diff --git a/src/h265.c b/src/h265.c
index ba764ee..bde26f8 100644
--- a/src/h265.c
+++ b/src/h265.c
@@ -70,6 +70,7 @@
 #include "surface.h"
 
 #include <assert.h>
+#include <errno.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -79,6 +80,7 @@
 #include <linux/videodev2.h>
 #include <linux/v4l2-controls.h>
 
+#include "utils.h"
 #include "v4l2.h"
 
 /*
@@ -627,6 +629,42 @@ int h265_set_controls(struct request_data *driver_data,
 			       surface_object->request_fd,
 			       controls, n);
 
+	/*
+	 * iter21 α-24 (diagnostic — gated by LIBVA_V4L2_REQ_GETBACK env):
+	 * immediately after S_EXT_CTRLS staging, attempt G_EXT_CTRLS readback
+	 * via the same request_fd for the SPS control. Outcome interpretation:
+	 *   EACCES        -> kernel disallows req readback; switch to kernel printk path.
+	 *   non-zero SPS  -> req->p_new HAS libva's payload; failure is in
+	 *                    v4l2_ctrl_request_setup application path.
+	 *   zero SPS      -> req->p_new doesn't have libva's payload; failure
+	 *                    is in v4l2_s_ext_ctrls request-staging itself.
+	 */
+	if (rc >= 0 && getenv("LIBVA_V4L2_REQ_GETBACK") != NULL) {
+		struct v4l2_ctrl_hevc_sps sps_rb;
+		struct v4l2_ext_control ctrl_rb = {
+			.id = V4L2_CID_STATELESS_HEVC_SPS,
+			.ptr = &sps_rb,
+			.size = sizeof(sps_rb),
+		};
+		int rb;
+
+		memset(&sps_rb, 0, sizeof(sps_rb));
+		rb = v4l2_get_controls(driver_data->video_fd,
+				       surface_object->request_fd,
+				       &ctrl_rb, 1);
+		if (rb < 0) {
+			request_log("α-24 G_EXT_CTRLS readback FAILED errno=%d (%s)\n",
+				    errno, strerror(errno));
+		} else {
+			uint8_t *p = (uint8_t *)&sps_rb;
+			request_log("α-24 G_EXT_CTRLS readback OK SPS[0..16]="
+				    "%02x %02x %02x %02x %02x %02x %02x %02x "
+				    "%02x %02x %02x %02x %02x %02x %02x %02x\n",
+				    p[0], p[1], p[2], p[3], p[4], p[5], p[6], p[7],
+				    p[8], p[9], p[10], p[11], p[12], p[13], p[14], p[15]);
+		}
+	}
+
 	free(slice_params_array);
 
 	if (rc < 0)