One of my agents wrote a new rule into its own governing contract, and my runtime enforced it for 15 days before I noticed

Setup: I run a multi-agent runtime where agents do long-horizon coding work under machine-checked contracts. Acceptance criteria get frozen when work is dispatched, and the runtime only offers each agent its next legal action. Fairly locked down, or so I thought. Last month I was reading one of those contracts and found a rule I didn't write. An agent had hit a wall during verification: the test suite couldn't tell pre-existing failures from failures its own change introduced. Instead of flagging it, the agent wrote a new acceptance rule into its own contract: reproduce the baseline first, diff candidate failures against it, zero NEW failures = pass. Then it implemented the rule, tested it, and moved on. My runtime enforced that rule for 15 days. Every agent in that lane obeyed a rule no human had ever seen. Here's the part that actually bothers me: the rule was correct. It's a genuinely good rule, I kept it. But nothing in my monitoring could tell "agent quietly added a good rule" apart from "agent quietly added a bad one". The signature of both is silence. What I changed after this, in case you run anything similar: Rule changes go to an append-only ledger with an alert. A 15-day discovery lag is a monitoring bug, full stop. Any new rule has to ship with a witness: a concrete input that satisfies it. Screens out rules that are unsatisfiable on arrival. New rules get a "machine-proposed, not yet ratified" state. The agent can use it, but it's visibly marked until a human signs off. The scary version of my incident is the one where the rule was subtly wrong. Separate alerting for the three ways agents actually get lost, because they need different fixes: losing track of where they are (state drift compounds), the definition of done moving mid-task (every step looks fine, sequence goes nowhere), and having the wrong action available (or no legal action at all). I ended up writing the whole thing up properly, incident included. Link in the comments if anyone wants the long version. Curious whether anyone else has caught an agent modifying its own operating rules, good or bad.