diff --git a/docs/guide/installation.md b/docs/guide/installation.md
index 612d92df..ae6e8ddc 100644
--- a/docs/guide/installation.md
+++ b/docs/guide/installation.md
@@ -536,6 +536,7 @@ After=network.target hapi-hub.service
 
 [Service]
 Type=simple
+KillMode=process
 ExecStart=/usr/local/bin/hapi runner start-sync
 Restart=always
 RestartSec=5
@@ -544,6 +545,8 @@ RestartSec=5
 WantedBy=default.target
 ```
 
+> **Why `KillMode=process`?** The runner spawns each agent session as a detached child process (`detached: true` in `cli/src/runner/run.ts`) so that sessions stay alive when the runner exits. Without `KillMode=process`, systemd's default `KillMode=control-group` sends SIGTERM to every PID in the runner's cgroup when the unit stops, defeating the detach and forcibly archiving every running session. `KillMode=process` preserves the contract: stopping or restarting the runner only signals the runner itself; agent sessions stay alive, and a fresh runner re-establishes control via the existing socket.io reconnect path. This applies to runner upgrades, manual restarts, and any reboot in which the runner unit is stopped before agents have finished.
+
 Enable and start:
 
 ```bash