diff --git a/syndicate/drivers/http.rkt b/syndicate/drivers/http.rkt
index 2deb425..f1bfef2 100644
--- a/syndicate/drivers/http.rkt
+++ b/syndicate/drivers/http.rkt
@@ -3,6 +3,8 @@
 ;;; SPDX-FileCopyrightText: Copyright © 2022 Tony Garnock-Jones <tonyg@leastfixedpoint.com>
 
 (provide (all-from-out syndicate/schemas/http)
+         (struct-out mime)
+         ->mime
          define-http-route
          define-plain-http-route
          headers-ref
@@ -251,6 +253,15 @@
 
 ;;---------------------------------------------------------------------------
 
+(struct mime (type data) #:prefab)
+
+(define (->mime v)
+  (match v
+    [(mime (? symbol?) (? bytes?)) v]
+    [(? string?) (mime 'text/plain (string->bytes/utf-8 v))]
+    [(? bytes?) (mime 'application/octet-stream v)]
+    [_ v]))
+
 (define-syntax define-plain-http-route
   (lambda (stx)
     (syntax-parse stx
@@ -265,7 +276,19 @@
            (lambda (req res)
              (match (HttpRequest-path req)
                [#,(match-quote-path-pattern #'(path-pattern-element ...))
-                (async body ...)]
+                (async (match (with-handlers [((lambda (e) #t)
+                                               (lambda (e)
+                                                 (send-http-response! res 500 "Internal Server Error")
+                                                 (raise e)))]
+                                (let () body ...))
+                         [(mime (? symbol? type) (? bytes? data))
+                          (log-syndicate/drivers/http-debug "REPLY: ~a ~v" type data)
+                          (send-http-response! res 200 "OK" #:mime-type type data)]
+                         [(? void?)
+                          (log-syndicate/drivers/http-debug "Assuming that reply sent previously")]
+                         [bad
+                          (log-syndicate/drivers/http-error "Bad MIME response: ~v" bad)
+                          (send-http-response! res 500 "Internal Server Error")]))]
                [_ (send-http-response! res 400 "Bad request path")]))))])))
 
 (define (define-http-route* ds host port method path-pattern handler)
@@ -333,7 +356,8 @@
   (let loop ((chunks chunks))
     (cond [(null? chunks) (send! res (HttpResponse-done ""))]
           [(null? (cdr chunks)) (send! res (HttpResponse-done (car chunks)))]
-          [else (send! res (HttpResponse-chunk (car chunks))) (loop (cdr chunks))])))
+          [else (send! res (HttpResponse-chunk (car chunks))) (loop (cdr chunks))]))
+  (void))
 
 ;;---------------------------------------------------------------------------
 
@@ -354,41 +378,29 @@
             (define info (let () parse-body ...)) ...
             (values info ...))))
       (log-syndicate/drivers/http-debug "INFO: ~a = ~a" 'info (pretty-format info)) ...
-      (call-with-values
-       (lambda () body ...)
-       (lambda vs
-         (match vs
-           [(list mime-type reply)
-            (log-syndicate/drivers/http-debug "REPLY: ~a ~a" mime-type (pretty-format reply))
-            (send-http-response! res 200 "OK" #:mime-type mime-type reply)]
-           [(or (list #f #f)
-                (list #f)
-                (list (? void?)))
-            (log-syndicate/drivers/http-debug "NO IMMEDIATE REPLY")]))))))
+      (->mime (let () body ...)))))
 
 (define-syntax-rule (define-json-route [pat ... res]
                       [(info parse-body ...) ...]
                       body ...)
   (define-http-route [pat ... res]
     [(info parse-body ...) ...]
-    (define reply (let () body ...))
-    (if (and reply (not (void? reply)))
-        (json-response reply)
-        (values #f #f))))
+    (match (let () body ...) [(? void?) (void)] [j (json-response j)])))
 
 (define (json-body req)
   (string->jsexpr (bytes->string/utf-8 (RequestBody-present-value (HttpRequest-body req)))))
 
 (define (json-response v)
-  (values 'application/json (jsexpr->string (->preserve v))))
+  (mime 'application/json (string->bytes/utf-8 (jsexpr->string (->preserve v)))))
 
 (define (send-json! res v)
   (send-http-response! res 200 "OK" #:mime-type 'application/json (jsexpr->string (->preserve v))))
 
-(define (resolve-req-relative-uri req rel)
+(define (resolve-req-relative-uri #:scheme [scheme "https"] req rel)
   (local-require net/url)
   (url->string (combine-url/relative
-                (string->url (format "https://~a/~a"
+                (string->url (format "~a://~a/~a"
+                                     scheme
                                      (HttpRequest-host req)
                                      (string-join (HttpRequest-path req) "/")))
                 rel)))