diff --git a/include/haproxy/mux_quic-t.h b/include/haproxy/mux_quic-t.h
index 9a69b7638..29d3897b1 100644
--- a/include/haproxy/mux_quic-t.h
+++ b/include/haproxy/mux_quic-t.h
@@ -140,7 +140,7 @@ struct qcs {
 struct qcc_app_ops {
 	int (*init)(struct qcc *qcc);
 	int (*attach)(struct qcs *qcs, void *conn_ctx);
-	int (*decode_qcs)(struct qcs *qcs, struct buffer *b, int fin);
+	ssize_t (*decode_qcs)(struct qcs *qcs, struct buffer *b, int fin);
 	size_t (*snd_buf)(struct stconn *sc, struct buffer *buf, size_t count, int flags);
 	void (*detach)(struct qcs *qcs);
 	int (*finalize)(void *ctx);
diff --git a/src/h3.c b/src/h3.c
index 655bf26a0..cbf12e4a7 100644
--- a/src/h3.c
+++ b/src/h3.c
@@ -145,10 +145,10 @@ DECLARE_STATIC_POOL(pool_head_h3s, "h3s", sizeof(struct h3s));
 
 /* Initialize an uni-stream <qcs> by reading its type from <b>.
  *
- * Returns 0 on success else non-zero.
+ * Returns the count of consumed bytes or a negative error code.
  */
-static int h3_init_uni_stream(struct h3c *h3c, struct qcs *qcs,
-                              struct buffer *b)
+static ssize_t h3_init_uni_stream(struct h3c *h3c, struct qcs *qcs,
+                                  struct buffer *b)
 {
 	/* decode unidirectional stream type */
 	struct h3s *h3s = qcs->ctx;
@@ -169,7 +169,7 @@ static int h3_init_uni_stream(struct h3c *h3c, struct qcs *qcs,
 	case H3_UNI_S_T_CTRL:
 		if (h3c->flags & H3_CF_UNI_CTRL_SET) {
 			qcc_emit_cc_app(qcs->qcc, H3_STREAM_CREATION_ERROR);
-			return 1;
+			return -1;
 		}
 		h3c->flags |= H3_CF_UNI_CTRL_SET;
 		h3s->type = H3S_T_CTRL;
@@ -183,7 +183,7 @@ static int h3_init_uni_stream(struct h3c *h3c, struct qcs *qcs,
 	case H3_UNI_S_T_QPACK_DEC:
 		if (h3c->flags & H3_CF_UNI_QPACK_DEC_SET) {
 			qcc_emit_cc_app(qcs->qcc, H3_STREAM_CREATION_ERROR);
-			return 1;
+			return -1;
 		}
 		h3c->flags |= H3_CF_UNI_QPACK_DEC_SET;
 		h3s->type = H3S_T_QPACK_DEC;
@@ -193,7 +193,7 @@ static int h3_init_uni_stream(struct h3c *h3c, struct qcs *qcs,
 	case H3_UNI_S_T_QPACK_ENC:
 		if (h3c->flags & H3_CF_UNI_QPACK_ENC_SET) {
 			qcc_emit_cc_app(qcs->qcc, H3_STREAM_CREATION_ERROR);
-			return 1;
+			return -1;
 		}
 		h3c->flags |= H3_CF_UNI_QPACK_ENC_SET;
 		h3s->type = H3S_T_QPACK_ENC;
@@ -207,21 +207,21 @@ static int h3_init_uni_stream(struct h3c *h3c, struct qcs *qcs,
 		 * streams that have unknown or unsupported types.
 		 */
 		qcs->flags |= QC_SF_READ_ABORTED;
-		return 1;
+		return -1;
 	};
 
 	h3s->flags |= H3_SF_UNI_INIT;
 
 	TRACE_LEAVE(H3_EV_H3S_NEW, qcs->qcc->conn, qcs);
-	return 0;
+	return len;
 }
 
 /* Parse an uni-stream <qcs> from <rxbuf> which does not contains H3 frames.
  * This may be used for QPACK encoder/decoder streams for example.
  *
- * Returns 0 on success else non-zero.
+ * Returns the number of consumed bytes or a negative error code.
  */
-static int h3_parse_uni_stream_no_h3(struct qcs *qcs, struct buffer *b)
+static ssize_t h3_parse_uni_stream_no_h3(struct qcs *qcs, struct buffer *b)
 {
 	struct h3s *h3s = qcs->ctx;
 
