diff --git a/pscp.c b/pscp.c
index d759fb4e..ec4dfa8a 100644
--- a/pscp.c
+++ b/pscp.c
@@ -67,6 +67,7 @@ static const SeatVtable pscp_seat_vt = {
     .eof = pscp_eof,
     .sent = nullseat_sent,
     .get_userpass_input = filexfer_get_userpass_input,
+    .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = console_connection_fatal,
diff --git a/psftp.c b/psftp.c
index ea52b23e..16beee56 100644
--- a/psftp.c
+++ b/psftp.c
@@ -49,6 +49,7 @@ static const SeatVtable psftp_seat_vt = {
     .eof = psftp_eof,
     .sent = nullseat_sent,
     .get_userpass_input = filexfer_get_userpass_input,
+    .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = console_connection_fatal,
diff --git a/putty.h b/putty.h
index 4492d515..d622574d 100644
--- a/putty.h
+++ b/putty.h
@@ -620,6 +620,8 @@ enum {
 #define BACKEND_NEEDS_TERMINAL      0x02   /* Backend must have terminal */
 #define BACKEND_SUPPORTS_NC_HOST    0x04   /* Backend can honour
                                               CONF_ssh_nc_host */
+#define BACKEND_NOTIFIES_SESSION_START 0x08 /* Backend will call
+                                               seat_notify_session_started */
 
 /* In (no)sshproxy.c */
 extern const bool ssh_proxy_supported;
@@ -932,6 +934,17 @@ struct SeatVtable {
      */
     int (*get_userpass_input)(Seat *seat, prompts_t *p, bufchain *input);
 
+    /*
+     * Notify the seat that the main session channel has been
+     * successfully set up.
+     *
+     * This is only used as part of the SSH proxying system, so it's
+     * not necessary to implement it in all backends. A backend must
+     * call this if it advertises the BACKEND_NOTIFIES_SESSION_START
+     * flag, and otherwise, doesn't have to.
+     */
+    void (*notify_session_started)(Seat *seat);
+
     /*
      * Notify the seat that the process running at the other end of
      * the connection has finished.
@@ -1138,6 +1151,8 @@ static inline void seat_sent(Seat *seat, size_t bufsize)
 static inline int seat_get_userpass_input(
     Seat *seat, prompts_t *p, bufchain *input)
 { return seat->vt->get_userpass_input(seat, p, input); }
+static inline void seat_notify_session_started(Seat *seat)
+{ seat->vt->notify_session_started(seat); }
 static inline void seat_notify_remote_exit(Seat *seat)
 { seat->vt->notify_remote_exit(seat); }
 static inline void seat_notify_remote_disconnect(Seat *seat)
@@ -1212,6 +1227,7 @@ size_t nullseat_output(
 bool nullseat_eof(Seat *seat);
 void nullseat_sent(Seat *seat, size_t bufsize);
 int nullseat_get_userpass_input(Seat *seat, prompts_t *p, bufchain *input);
+void nullseat_notify_session_started(Seat *seat);
 void nullseat_notify_remote_exit(Seat *seat);
 void nullseat_notify_remote_disconnect(Seat *seat);
 void nullseat_connection_fatal(Seat *seat, const char *message);
diff --git a/ssh/mainchan.c b/ssh/mainchan.c
index 70033a7a..2e690547 100644
--- a/ssh/mainchan.c
+++ b/ssh/mainchan.c
@@ -129,6 +129,7 @@ static void mainchan_open_confirmation(Channel *chan)
 
     seat_update_specials_menu(mc->ppl->seat);
     ppl_logevent("Opened main channel");
+    seat_notify_session_started(mc->ppl->seat);
 
     if (mc->is_simple)
         sshfwd_hint_channel_is_simple(mc->sc);
diff --git a/ssh/server.c b/ssh/server.c
index e6d7f605..1517522b 100644
--- a/ssh/server.c
+++ b/ssh/server.c
@@ -109,6 +109,7 @@ static const SeatVtable server_seat_vt = {
     .eof = nullseat_eof,
     .sent = nullseat_sent,
     .get_userpass_input = nullseat_get_userpass_input,
+    .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = nullseat_connection_fatal,
diff --git a/ssh/sesschan.c b/ssh/sesschan.c
index e1496023..527b7603 100644
--- a/ssh/sesschan.c
+++ b/ssh/sesschan.c
@@ -188,6 +188,7 @@ static const SeatVtable sesschan_seat_vt = {
     .eof = sesschan_seat_eof,
     .sent = nullseat_sent,
     .get_userpass_input = nullseat_get_userpass_input,
+    .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = sesschan_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = sesschan_connection_fatal,
diff --git a/ssh/ssh.c b/ssh/ssh.c
index 4ad10169..aee04db9 100644
--- a/ssh/ssh.c
+++ b/ssh/ssh.c
@@ -1230,7 +1230,7 @@ const BackendVtable ssh_backend = {
     .id = "ssh",
     .displayname = "SSH",
     .protocol = PROT_SSH,
-    .flags = BACKEND_SUPPORTS_NC_HOST,
+    .flags = BACKEND_SUPPORTS_NC_HOST | BACKEND_NOTIFIES_SESSION_START,
     .default_port = 22,
 };
 
