Summary

The built-in heartbeat feature accumulates conversation turns in the main session with no way to isolate it. Over time (2+ days of hourly heartbeats), the session .jsonl file grows to 10–15MB, corrupts, and triggers an automatic reset — silently wiping all agent context.

Root Cause

heartbeat.session config only supports "main" — there is no "isolated" option. Every heartbeat turn is permanently stored in the main session history and re-sent to the LLM on every subsequent request, inflating token cost and file size indefinitely.

Reproduction

1. Set heartbeat.every: "1h" (or any interval)
2. Run OpenClaw for 2+ days
3. Observe ~/.openclaw/agents/main/sessions/<main-session-id>.jsonl growing 5–15MB
4. Eventually the session file corrupts (header truncation) and OpenClaw auto-resets — all context lost

Impact

• Context window bloated by hundreds of accumulated heartbeat turns
• Token cost inflates proportionally on every new request
• Silent context loss when session eventually resets
• No warning, no user-facing indicator

Workaround

Disable built-in heartbeat entirely (heartbeat.every: "0m") and replace with a cron job using sessionTarget: "isolated":

{
  "schedule": {"kind": "every", "everyMs": 3600000},
  "payload": {"kind": "agentTurn", "model": "<cheap-model>", "message": "..."},
  "sessionTarget": "isolated"
}

This works but is a significant usability gap — the built-in heartbeat is the obvious first-choice for periodic monitoring.

Requested Fix

Add heartbeat.session: "isolated" config option that runs heartbeat in a fresh isolated session, preventing main session pollution. Alternatively, add an automatic session file size cap or rolling compaction triggered by file size rather than token count alone.

Environment

• OpenClaw version: 2026.2.13
• Platform: Linux x64
• Heartbeat interval: hourly
• Time to failure: ~2 days