diff --git a/pins.env b/pins.env
index a07fbcd..a3dc370 100644
--- a/pins.env
+++ b/pins.env
@@ -14,9 +14,30 @@
 # Why build it at all: Debian has no Rockchip encoder in any suite, and Radxa's
 # `gstreamer1.0-rockchip1_1.14-4` is **decode-only** — measured on a Zero 3W, it provides
 # `mppvideodec` and `mppjpegdec` and nothing else. 1.14.4 predates the encoders.
+# **Not the branch tip, and this is the interesting part of the pin.**
+#
+# Branch HEAD (dcbcd645, 2026-05-21) does not compile against the MPP Radxa ships. Commit
+# 666203a8 — "mppjpegdec: Get the required size from mpp_sys_cfg", 2026-03-06 — started using
+# `MppSysCfg` and `mpp_sys_cfg_*`, which do not exist in librockchip-mpp 1.5.0-1:
+#
+#   gstmppjpegdec.c:219:3: error: unknown type name 'MppSysCfg'
+#   gstmppjpegdec.c:235:7: error: implicit declaration of function 'mpp_sys_cfg_get'
+#
+# So this is 1ceaf42c, its parent. Everything else in the plugin builds fine against 1.5.0 —
+# `gstmppenc.c`, `gstmppjpegenc.c`, `gstmppvideodec.c` all compiled; the failure is confined to
+# the JPEG *decoder*, which nothing here needs.
+#
+# What pinning behind it costs is three commits, and none of them matter to us: a `mppvideodec`
+# input-packet-leak fix (a decoder, not the encode path), an `rkximage` aspect-ratio fix (a sink
+# this build disables), and the breaking change itself.
+#
+# The alternative is building a newer librockchip-mpp too — `JeffyCN/mirrors` has `mpp` and
+# `mpp-dev` branches — and then shipping that to the robot as well. Rejected for now: 1.5.0-1 is
+# the version hardware H.264 was *proven* on (`mpi_enc_test`, 720p, a bitstream that decodes
+# clean), and replacing it would invalidate that proof to gain a decoder fix we do not use.
 GST_ROCKCHIP_REPO=https://github.com/JeffyCN/mirrors.git
 GST_ROCKCHIP_BRANCH=gstreamer-rockchip
-GST_ROCKCHIP_REF=dcbcd6454ef892e385b3a782600369eb6c0719db
+GST_ROCKCHIP_REF=1ceaf42ca3189c2c0ca917181cf3cb6db8c4df45
 
 # ── gst-plugins-rs: webrtcsink, webrtcsrc, and the rtp sibling the same stack wants ──
 #