@@ -231,11 +231,11 @@ static int h3_parse_uni_stream_no_h3(struct qcs *qcs, struct buffer *b)
 	switch (h3s->type) {
 	case H3S_T_QPACK_DEC:
 		if (qpack_decode_dec(qcs, NULL))
-			return 1;
+			return -1;
 		break;
 	case H3S_T_QPACK_ENC:
 		if (qpack_decode_enc(qcs, NULL))
-			return 1;
+			return -1;
 		break;
 	case H3S_T_UNKNOWN:
 	default:
@@ -243,6 +243,7 @@ static int h3_parse_uni_stream_no_h3(struct qcs *qcs, struct buffer *b)
 		ABORT_NOW();
 	}
 
+	/* TODO adjust return code */
 	return 0;
 }
 
@@ -320,10 +321,10 @@ static int h3_is_frame_valid(struct h3c *h3c, struct qcs *qcs, uint64_t ftype)
  * in a local HTX buffer and transfer to the stream connector layer. <fin> must be
  * set if this is the last data to transfer from this stream.
  *
- * Returns the number of bytes handled or a negative error code.
+ * Returns the number of consumed bytes or a negative error code.
  */
-static int h3_headers_to_htx(struct qcs *qcs, const struct buffer *buf,
-                             uint64_t len, char fin)
+static ssize_t h3_headers_to_htx(struct qcs *qcs, const struct buffer *buf,
+                                 uint64_t len, char fin)
 {
 	struct buffer htx_buf = BUF_NULL;
 	struct buffer *tmp = get_trash_chunk();
@@ -418,10 +419,10 @@ static int h3_headers_to_htx(struct qcs *qcs, const struct buffer *buf,
  * HTX buffer. <fin> must be set if this is the last data to transfer from this
  * stream.
  *
- * Returns the number of bytes handled or a negative error code.
+ * Returns the number of consumed bytes or a negative error code.
  */
-static int h3_data_to_htx(struct qcs *qcs, const struct buffer *buf,
-                          uint64_t len, char fin)
+static ssize_t h3_data_to_htx(struct qcs *qcs, const struct buffer *buf,
+                              uint64_t len, char fin)
 {
 	struct buffer *appbuf;
 	struct htx *htx = NULL;
@@ -484,10 +485,10 @@ static int h3_data_to_htx(struct qcs *qcs, const struct buffer *buf,
 
 /* Parse a SETTINGS frame of length <len> of payload <buf>.
  *
- * Returns the number of bytes handled or a negative error code.
+ * Returns the number of consumed bytes or a negative error code.
  */
-static size_t h3_parse_settings_frm(struct h3c *h3c, const struct buffer *buf,
-                                    size_t len)
+static ssize_t h3_parse_settings_frm(struct h3c *h3c, const struct buffer *buf,
+                                     size_t len)
 {
 	struct buffer b;
 	uint64_t id, value;
@@ -568,26 +569,30 @@ static size_t h3_parse_settings_frm(struct h3c *h3c, const struct buffer *buf,
  *
  * Returns 0 on success else non-zero.
  */
-static int h3_decode_qcs(struct qcs *qcs, struct buffer *b, int fin)
+static ssize_t h3_decode_qcs(struct qcs *qcs, struct buffer *b, int fin)
 {
 	struct h3s *h3s = qcs->ctx;
 	struct h3c *h3c = h3s->h3c;
-	ssize_t ret;
+	ssize_t total = 0, ret;
 
 	h3_debug_printf(stderr, "%s: STREAM ID: %lu\n", __func__, qcs->id);
 	if (!b_data(b))
 		return 0;
 
 	if (quic_stream_is_uni(qcs->id) && !(h3s->flags & H3_SF_UNI_INIT)) {
-		if (h3_init_uni_stream(h3c, qcs, b))
-			return 1;
+		if ((ret = h3_init_uni_stream(h3c, qcs, b)) < 0)
+			return -1;
+
+		total += ret;
 	}
 
