diff --git a/.gitea/workflows/build-kernel.yml b/.gitea/workflows/build-kernel.yml
index 8b4978b..7add102 100644
--- a/.gitea/workflows/build-kernel.yml
+++ b/.gitea/workflows/build-kernel.yml
@@ -27,8 +27,14 @@ jobs:
       # Don't cancel the other versions if one fails to build.
       fail-fast: false
       matrix:
-        kernel_version:
-          - "1:6.12.93-1+rpt1"
+        # Each raspberrypi archive suite only carries its own latest kernel, so
+        # the Debian base suite is paired with the kernel version it can install:
+        #   bookworm -> 6.12.x, trixie -> 6.18.x.
+        include:
+          - kernel_version: "1:6.12.93-1+rpt1"
+            suite: bookworm
+          - kernel_version: "1:6.18.34-1+rpt1"
+            suite: trixie
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -38,6 +44,7 @@ jobs:
         env:
           HEADERS_PKG: ${{ env.HEADERS_PKG }}
           KERNEL_VERSION: ${{ matrix.kernel_version }}
+          DEBIAN_SUITE: ${{ matrix.suite }}
         run: |
           set -eux
           # arm64 emulation for the builder image (no-op if already registered).
@@ -46,6 +53,7 @@ jobs:
           fi
           # Builder image: toolchain + matching raspberrypi kernel headers.
           docker build --platform linux/arm64 \
+            --build-arg DEBIAN_SUITE="$DEBIAN_SUITE" \
             --build-arg HEADERS_PKG="$HEADERS_PKG" \
             --build-arg KERNEL_VERSION="$KERNEL_VERSION" \
             -t iec-kbuild .
diff --git a/docs/kernel-notes.md b/docs/kernel-notes.md
index 7f324b7..fae5df7 100644
--- a/docs/kernel-notes.md
+++ b/docs/kernel-notes.md
@@ -168,14 +168,27 @@ Configurable via env vars (kernel version is configurable as requested):
 
 | Var | Default | Purpose |
 |-----|---------|---------|
+| `SUITE` | `bookworm` | Debian/RPi OS suite; **must match the kernel** — `bookworm` ⇒ 6.12.x, `trixie` ⇒ 6.18.x |
 | `HEADERS_PKG` | `linux-headers-rpi-v8` | headers package; use `-v7`/`-v6` for 32-bit, or `raspberrypi-kernel-headers` |
 | `KERNEL_VERSION` | *(latest)* | exact version pin, e.g. `1:6.6.51-1+rpt3` |
 | `IMAGE` | `iec-kbuild` | builder image tag |
 
 ```bash
-KERNEL_VERSION=1:6.6.51-1+rpt3 ./build-in-docker.sh
+SUITE=trixie KERNEL_VERSION=1:6.18.34-1+rpt1 ./build-in-docker.sh
 ```
 
+**`SUITE` must match `KERNEL_VERSION`.** Each raspberrypi archive suite carries
+only its own latest kernel, so a trixie-era version against a `bookworm` base
+fails with `Version '…' was not found`. Find the Pi's suite with
+`. /etc/os-release; echo "$VERSION_CODENAME"` (or `lsb_release -cs`).
+
+**trixie SHA1 caveat.** On trixie, apt verifies signatures with Sequoia (`sqv`),
+whose crypto policy rejects SHA1 since 2026-02-01. The raspberrypi archive key's
+binding self-signature is SHA1, so trixie's apt rejects the repo as *"not
+signed"* (`Policy rejected … SHA1 is not considered secure …`). The Dockerfile
+works around this by marking the raspberrypi repo `trusted=yes` **only when
+`DEBIAN_SUITE=trixie`**; bookworm (gpgv) keeps full signature verification.
