diff --git a/protocol/river-control-unstable-v1.xml b/protocol/river-control-unstable-v1.xml
index d770d7d..838bba5 100644
--- a/protocol/river-control-unstable-v1.xml
+++ b/protocol/river-control-unstable-v1.xml
@@ -19,32 +19,57 @@
   <interface name="zriver_control_v1" version="1">
     <description summary="run compositor commands">
       This interface allows clients to run compositor commands and receive a
-      success/failure response with a failure message if needed.
+      success/failure response with output or a failure message respectively.
+
+      Each command is built up in a series of add_argument requests and
+      executed with a run_command request. The first argument is the command
+      to be run.
+
+      A complete list of commands should be made available in the man page of
+      the compositor.
     </description>
 
-    <request name="run_command">
-      <description summary="run a compositor command">
-        A complete list of commands will be found in the man page for the
-        compositor.
+    <request name="destroy" type="destructor">
+      <description summary="destroy the river_control object">
+        This request indicates that the client will not use the
+        river_control object any more. Objects that have been created
+        through this instance are not affected.
       </description>
-      <arg name="command" type="array" summary="the command to run as a series
-        of null-terminated strings"/>
+    </request>
+
+    <request name="add_argument">
+      <description summary="add an argument to the current command">
+        Arguments are stored by the server in the order they were sent until
+        the run_command request is made.
+      </description>
+      <arg name="argument" type="string" summary="the argument to add"/>
+    </request>
+
+    <request name="run_command">
+      <description summary="run the current command">
+        Execute the command built up using the add_argument request for the
+        given seat.
+      </description>
+      <arg name="seat" type="object" interface="wl_seat"/>
       <arg name="callback" type="new_id" interface="zriver_command_callback_v1"
-        summary="callback object to receive success/error events"/>
+        summary="callback object"/>
     </request>
   </interface>
 
   <interface name="zriver_command_callback_v1" version="1">
     <description summary="callback object">
       This object is created by the run_command request. Exactly one of the
-      success or failure events will be sent.
+      success or failure events will be sent. This object will be destroyed
+      by the compositor after one of the events is sent.
     </description>
 
     <event name="success">
       <description summary="command successful">
-        Send when the command has been successfully received and validated by
-        the server and will be carried out.
+        Sent when the command has been successfully received and executed by
+        the compositor. Some commands may produce output, in which case the
+        output argument will be a non-empty string.
       </description>
+      <arg name="output" type="string" summary="the output of the command"/>
     </event>
 
     <event name="failure">