diff --git a/.gitignore b/.gitignore
index f6fcba1..20bd53e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -23,3 +23,4 @@ spike-webswing/fontconfig.properties
 # behind the numbers in README "Findings".
 results/watch/
 results/*.log
+results/*.pid
diff --git a/README.md b/README.md
index fecccea..48f2d23 100644
--- a/README.md
+++ b/README.md
@@ -27,6 +27,12 @@ Two people can run side by side by picking different ports and an instance name,
 which namespaces the generated configs and logs:
 
     INSTANCE=claude PORT_WS=8090 PORT_MM=8860 ./spike-webswing/start.sh
+    ./spike-webswing/stop.sh claude
+
+**Stop instances with `stop.sh`, never `pkill -f spike-webswing/start.sh`** — that
+pattern matches every instance on the box and will kill someone else's live game.
+`start.sh` publishes `results/<instance>.pid` with the process group ids of its
+children, and `stop.sh` signals exactly those.
 
 Still shared between instances: MegaMek's `mmconf/` (both seed the same
 `clientsettings.xml`, so identical content) and `logs/`.
diff --git a/spike-webswing/start.sh b/spike-webswing/start.sh
index f232508..50e96db 100755
--- a/spike-webswing/start.sh
+++ b/spike-webswing/start.sh
@@ -29,6 +29,7 @@ INSTANCE="${INSTANCE:-default}"
 WS_CONFIG="$WS/webswing-$INSTANCE.config"
 WS_JETTY="$WS/jetty-$INSTANCE.properties"
 WS_LOG="logs/ws-$INSTANCE.log"
+PID_FILE="$ROOT/results/$INSTANCE.pid"
 RAW_LOG="$ROOT/results/host-$INSTANCE-raw.log"
 
 [ -x "$JAVA" ] || { echo "ERROR: JDK missing; run scripts/fetch-deps.sh" >&2; exit 1; }
@@ -93,6 +94,7 @@ cleanup() {
   [ -n "$TAIL_PID" ] && kill "$TAIL_PID" 2>/dev/null || true
   kill_group "$HOST_PID"
   kill_group "$WS_PID"   # its group includes any MegaMek client it spawned
+  rm -f "$PID_FILE"
   echo "done."
 }
 trap cleanup INT TERM EXIT
@@ -159,5 +161,16 @@ grep --line-buffered -vE "^(ERROR|WARN|INFO) " < <(tail -f -n +1 "$RAW_LOG") \
   | tee "$ROOT/results/host-$INSTANCE.log" &
 TAIL_PID=$!
 
+# Both children were started with setsid, so their PID is also their process
+# group id and can be signalled as a group.
+cat > "$PID_FILE" <<PIDS
+INSTANCE=$INSTANCE
+PORT_WS=$PORT_WS
+PORT_MM=$PORT_MM
+SCRIPT_PID=$$
+WEBSWING_PGID=$WS_PID
+HOST_PGID=$HOST_PID
+PIDS
+
 # Blocks until the game reaches VICTORY (HostForHuman exits 0) or Ctrl-C.
 wait "$HOST_PID" || true
diff --git a/spike-webswing/stop.sh b/spike-webswing/stop.sh
new file mode 100755
index 0000000..3aa1619
--- /dev/null
+++ b/spike-webswing/stop.sh
@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+# Stop one spike instance, and only that one.
+#
+#   ./spike-webswing/stop.sh            # stops the "default" instance
+#   ./spike-webswing/stop.sh claude     # stops the "claude" instance
+#
+# Use this instead of `pkill -f spike-webswing/start.sh`, which matches every
+# instance on the box and will kill someone else's live game. start.sh publishes
+# results/<instance>.pid holding the process group ids of its two children, so we
+# can signal exactly those.
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+INSTANCE="${1:-${INSTANCE:-default}}"
+PID_FILE="$ROOT/results/$INSTANCE.pid"
+
+if [ ! -f "$PID_FILE" ]; then
+  echo "No PID file for instance '$INSTANCE' ($PID_FILE)."
+  echo "Running instances:"
+  shopt -s nullglob
+  found=0
+  for f in "$ROOT"/results/*.pid; do
+    echo "  $(basename "${f%.pid}")"
+    found=1
+  done
+  [ "$found" = "0" ] && echo "  (none)"
+  exit 1
+fi
+
+# shellcheck disable=SC1090
+. "$PID_FILE"
+
+kill_group() {
+  local pid="${1:-}" what="$2"
+  [ -n "$pid" ] || return 0
+  kill -0 "$pid" 2>/dev/null || return 0
+  echo "  stopping $what (pgid $pid)"
+  kill -TERM "-$pid" 2>/dev/null || kill -TERM "$pid" 2>/dev/null || true
+  for _ in $(seq 20); do kill -0 "$pid" 2>/dev/null || return 0; sleep 0.25; done
+  kill -KILL "-$pid" 2>/dev/null || kill -KILL "$pid" 2>/dev/null || true
+}
+
+echo "stopping instance '$INSTANCE' (web :${PORT_WS:-?}, game :${PORT_MM:-?})"
+
+# The script's own EXIT trap would normally do this, but stop it first so it does
+# not race us, then clean up the children directly in case it was already gone.
+if [ -n "${SCRIPT_PID:-}" ] && kill -0 "$SCRIPT_PID" 2>/dev/null; then
+  kill -TERM "$SCRIPT_PID" 2>/dev/null || true
+  for _ in $(seq 20); do kill -0 "$SCRIPT_PID" 2>/dev/null || break; sleep 0.25; done
+fi
+kill_group "${HOST_PGID:-}" "MegaMek host"
+kill_group "${WEBSWING_PGID:-}" "Webswing"
+
+rm -f "$PID_FILE"
+echo "done."