diff --git a/lib/server/connection.ml b/lib/server/connection.ml
index 953694e..f0f4719 100644
--- a/lib/server/connection.ml
+++ b/lib/server/connection.ml
@@ -71,6 +71,7 @@ let notonchannel chan     = "442", [chan; "You're not on that channel"]
 let notregistered         = "451", ["You have not registered"]
 let needmoreparams cmd    = "461", [cmd; "Not enough parameters"]
 let alreadyregistered     = "462", ["Unauthorized command (already registered)"]
+let modeunknownflag       = "501", ["Unknown MODE flag"]
 
 
 (* user registration *)
@@ -226,14 +227,31 @@ let on_msg_part t name =
 
 let user_get_mode user me =
   let modestr = Fmt.str "+%a" Irc.Mode.Set.pp (User.mode user) in
-  let msg = Irc.Msg.make "MODE" [User.nick user; modestr] in
+  let msg = Irc.Msg.make "MODE" [User.nick user; modestr] ~always_trailing:true in
   Router.relay msg ~from:me `to_self;
   Ok ()
 
-let user_set_mode user me modestr args =
-  let _ = me, user, modestr, args in
-  (* TODO *)
-  Error (tryagain "MODE")
+let user_set_mode user me modestr _args =
+  (* TODO: only +o can set modes for users besides themselves *)
+  let* set = try Ok (Irc.Mode.Parse.user modestr)
+    with Irc.Mode.Parse.Error ->
+      Error modeunknownflag
+  in
+  let mode = User.mode user in
+  (* TODO: only +o can set +o mode *)
+  let mode, modestr =
+    let open Irc.Mode.Set in
+    let add = diff set.add mode in
+    let rem = inter set.rem mode in
+    let mode = union mode add in
+    let mode = diff mode rem in
+    mode, Fmt.str "%a" Irc.Mode.Parse.pp_user_mode_set { add; rem }
+  in
+  let msg = Irc.Msg.make "MODE" [User.nick user; modestr] ~always_trailing:true in
+  (* TODO: if setting mode for user besides self, notify them too *)
+  Router.relay msg ~from:me `to_self;
+  User.set_mode user mode;
+  Ok ()
 
 let chan_get_mode chan me =
   let _ = me, chan in
diff --git a/lib/server/router.ml b/lib/server/router.ml
index fafc840..044a6a6 100644
--- a/lib/server/router.ml
+++ b/lib/server/router.ml
@@ -100,6 +100,9 @@ module User = struct
       `nick_set
     end
 
+  let set_mode t new_mode =
+    t.mode <- new_mode
+
   let rec part_all t =
     match Dllist.take_l t.membership with
     | m ->