diff --git a/lib/aws.mli b/lib/aws.mli
index e618e5a36..ed79709aa 100644
--- a/lib/aws.mli
+++ b/lib/aws.mli
@@ -285,6 +285,10 @@ module Util : sig
   val option_all : 'a option list -> 'a list option
   (** If all values in list are Some v, produce Some (list_filter_opt
       list), else produce None. *)
+
+  val string_starts_with : string -> string -> bool
+  (** Judges whether s starts with prefix.
+      It is ported from stdlib in OCaml 4.13. *)
 end
 
 (** This module contains the V2 and V4 Authorization header AWS signature algorithm. *)
diff --git a/lib/endpoints.ml b/lib/endpoints.ml
index b28c0d4d9..3ba3117b3 100644
--- a/lib/endpoints.ml
+++ b/lib/endpoints.ml
@@ -2049,5 +2049,7 @@ let endpoint_of svc_name region =
   | _ -> None
 let url_of svc_name region =
   match endpoint_of svc_name region with
-  | Some var -> Some ("https://" ^ var)
-  | None -> None
\ No newline at end of file
+  | Some var when Util.string_starts_with "localhost" var ->
+      Some "http://localhost:8000"
+  | Some var -> Some (String.concat "" ["https://"; var])
+  | None -> None
diff --git a/lib/util.ml b/lib/util.ml
index c45a805f0..5131282c2 100644
--- a/lib/util.ml
+++ b/lib/util.ml
@@ -37,3 +37,15 @@ let rec option_all = function
   | [] -> Some []
   | Some v :: xs -> option_bind (option_all xs) (fun rest -> Some (v :: rest))
   | None :: _ -> None
+
+let string_starts_with prefix s =
+  let open String in
+  let len_s = length s and len_pre = length prefix in
+  let rec aux i =
+    if i = len_pre
+    then true
+    else if unsafe_get s i <> unsafe_get prefix i
+    then false
+    else aux (i + 1)
+  in
+  len_s >= len_pre && aux 0
diff --git a/src/endpoint_gen.ml b/src/endpoint_gen.ml
index ee688f833..8a575b230 100644
--- a/src/endpoint_gen.ml
+++ b/src/endpoint_gen.ml
@@ -54,10 +54,26 @@ let write_url_of =
       (fun2
          "svc_name"
          "region"
-         (matchoption
+         (match_
             (app2 "endpoint_of" (ident "svc_name") (ident "region"))
-            (app1 "Some" (app2 "^" (str "https://") (ident "var")))
-            (ident "None"))))
+            [ (let some_v = "var" in
+               casearm
+                 (lid "Some")
+                 (Some (pvar some_v))
+                 ~guard:(app2 "Util.string_starts_with" (str "localhost") (ident some_v))
+                 (app1 "Some" (str "http://localhost:8000")))
+            ; (let some_v = "var" in
+               casearm
+                 (lid "Some")
+                 (Some (pvar some_v))
+                 (app1
+                    "Some"
+                    (app2
+                       "String.concat"
+                       (str "")
+                       (list [ ident some_v; str "https://" ]))))
+            ; casearm (lid "None") None (ident "None")
+            ])))
 
 let main input outdir =
   log "Start processing endpoints";
diff --git a/src/syntax.ml b/src/syntax.ml
index 49aa629b7..1b20130c2 100644
--- a/src/syntax.ml
+++ b/src/syntax.ml
@@ -258,6 +258,12 @@ let withty _nm0 nm1 =
 (* if cond then thn else els *)
 let ifthen cond thn els = Exp.ifthenelse cond thn (Some els)
 
+let match_ = Exp.match_
+
+let casearm ?guard lid pat body = Exp.case (Pat.construct lid pat) ?guard body
+
+let pvar v = Pat.var (strloc v)
+
 (* match exp with | Constructor -> body | Constructor -> body ... *)
 let matchvar exp branches =
   Exp.match_
@@ -266,14 +272,14 @@ let matchvar exp branches =
 
 (* match exp with | "String" -> body ... | _ -> els ... *)
 let matchstrs exp branches els =
-  Exp.match_
+  match_
     exp
     (List.map (fun (nm, body) -> Exp.case (Pat.constant (Const.string nm)) body) branches
     @ [ Exp.case (Pat.any ()) els ])
 
 (* match exp with | Some var -> some_body | None -> none_body *)
 let matchoption exp some_body none_body =
-  Exp.match_
+  match_
     exp
     [ Exp.case (construct (lid "Some") (Some (Pat.var (strloc "var")))) some_body
     ; Exp.case (construct (lid "None") None) none_body