          file (uri->string uri) size)
+                                          file (uri->string uri)
+                                          (response-content-length
+                                           response))
                                          (progress-reporter/file
-                                          (uri-abbreviation uri) size)))
+                                          (uri-abbreviation uri)
+                                          (response-content-length
+                                           response))))
               (newline)))
           (close-port port)
           file)))
diff --git a/guix/http-client.scm b/guix/http-client.scm
index 4e0cc59e91..4622dbdcb6 100644
--- a/guix/http-client.scm
+++ b/guix/http-client.scm
@@ -83,11 +83,11 @@
                      (headers '((user-agent . "GNU Guile")))
                      (log-port (current-error-port))
                      timeout)
-  "Return an input port containing the data at URI, and the expected number of
-bytes available or #f.  If TEXT? is true, the data at URI is considered to be
-textual.  Follow any HTTP redirection.  When BUFFERED? is #f, return an
-unbuffered port, suitable for use in `filtered-port'.  HEADERS is an alist of
-extra HTTP headers.
+  "Return an input port containing the data at URI, and the HTTP response from
+the server.  If TEXT? is true, the data at URI is considered to be textual.
+Follow any HTTP redirection.  When BUFFERED? is #f, return an unbuffered port,
+suitable for use in `filtered-port'.  HEADERS is an alist of extra HTTP
+headers.
 
 When KEEP-ALIVE? is true, the connection is marked as 'keep-alive' and PORT is
 not closed upon completion.
@@ -126,7 +126,7 @@ Raise an '&http-get-error' condition if downloading fails."
                      (response-code resp)))
         (case code
           ((200)
-           (values data (response-content-length resp)))
+           (values data resp))
           ((301                                   ; moved permanently
             302                                   ; found (redirection)
             303                                   ; see other
diff --git a/guix/scripts/challenge.scm b/guix/scripts/challenge.scm
index d38171b868..a1f90b7844 100644
--- a/guix/scripts/challenge.scm
+++ b/guix/scripts/challenge.scm
@@ -44,6 +44,7 @@
   #:use-module (ice-9 format)
   #:use-module (ice-9 ftw)
   #:use-module (web uri)
+  #:use-module (web response)
   #:export (compare-contents
 
             comparison-report?
@@ -257,11 +258,14 @@ in the nar."
   "Call PROC with an input port from which it can read the nar pointed to by
 NARINFO."
   (let* ((uri compression size (narinfo-best-uri narinfo))
-         (port actual-size     (http-fetch uri)))
+         (port response        (http-fetch uri)))
     (define reporter
       (progress-reporter/file (narinfo-path narinfo)
                               (and size
-                                   (max size (or actual-size 0))) ;defensive
+                                   (max size (or
+                                              (response-content-length
+                                               response)
+                                              0))) ;defensive
                               #:abbreviation (const (uri-host uri))))
 
     (define result
diff --git a/guix/scripts/substitute.scm b/guix/scripts/substitute.scm
index aa702abc60..2f86e3c55e 100755
--- a/guix/scripts/substitute.scm
+++ b/guix/scripts/substitute.scm
@@ -61,6 +61,7 @@
   #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-71)
   #:use-module (web uri)
+  #:use-module (web response)
   #:use-module (guix http-client)
   #:export (%allow-unauthenticated-substitutes?
             %reply-file-descriptor
@@ -496,10 +497,14 @@ STATUS-PORT."
                     (uri->string uri))
            (warning (G_ "try `--no-substitutes' if the problem persists~%")))
          (with-cached-connection uri port
-           (http-fetch uri #:text? #f
-                       #:port port
-                       #:keep-alive? #t
-                       #:buffered? #f))))
+           (let ((raw
+                  response
+                  (http-fetch uri #:text? #f
+                              #:port port
+                              #:keep-alive? #t
+                              #:buffered? #f)))
+             (values raw
+                     (response-content-length response))))))
       (else
        (raise
         (formatted-message