 	if (quic_stream_is_uni(qcs->id) && (h3s->flags & H3_SF_UNI_NO_H3)) {
 		/* For non-h3 STREAM, parse it and return immediately. */
-		if (h3_parse_uni_stream_no_h3(qcs, b))
-			return 1;
-		return 0;
+		if ((ret = h3_parse_uni_stream_no_h3(qcs, b)) < 0)
+			return -1;
+
+		total += ret;
+		return total;
 	}
 
 	while (b_data(b) && !(qcs->flags & QC_SF_DEM_FULL)) {
@@ -605,6 +610,7 @@ static int h3_decode_qcs(struct qcs *qcs, struct buffer *b, int fin)
 
 			h3s->demux_frame_type = ftype;
 			h3s->demux_frame_len = flen;
+			total += hlen;
 
 			if (!h3_is_frame_valid(h3c, qcs, ftype)) {
 				qcc_emit_cc_app(qcs->qcc, H3_FRAME_UNEXPECTED);
@@ -679,6 +685,7 @@ static int h3_decode_qcs(struct qcs *qcs, struct buffer *b, int fin)
 			BUG_ON(h3s->demux_frame_len < ret);
 			h3s->demux_frame_len -= ret;
 			b_del(b, ret);
+			total += ret;
 		}
 	}
 
@@ -686,7 +693,7 @@ static int h3_decode_qcs(struct qcs *qcs, struct buffer *b, int fin)
 	 * However, currently, io-cb of MUX does not handle Rx.
 	 */
 
-	return 0;
+	return total;
 }
 
 /* Returns buffer for data sending.
diff --git a/src/hq_interop.c b/src/hq_interop.c
index 4b4b5222d..5000a7e2e 100644
--- a/src/hq_interop.c
+++ b/src/hq_interop.c
@@ -8,7 +8,7 @@
 #include <haproxy/http.h>
 #include <haproxy/mux_quic.h>
 
-static int hq_interop_decode_qcs(struct qcs *qcs, struct buffer *b, int fin)
+static ssize_t hq_interop_decode_qcs(struct qcs *qcs, struct buffer *b, int fin)
 {
 	struct htx *htx;
 	struct htx_sl *sl;
@@ -62,7 +62,7 @@ static int hq_interop_decode_qcs(struct qcs *qcs, struct buffer *b, int fin)
 
 	sl = htx_add_stline(htx, HTX_BLK_REQ_SL, 0, ist("GET"), path, ist("HTTP/1.0"));
 	if (!sl)
-		return 1;
+		return -1;
 
 	sl->flags |= HTX_SL_F_BODYLESS;
 	sl->info.req.meth = find_http_meth("GET", 3);
@@ -72,15 +72,14 @@ static int hq_interop_decode_qcs(struct qcs *qcs, struct buffer *b, int fin)
 
 	sc = qc_attach_sc(qcs, &htx_buf);
 	if (!sc)
-		return 1;
+		return -1;
 
-	b_reset(b);
 	b_free(&htx_buf);
 
 	if (fin)
 		htx->flags |= HTX_FL_EOM;
 
-	return 0;
+	return b_data(b);
 }
 
 static struct buffer *mux_get_buf(struct qcs *qcs)
diff --git a/src/mux_quic.c b/src/mux_quic.c
index 28a1896a5..c60fdfca8 100644
--- a/src/mux_quic.c
+++ b/src/mux_quic.c
@@ -439,24 +439,19 @@ static void qcs_consume(struct qcs *qcs, uint64_t bytes)
 static int qcc_decode_qcs(struct qcc *qcc, struct qcs *qcs)
 {
 	struct buffer b;
-	size_t data, done;
-	int ret;
+	ssize_t ret;
 
 	TRACE_ENTER(QMUX_EV_QCS_RECV, qcc->conn, qcs);
 
 	b = qcs_b_dup(&qcs->rx.ncbuf);
-	data = b_data(&b);
-
 	ret = qcc->app_ops->decode_qcs(qcs, &b, qcs->flags & QC_SF_FIN_RECV);
-	if (ret) {
+	if (ret < 0) {
 		TRACE_DEVEL("leaving on decoding error", QMUX_EV_QCS_RECV, qcc->conn, qcs);
 		return 1;
 	}
 
-	BUG_ON_HOT(data < b_data(&b));
-	done = data - b_data(&b);
-	if (done) {
-		qcs_consume(qcs, done);
+	if (ret) {
+		qcs_consume(qcs, ret);
 		qcs_notify_recv(qcs);
 	}