+
 **Find the exact values for *your* Pi** — run this on the Pi (e.g. over SSH); it
 prints the two lines ready to copy into the `build-in-docker.sh` invocation:
 
@@ -183,19 +196,21 @@ prints the two lines ready to copy into the `build-in-docker.sh` invocation:
 pkg="linux-headers-$(uname -r | sed 's/.*+rpt-//')"; echo "HEADERS_PKG=$pkg KERNEL_VERSION=$(dpkg-query -W -f='${Version}' "$pkg")"
 ```
 
-On the project's Pi Zero 2 W (`chris@10.1.0.41`, kernel `6.12.93+rpt-rpi-v8`)
-this currently prints:
+On the Pi at `chris@10.1.0.41` (a Pi 3B+, kernel `6.12.93+rpt-rpi-v8`; same
+arm64/`-v8` headers as the Zero 2 W) this currently prints:
 
 ```bash
 HEADERS_PKG=linux-headers-rpi-v8 KERNEL_VERSION=1:6.12.93-1+rpt1
 ```
 
-**vermagic caveat:** the raspberrypi apt archive normally serves only the
-*latest* kernel in its pool, so pinning `KERNEL_VERSION` to an old release may
-not be downloadable. The reliable strategy is to keep the Pi current
-(`sudo apt full-upgrade`) and build with the default (latest) — then the Pi and
-the container agree. If you must target an older/specific kernel, copy the Pi's
-`/lib/modules/$(uname -r)/build` tree into the container instead of using apt.
+**vermagic caveat:** each raspberrypi apt archive **suite** normally serves only
+the *latest* kernel in its pool (`bookworm` ⇒ 6.12.x, `trixie` ⇒ 6.18.x), so the
+container's `SUITE` must match the Pi's release, and pinning `KERNEL_VERSION` to
+an old release within a suite may not be downloadable. The reliable strategy is
+to keep the Pi current (`sudo apt full-upgrade`) and build with the matching
+`SUITE` + default (latest) version — then the Pi and the container agree. If you
+must target an older/specific kernel, copy the Pi's `/lib/modules/$(uname
+-r)/build` tree into the container instead of using apt.
 
 `uname -r` is **not** used inside the container (it reports the host kernel under
 emulation); the entrypoint derives `KDIR` from the installed headers under
diff --git a/kernel/Dockerfile b/kernel/Dockerfile
index 7531691..b8b592e 100644
--- a/kernel/Dockerfile
+++ b/kernel/Dockerfile
@@ -9,18 +9,38 @@
 #   docker run --privileged --rm tonistiigi/binfmt --install arm64
 #
 # Kernel version is configurable via build args:
+#   --build-arg DEBIAN_SUITE=trixie                # Debian/RPi OS suite (see notes)
 #   --build-arg HEADERS_PKG=linux-headers-rpi-v8   # 64-bit Pi Zero 2 W (default)
 #   --build-arg KERNEL_VERSION=1:6.6.51-1+rpt3      # optional exact pin (see notes)
 #
+# DEBIAN_SUITE must match the kernel you target: each raspberrypi archive suite
+# only carries its own latest kernel (bookworm -> 6.12.x, trixie -> 6.18.x), so
+# pinning a trixie-era KERNEL_VERSION against a bookworm base will fail with
+# "Version ... was not found". Keep this in sync with the Pi's release.
+#
 # Build context is this kernel/ directory.
-FROM --platform=linux/arm64 debian:bookworm
+ARG DEBIAN_SUITE=bookworm
+FROM --platform=linux/arm64 debian:${DEBIAN_SUITE}
+
+# Re-declare after FROM so it is in scope for the RUN below (ARGs before FROM
+# are only visible to the FROM line itself).
+ARG DEBIAN_SUITE=bookworm
 
 # The raspberrypi kernel/headers live in the raspberrypi.com archive, not Debian.
+#
+# On trixie, apt verifies signatures with Sequoia (sqv), whose crypto policy
+# rejects SHA1 since 2026-02-01. The raspberrypi archive key's binding
+# self-signature is SHA1, so trixie's apt refuses the repo as "not signed"
+# (bookworm's gpgv doesn't enforce this). We mark the repo trusted=yes *only on
+# trixie* to bypass that check; bookworm keeps full signature verification. The
+# key is still fetched over HTTPS and pinned via signed-by where it is honoured.
