diff --git a/include/types/connection.h b/include/types/connection.h
index 6cd8a3a..f9757e5 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -298,7 +298,7 @@
 	int  (*wake)(struct connection *conn);        /* mux-layer callback to report activity, mandatory */
 	void (*update_poll)(struct conn_stream *cs);  /* commit cs flags to mux/conn */
 	int  (*rcv_buf)(struct conn_stream *cs, struct buffer *buf, int count); /* Called from the upper layer to get data */
-	int  (*snd_buf)(struct conn_stream *cs, struct buffer *buf, int flags); /* Called from the upper layer to send data */
+	size_t (*snd_buf)(struct conn_stream *cs, const struct buffer *buf, size_t count, int flags); /* Called from the upper layer to send data */
 	int  (*rcv_pipe)(struct conn_stream *cs, struct pipe *pipe, unsigned int count); /* recv-to-pipe callback */
 	int  (*snd_pipe)(struct conn_stream *cs, struct pipe *pipe); /* send-to-pipe callback */
 	void (*shutr)(struct conn_stream *cs, enum cs_shr_mode);     /* shutr function */
diff --git a/src/checks.c b/src/checks.c
index 0150bbb..9cbc5de 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -746,7 +746,9 @@
 		goto out_unlock;
 
 	if (check->bo->o) {
-		conn->mux->snd_buf(cs, check->bo, 0);
+		b_del(check->bo, conn->mux->snd_buf(cs, check->bo, check->bo->o, 0));
+		b_realign_if_empty(check->bo);
+
 		if (conn->flags & CO_FL_ERROR || cs->flags & CS_FL_ERROR) {
 			chk_report_conn_err(check, errno, 0);
 			__cs_stop_both(cs);
@@ -2669,9 +2671,14 @@
 		    (&check->current_step->list == head ||
 		     check->current_step->action != TCPCHK_ACT_SEND ||
 		     check->current_step->string_len >= buffer_total_space(check->bo))) {
+			int ret;
 
 			__cs_want_send(cs);
-			if (conn->mux->snd_buf(cs, check->bo, 0) <= 0) {
+			ret = conn->mux->snd_buf(cs, check->bo, check->bo->o, 0);
+			b_del(check->bo, ret);
+			b_realign_if_empty(check->bo);
+
+			if (ret <= 0) {
 				if (conn->flags & CO_FL_ERROR || cs->flags & CS_FL_ERROR) {
 					chk_report_conn_err(check, errno, 0);
 					__cs_stop_both(cs);
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 2251a8e..4a39b51 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -3378,10 +3378,9 @@
 }
 
 /* Called from the upper layer, to send data */
-static int h2_snd_buf(struct conn_stream *cs, struct buffer *buf, int flags)
+static size_t h2_snd_buf(struct conn_stream *cs, const struct buffer *buf, size_t count, int flags)
 {
 	struct h2s *h2s = cs->ctx;
-	size_t count = buf->o;
 	size_t total = 0;
 	size_t ret;
 
@@ -3452,7 +3451,6 @@
 			LIST_ADDQ(&h2s->h2c->send_list, &h2s->list);
 	}
 
-	b_del(buf, total);
 	return total;
 }
 
diff --git a/src/mux_pt.c b/src/mux_pt.c
index 688b7d8..d65c4e2 100644
--- a/src/mux_pt.c
+++ b/src/mux_pt.c
@@ -172,15 +172,9 @@
 }
 
 /* Called from the upper layer, to send data */
-static int mux_pt_snd_buf(struct conn_stream *cs, struct buffer *buf, int flags)
+static size_t mux_pt_snd_buf(struct conn_stream *cs, const struct buffer *buf, size_t count, int flags)
 {
-	int ret = cs->conn->xprt->snd_buf(cs->conn, buf, buf->o, flags);
-
-	if (ret > 0)
-		b_del(buf, ret);
-
-	b_realign_if_empty(buf);
-	return ret;
+	return cs->conn->xprt->snd_buf(cs->conn, buf, count, flags);
 }
 
 #if defined(CONFIG_HAP_LINUX_SPLICE)
diff --git a/src/stream_interface.c b/src/stream_interface.c
index 62cbf71..4a095d8 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -683,11 +683,14 @@
 		if (oc->flags & CF_STREAMER)
 			send_flag |= CO_SFL_STREAMER;
 
-		ret = conn->mux->snd_buf(cs, oc->buf, send_flag);
+		ret = conn->mux->snd_buf(cs, oc->buf, co_data(oc), send_flag);
 		if (ret > 0) {
 			oc->flags |= CF_WRITE_PARTIAL | CF_WROTE_DATA | CF_WRITE_EVENT;
 
-			if (!oc->buf->o) {
+			b_del(oc->buf, ret);
+			c_realign_if_empty(oc);
+
+			if (!co_data(oc)) {
 				/* Always clear both flags once everything has been sent, they're one-shot */
 				oc->flags &= ~(CF_EXPECT_MORE | CF_SEND_DONTWAIT);
 			}