diff --git a/src/v4l2.c b/src/v4l2.c
index 9e2ce8b..7e09ef9 100644
--- a/src/v4l2.c
+++ b/src/v4l2.c
@@ -433,6 +433,7 @@ static int v4l2_ioctl_controls(int video_fd, int request_fd, unsigned long ioc,
 			       unsigned int num_controls)
 {
 	struct v4l2_ext_controls controls;
+	int rc;
 
 	memset(&controls, 0, sizeof(controls));
 
@@ -444,7 +445,24 @@ static int v4l2_ioctl_controls(int video_fd, int request_fd, unsigned long ioc,
 		controls.request_fd = request_fd;
 	}
 
-	return ioctl(video_fd, ioc, &controls);
+	rc = ioctl(video_fd, ioc, &controls);
+
+	/*
+	 * iter18 α-22 diagnostic: dump error_idx + request_fd after each
+	 * S_EXT_CTRLS to detect silent partial failure (mechanism 5).
+	 * The kernel sets error_idx to the failing control's index or
+	 * count (== num_controls) for "validation passed all" or
+	 * "system-wide failure". Log unconditionally so we can correlate.
+	 */
+	if (ioc == VIDIOC_S_EXT_CTRLS) {
+		request_log("α-22: S_EXT_CTRLS rc=%d errno=%d count=%u "
+			    "error_idx=%u request_fd=%d which=0x%x\n",
+			    rc, rc < 0 ? errno : 0, controls.count,
+			    controls.error_idx, controls.request_fd,
+			    controls.which);
+	}
+
+	return rc;
 }
 
 int v4l2_get_controls(int video_fd, int request_fd,