docs(gateway): add uptime monitoring guidance to health check docs (fixes #55768) (#92608)

docs/gateway/health.md

@@ -42,6 +42,21 @@ health commands above for live connectivity checks.
 - `channels.<provider>.accounts.<accountId>.healthMonitor.enabled`: multi-account override that wins over the channel-level setting.
 - These per-channel overrides apply to the built-in channel monitors that expose them today: Discord, Google Chat, iMessage, Microsoft Teams, Signal, Slack, Telegram, and WhatsApp.
 
+## Uptime monitoring
+
+External uptime monitoring services should use the dedicated `/health` endpoint, not `/v1/chat/completions`.
+
+- **DO use:** `GET /health` — instant response, no session created, no LLM call, returns `{"ok":true,"status":"live"}`
+- **DON'T use:** `/v1/chat/completions` for health checks — each request creates a full agent session with skill snapshot, context assembly, and LLM calls
+
+When no `x-openclaw-session-key` header or `user` field is provided, `/v1/chat/completions` generates a new random session for each request. Monitoring services that ping every 15 minutes create ~96 sessions/day, each consuming 4–22KB. Over time this causes session store bloat and can lead to context window overflow.
+
+### Monitoring service setup examples
+
+- **BetterStack:** Set health check URL to `https://<your-gateway-host>:<port>/health`
+- **UptimeRobot:** Add a new HTTP monitor with URL `https://<your-gateway-host>:<port>/health`
+- **Generic:** Any HTTP GET to `/health` returns 200 with `{"ok":true}` when the gateway is healthy
+
 ## When something fails
 
 - `logged out` or status 409–515 → relink with `openclaw channels logout` then `openclaw channels login`.