diff --git a/goblins/ocapn/captp.rkt b/goblins/ocapn/captp.rkt
index 2c2312f..19d00e7 100644
--- a/goblins/ocapn/captp.rkt
+++ b/goblins/ocapn/captp.rkt
@@ -64,6 +64,7 @@
 ;; For the handoff stuff:
 ;;   https://dustycloud.org/tmp/captp-handoff-musings.org.txt
 ;;   https://dustycloud.org/misc/3vat-handoff-scaled.jpg
+(define captp-version 0.1)
 
 (struct captp-session-severed ())
 
@@ -190,7 +191,8 @@
 
 ;; machinetp operations/descriptions
 (define-recordable-struct mtp:op:start-session
-  (handoff-pubkey
+  (captp-version
+   handoff-pubkey
    ;; a sig-envelope signed by handoff-pubkey with a <my-location $location-data>
    acceptable-location
    acceptable-location-sig)
@@ -1419,9 +1421,15 @@
           ;; TODO: Shouldn't the netlayer actually interpret this message
           ;;   before it gets here?  Ie, at this stage, we're already
           ;;   "confident" this is from the right location
-          [(mtp:op:start-session remote-encoded-pubkey
+          [(mtp:op:start-session remote-captp-version
+                                 remote-encoded-pubkey
                                  (? ocapn-machine? claimed-remote-location)
                                  remote-location-sig)
+           (unless (= remote-captp-version captp-version)
+             (error (format "CapTP version is incompatible (our version: ~a, remove version: ~a)"
+                            captp-version
+                            remote-captp-version)))
+
            (define remote-handoff-pubkey
              (datum->pk-key (gcrypt->racket/public-key remote-encoded-pubkey) 'rkt-public))
            ;; TODO: I guess we didn't know by the time this was opened
@@ -1512,7 +1520,8 @@
 
       ;; Now we'll need to send our side of the start-session and get the
       ;; other side... which will be handled by the ^setup-completer above
-      (send-to-remote (mtp:op:start-session handoff-pubkey
+      (send-to-remote (mtp:op:start-session captp-version
+                                            handoff-pubkey
                                             our-location
                                             our-location-sig))