@@ -1255,5 +1255,5 @@ const BackendVtable sshconn_backend = {
     .id = "ssh-connection",
     .displayname = "Bare ssh-connection",
     .protocol = PROT_SSHCONN,
-    .flags = BACKEND_SUPPORTS_NC_HOST,
+    .flags = BACKEND_SUPPORTS_NC_HOST | BACKEND_NOTIFIES_SESSION_START,
 };
diff --git a/sshproxy.c b/sshproxy.c
index 9c6708ab..feb9b077 100644
--- a/sshproxy.c
+++ b/sshproxy.c
@@ -66,6 +66,7 @@ typedef struct SshProxy {
     bool rcvd_eof_ssh_to_socket, sent_eof_ssh_to_socket;
 
     SockAddr *addr;
+    int port;
 
     /* Traits implemented: we're a Socket from the point of view of
      * the client connection, and a Seat from the POV of the SSH
@@ -245,6 +246,12 @@ static void try_send_ssh_to_socket(void *ctx)
     }
 }
 
+static void sshproxy_notify_session_started(Seat *seat)
+{
+    SshProxy *sp = container_of(seat, SshProxy, seat);
+    plug_log(sp->plug, PLUGLOG_CONNECT_SUCCESS, sp->addr, sp->port, NULL, 0);
+}
+
 static size_t sshproxy_output(Seat *seat, bool is_stderr,
                               const void *data, size_t len)
 {
@@ -431,6 +438,7 @@ static const SeatVtable SshProxy_seat_vt = {
     .eof = sshproxy_eof,
     .sent = sshproxy_sent,
     .get_userpass_input = sshproxy_get_userpass_input,
+    .notify_session_started = sshproxy_notify_session_started,
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = sshproxy_notify_remote_disconnect,
     .connection_fatal = sshproxy_connection_fatal,
@@ -470,6 +478,7 @@ Socket *sshproxy_new_connection(SockAddr *addr, const char *hostname,
     bufchain_init(&sp->ssh_to_socket);
 
     sp->addr = addr;
+    sp->port = port;
 
     sp->conf = conf_new();
     /* Try to treat proxy_hostname as the title of a saved session. If
@@ -513,6 +522,14 @@ Socket *sshproxy_new_connection(SockAddr *addr, const char *hostname,
         return &sp->sock;
     }
 
+    /*
+     * We also expect that the backend will announce a willingness to
+     * notify us that the session has started. Any backend providing
+     * NC_HOST should also provide this.
+     */
+    assert(backvt->flags & BACKEND_NOTIFIES_SESSION_START &&
+           "Backend provides NC_HOST without SESSION_START!");
+
     /*
      * Turn off SSH features we definitely don't want. It would be
      * awkward and counterintuitive to have the proxy SSH connection
diff --git a/unix/plink.c b/unix/plink.c
index a1f640a1..f8acd5ec 100644
--- a/unix/plink.c
+++ b/unix/plink.c
@@ -391,6 +391,7 @@ static const SeatVtable plink_seat_vt = {
     .eof = plink_eof,
     .sent = nullseat_sent,
     .get_userpass_input = plink_get_userpass_input,
+    .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = console_connection_fatal,
diff --git a/unix/window.c b/unix/window.c
index e5b541c8..cb6e831d 100644
--- a/unix/window.c
+++ b/unix/window.c
@@ -392,6 +392,7 @@ static const SeatVtable gtk_seat_vt = {
     .eof = gtk_seat_eof,
     .sent = nullseat_sent,
     .get_userpass_input = gtk_seat_get_userpass_input,
+    .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = gtk_seat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = gtk_seat_connection_fatal,
diff --git a/utils/nullseat.c b/utils/nullseat.c
index 5e601669..907d9176 100644
--- a/utils/nullseat.c
+++ b/utils/nullseat.c
@@ -10,6 +10,7 @@ bool nullseat_eof(Seat *seat) { return true; }
 void nullseat_sent(Seat *seat, size_t bufsize) {}
 int nullseat_get_userpass_input(
     Seat *seat, prompts_t *p, bufchain *input) { return 0; }
+void nullseat_notify_session_started(Seat *seat) {}
 void nullseat_notify_remote_exit(Seat *seat) {}
 void nullseat_notify_remote_disconnect(Seat *seat) {}
 void nullseat_connection_fatal(Seat *seat, const char *message) {}
diff --git a/windows/plink.c b/windows/plink.c
index 5bb001be..1587cbd3 100644
--- a/windows/plink.c
+++ b/windows/plink.c
@@ -85,6 +85,7 @@ static const SeatVtable plink_seat_vt = {
     .eof = plink_eof,
     .sent = nullseat_sent,
     .get_userpass_input = plink_get_userpass_input,
+    .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = console_connection_fatal,
diff --git a/windows/window.c b/windows/window.c
index 9a273d83..6c288600 100644
--- a/windows/window.c
+++ b/windows/window.c
@@ -333,6 +333,7 @@ static const SeatVtable win_seat_vt = {
     .eof = win_seat_eof,
     .sent = nullseat_sent,
     .get_userpass_input = win_seat_get_userpass_input,
+    .notify_session_started = nullseat_notify_session_started,
     .notify_remote_exit = win_seat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = win_seat_connection_fatal,