diff --git a/scripts/ci/docker-prune.service b/scripts/ci/docker-prune.service
index 372117c..560a9c0 100644
--- a/scripts/ci/docker-prune.service
+++ b/scripts/ci/docker-prune.service
@@ -2,32 +2,31 @@
 #
 # Why this exists: every CI push builds and sha-<commit>-tags a Docker image per pipeline
 # (rust-ci, web, docs, fedora-rpm, fedora44-rpm, ...). Those tags are never dangling, so a
-# plain `docker image prune` SKIPS them and they accumulate — that is what filled the disk.
-# Host-level, not per-repo CI, because the runner is shared (punktfunk + other orgs all benefit).
+# plain `docker image prune` SKIPS them and they accumulate. Host-level, not per-repo CI,
+# because the runner is shared (punktfunk + other orgs all benefit).
 #
-# Two tiers: trim anything older than 12h normally, AND — because a push-burst can fill 99 GB
-# WITHIN that 12h window (a fast iteration session hit 100% and poisoned the cargo cache with a
-# truncated, half-saved target/) — a burst guard that prunes ALL idle images + cache once the
-# disk is >85% full. Images IN USE by a running container are always protected.
+# THE BIG ONE (2026-06-19): the act_runner CACHE SERVER store lives in the long-running runner
+# container's WRITABLE LAYER (HOME/.cache/actcache/cache inside gitea-runner-runner-1,
+# `cache.dir: ""` -> defaults under /root). `docker prune` can NEVER see it — only stopped
+# containers + unused images/cache are prunable, not a 13-day-up container's layer. That store
+# grew to ~66 GB and filled a 125 GB disk on its own. docker-prune.sh caps it by clearing the
+# blobs in-place (act_runner repopulates; keys are content-hashed).
+#
+# The logic is in docker-prune.sh, NOT inline ExecStart=, because systemd does its own
+# $-expansion on ExecStart and would empty the shell vars / $(...) before sh runs them.
 #
 # Install on the runner host (root):
+#   install -m755 scripts/ci/docker-prune.sh /usr/local/bin/ci-docker-prune.sh
 #   cp scripts/ci/docker-prune.{service,timer} /etc/systemd/system/
 #   systemctl daemon-reload && systemctl enable --now docker-prune.timer
 # See also scripts/ci/setup-macos-runner.sh for the macOS runner.
 
 [Unit]
-Description=Prune aged Docker images / build cache (CI runner disk hygiene)
+Description=Prune aged Docker images/cache + cap the act_runner cache (CI runner disk hygiene)
 Documentation=https://git.unom.io/unom/punktfunk
 Wants=docker.service
 After=docker.service
 
 [Service]
 Type=oneshot
-# '-' prefix: each step is independent — a no-op/failure never blocks the others.
-ExecStart=-/usr/bin/docker image prune -af --filter until=12h
-ExecStart=-/usr/bin/docker builder prune -af --filter until=12h
-ExecStart=-/usr/bin/docker buildx prune -af --filter until=12h
-ExecStart=-/usr/bin/docker container prune -f --filter until=12h
-# Burst guard: if STILL >85% full, prune every idle image + all build cache (in-use protected),
-# so a push-storm can't drive CI into ENOSPC (which truncates and poisons the actions/cargo cache).
-ExecStart=-/bin/sh -c 'P=$(df --output=pcent / | tr -dc 0-9); [ "$P" -ge 85 ] && { docker image prune -af; docker builder prune -af; docker buildx prune -af; } || true'
+ExecStart=/usr/local/bin/ci-docker-prune.sh
diff --git a/scripts/ci/docker-prune.sh b/scripts/ci/docker-prune.sh
new file mode 100644
index 0000000..05b80aa
--- /dev/null
+++ b/scripts/ci/docker-prune.sh
@@ -0,0 +1,42 @@
+#!/usr/bin/env bash
+# CI runner disk hygiene — invoked by docker-prune.service (every 30 min). Lives in a real script
+# rather than inline ExecStart= lines because systemd does its OWN $-expansion on ExecStart and
+# empties shell vars / $(...) before /bin/sh sees them (silently breaking the logic under `|| true`).
+#
+# See docker-prune.service for the full why. The headline: the act_runner cache server's blob store
+# lives INSIDE the long-running runner container's writable layer, where `docker prune` can't reach
+# it — left alone it grows to tens of GB and fills the disk on its own.
+set -u
+export PATH=/usr/bin:/bin:/usr/local/bin:$PATH
+
+RUNNER=$(docker ps -q -f name=gitea-runner-runner | head -1)
+ACTCACHE=/root/.cache/actcache/cache    # path INSIDE the runner container (HOME=/root there)
+CAP_MB=20000                            # clear the actcache once its blob dir exceeds ~20 GB
+BURST_PCT=80                            # full clear once the disk is this % full
+
+# 1) Routine: trim aged images / build cache / stopped containers. sha-<commit> tags aren't
+#    dangling, so -a is required; until=6h keeps very recent ones for quick re-runs.
+docker image prune     -af --filter until=6h || true
+docker builder prune   -af --filter until=6h || true
+docker buildx prune    -af --filter until=6h || true
+docker container prune  -f --filter until=6h || true
+
+# 2) Cap the act_runner cache server store (the real disk filler). Clearing the blobs is safe —
+#    act_runner repopulates it and cache keys are content-hashed, so this only drops stale entries.
+if [ -n "$RUNNER" ]; then
+  SZ=$(docker exec "$RUNNER" du -sm "$ACTCACHE" 2>/dev/null | cut -f1)
+  if [ -n "${SZ:-}" ] && [ "$SZ" -ge "$CAP_MB" ]; then
+    docker exec "$RUNNER" sh -c "rm -rf $ACTCACHE/*" && echo "actcache cleared (was ${SZ} MB)"
+  fi
+fi
+
+# 3) Burst guard: a push-storm can fill the disk within one interval. Once >=BURST_PCT% full, prune
+#    ALL idle images/cache AND clear the actcache, regardless of age. In-use images are protected.
+PCT=$(df --output=pcent / | tr -dc '0-9')
+if [ -n "$PCT" ] && [ "$PCT" -ge "$BURST_PCT" ]; then
+  echo "disk ${PCT}% >= ${BURST_PCT}% — burst clear"
+  docker image prune -af   || true
+  docker builder prune -af || true
+  docker buildx prune -af  || true
+  [ -n "$RUNNER" ] && docker exec "$RUNNER" sh -c "rm -rf $ACTCACHE/*" || true
+fi
diff --git a/scripts/ci/docker-prune.timer b/scripts/ci/docker-prune.timer
index 104d8d0..dacb784 100644
--- a/scripts/ci/docker-prune.timer
+++ b/scripts/ci/docker-prune.timer
@@ -1,12 +1,13 @@
-# Runs docker-prune.service hourly (the burst guard needs to react within the hour, not every 6h).
+# Runs docker-prune.service every 30 min. The runner is hammered with build bursts that can refill
+# the disk fast (and the actcache cap needs to react well within an hour), so 30 min beats hourly.
 # Persistent=true catches up after downtime. Install: see the header of docker-prune.service.
 
 [Unit]
-Description=Run docker-prune hourly (CI runner disk hygiene + burst guard)
+Description=Run docker-prune every 30 min (CI runner disk hygiene + actcache cap + burst guard)
 
 [Timer]
-OnCalendar=hourly
-RandomizedDelaySec=300
+OnCalendar=*:0/30
+RandomizedDelaySec=120
 Persistent=true
 
 [Install]