diff --git a/include/haproxy/applet-t.h b/include/haproxy/applet-t.h
index adeb9da12..8b31e02bc 100644
--- a/include/haproxy/applet-t.h
+++ b/include/haproxy/applet-t.h
@@ -82,6 +82,7 @@ static forceinline char *appctx_show_flags(char *buf, size_t len, const char *de
 }
 
 #define APPLET_FL_NEW_API 0x00000001 /* Set if the applet is based on the new API (using applet's buffers) */
+#define APPLET_FL_WARNED  0x00000002 /* Set when warning was already emitted about a legacy applet */
 
 /* Applet descriptor */
 struct applet {
diff --git a/src/applet.c b/src/applet.c
index 1cd10e038..6bfff12de 100644
--- a/src/applet.c
+++ b/src/applet.c
@@ -19,6 +19,7 @@
 #include <haproxy/channel.h>
 #include <haproxy/htx.h>
 #include <haproxy/list.h>
+#include <haproxy/log.h>
 #include <haproxy/sc_strm.h>
 #include <haproxy/stconn.h>
 #include <haproxy/stream.h>
@@ -232,6 +233,13 @@ struct appctx *appctx_new_on(struct applet *applet, struct sedesc *sedesc, int t
 
 	TRACE_ENTER(APPLET_EV_NEW);
 
+	if (unlikely(!(applet->flags & (APPLET_FL_NEW_API|APPLET_FL_WARNED)))) {
+		send_log(NULL, LOG_WARNING,
+			 "Applet '%s' is based on a deprecated API. Please report this error to developers\n",
+			 applet->name);
+		applet->flags |= APPLET_FL_WARNED;
+	}
+
 	appctx = pool_zalloc(pool_head_appctx);
 	if (unlikely(!appctx)) {
 		TRACE_ERROR("APPCTX allocation failure", APPLET_EV_NEW|APPLET_EV_ERR);