 RUN apt-get update \
  && apt-get install -y --no-install-recommends ca-certificates curl gnupg \
  && curl -fsSL https://archive.raspberrypi.com/debian/raspberrypi.gpg.key \
       | gpg --dearmor -o /usr/share/keyrings/raspberrypi-archive-keyring.gpg \
- && echo "deb [signed-by=/usr/share/keyrings/raspberrypi-archive-keyring.gpg] http://archive.raspberrypi.com/debian/ bookworm main" \
+ && opts="signed-by=/usr/share/keyrings/raspberrypi-archive-keyring.gpg" \
+ && if [ "$DEBIAN_SUITE" = "trixie" ]; then opts="$opts trusted=yes"; fi \
+ && echo "deb [$opts] http://archive.raspberrypi.com/debian/ ${DEBIAN_SUITE} main" \
       > /etc/apt/sources.list.d/raspi.list \
  && rm -rf /var/lib/apt/lists/*
 
diff --git a/kernel/build-in-docker.sh b/kernel/build-in-docker.sh
index a905d3d..30c8966 100755
--- a/kernel/build-in-docker.sh
+++ b/kernel/build-in-docker.sh
@@ -9,19 +9,22 @@
 #   ./build-in-docker.sh clean          # clean build artifacts
 #
 # Configurable via environment variables:
+#   SUITE           Debian/RPi OS suite (default: bookworm; use trixie for 6.18.x)
 #   HEADERS_PKG     headers package (default: linux-headers-rpi-v8, 64-bit Zero 2 W)
 #                   e.g. linux-headers-rpi-v7 / -v6 for 32-bit, raspberrypi-kernel-headers
 #   KERNEL_VERSION  exact version to pin, e.g. 1:6.6.51-1+rpt3 (default: latest in repo)
 #   IMAGE           builder image tag (default: iec-kbuild)
 #
-# NOTE: the raspberrypi apt archive generally serves only the *latest* kernel in
-# its pool, so pinning KERNEL_VERSION to an old release may fail to download. The
+# NOTE: each raspberrypi apt archive suite serves only the *latest* kernel in its
+# pool (bookworm -> 6.12.x, trixie -> 6.18.x), so SUITE must match the kernel you
+# target and pinning KERNEL_VERSION to an old release may fail to download. The
 # reliable match strategy is to keep the Pi current (`sudo apt full-upgrade`) and
-# build with the default (latest). See docs/kernel-notes.md.
+# build with the matching SUITE + default (latest) version. See docs/kernel-notes.md.
 set -euo pipefail
 cd "$(dirname "$0")"
 
 IMAGE="${IMAGE:-iec-kbuild}"
+SUITE="${SUITE:-bookworm}"
 HEADERS_PKG="${HEADERS_PKG:-linux-headers-rpi-v8}"
 KERNEL_VERSION="${KERNEL_VERSION:-}"
 
@@ -32,8 +35,9 @@ if [ ! -e /proc/sys/fs/binfmt_misc/qemu-aarch64 ]; then
 fi
 
 # 2. Build the builder image (installs toolchain + matching kernel headers).
-echo "==> building image '$IMAGE' (HEADERS_PKG=$HEADERS_PKG KERNEL_VERSION=${KERNEL_VERSION:-latest})"
+echo "==> building image '$IMAGE' (SUITE=$SUITE HEADERS_PKG=$HEADERS_PKG KERNEL_VERSION=${KERNEL_VERSION:-latest})"
 docker build --platform linux/arm64 \
+	--build-arg DEBIAN_SUITE="$SUITE" \
 	--build-arg HEADERS_PKG="$HEADERS_PKG" \
 	--build-arg KERNEL_VERSION="$KERNEL_VERSION" \
 	-t "$IMAGE" .