diff --git a/main.sh b/main.sh
index 5b44aff..ac8fadf 100755
--- a/main.sh
+++ b/main.sh
@@ -1090,8 +1090,11 @@ prune() { #{{{
   update_diagnostic "system.docker.reclaimed_containers_images" "$reclaimed_containers_images"
   update_diagnostic "system.docker.prune_filter" "$prune_filter"
 
+  # Volumes only. `system prune --volumes` also sweeps every non-running
+  # container regardless of age, deleting freshly created ECS task containers
+  # before the agent can start them.
   local reclaimed_volumes
-  reclaimed_volumes=$($CONTAINER_ORCHESTRATOR system prune --volumes -f |
+  reclaimed_volumes=$($CONTAINER_ORCHESTRATOR volume prune -f |
     cut -d: -f2 | tr -d ' ')
   update_diagnostic "system.docker.reclaimed_volumes" "$reclaimed_volumes"
 
diff --git a/test/mocks/bin/docker b/test/mocks/bin/docker
index 955fe1e..7a1d724 100755
--- a/test/mocks/bin/docker
+++ b/test/mocks/bin/docker
@@ -4,6 +4,7 @@
 #   ps        -> $MOCK_DOCKER_PS_OUTPUT       (default: empty)
 #   inspect   -> $MOCK_DOCKER_INSPECT_OUTPUT  (default: healthy)
 #   system    -> $MOCK_DOCKER_SYSTEM_OUTPUT   (default: "Total reclaimed space: 0B")
+#   volume    -> $MOCK_DOCKER_VOLUME_OUTPUT   (default: "Total reclaimed space: 0B")
 # Exit code override: $MOCK_DOCKER_EXIT (default: 0).
 printf '%s\n' "docker $*" >>"${MOCK_CALL_LOG}"
 
@@ -18,6 +19,9 @@ case "$1" in
   system)
     echo "${MOCK_DOCKER_SYSTEM_OUTPUT:-Total reclaimed space: 0B}"
     ;;
+  volume)
+    echo "${MOCK_DOCKER_VOLUME_OUTPUT:-Total reclaimed space: 0B}"
+    ;;
 esac
 
 exit "${MOCK_DOCKER_EXIT:-0}"
diff --git a/test/unit/prune.bats b/test/unit/prune.bats
new file mode 100644
index 0000000..ee0b526
--- /dev/null
+++ b/test/unit/prune.bats
@@ -0,0 +1,65 @@
+#!/usr/bin/env bats
+#
+# Unit tests for prune(), which the `0 */4 * * *` crontab entry installed by
+# setup_cron() invokes on every registered runner.
+#
+# The container/image sweep is age-guarded with --filter until=4h. The volume
+# sweep must never be able to delete a container: an unfiltered
+# `docker system prune` removes every non-running container, including one in
+# `created` state, which races the ECS agent between docker create and docker
+# start and fails the task with CannotStartContainerError / No such container.
+
+setup() {
+  load "${BATS_TEST_DIRNAME}/../helpers/load.bash"
+  load_main
+  CONTAINER_ORCHESTRATOR="docker"
+  init_diagnostic_dir
+}
+
+# Every recorded `docker ... prune` invocation, one per line.
+prune_calls() {
+  mock_calls docker | grep -F "prune" || true
+}
+
+@test "prune age-guards the container and image sweep" {
+  run prune
+  assert_success
+
+  assert_called docker "system prune -f --filter until=4h"
+}
+
+@test "prune never issues a system prune without an age filter" {
+  run prune
+  assert_success
+
+  local unguarded
+  unguarded="$(prune_calls | grep -F "system prune" | grep -vF -- "--filter" || true)"
+  assert_equal "$unguarded" ""
+}
+
+@test "prune reclaims volumes without a container-deleting sweep" {
+  run prune
+  assert_success
+
+  assert_called docker "volume prune"
+
+  local container_deleting
+  container_deleting="$(prune_calls | grep -F "system prune --volumes" || true)"
+  assert_equal "$container_deleting" ""
+}
+
+@test "prune records the filter it actually applied to the guarded sweep" {
+  run prune
+  assert_success
+
+  assert_equal "$(jq -r '.system.docker.prune_filter' "$SG_DIAGNOSTIC_FILE")" "until=4h"
+}
+
+@test "prune records reclaimed space for both sweeps" {
+  run prune
+  assert_success
+
+  refute [ "$(jq -r '.system.docker.last_prune' "$SG_DIAGNOSTIC_FILE")" = "null" ]
+  refute [ "$(jq -r '.system.docker.reclaimed_containers_images' "$SG_DIAGNOSTIC_FILE")" = "null" ]
+  refute [ "$(jq -r '.system.docker.reclaimed_volumes' "$SG_DIAGNOSTIC_FILE")" = "null" ]
+}