diff --git a/docs/AUTONOMOUS-LOOP.md b/docs/AUTONOMOUS-LOOP.md
index 8fb3479f..9d548c8a 100644
--- a/docs/AUTONOMOUS-LOOP.md
+++ b/docs/AUTONOMOUS-LOOP.md
@@ -103,6 +103,69 @@ fires one final time, then deletes itself."* For runs
 longer than 7 days, the `long-term-rescheduler` skill rotates
 crons near expiry via `CronDelete` + `CronCreate`.
 
+## Invariant — rediscoverable from `main` alone
+
+This discipline serves a load-bearing invariant:
+
+> **A fresh agent reading `main` alone — no chat history,
+> no in-session memory, no out-of-band context — can pick
+> up the next tick and continue the work cleanly.**
+
+The every-tick checklist below is *a* mechanism that
+preserves this invariant; it is not itself the invariant.
+The number of checklist steps is incidental — discoverability
+from `main` alone is what's load-bearing.
+
+Concretely, every change to this file MUST preserve (or
+strengthen) the invariant. The check is: *can a freshly
+spawned agent, reading only committed-on-`main` artifacts,
+find the prior tick's state and act on it?* If yes, the
+invariant holds. If no, the change has broken it.
+
+Four properties make the invariant true today:
+
+- **Tick-history is on `main`.** Each tick lands a row in
+  `docs/hygiene-history/ticks/YYYY/MM/DD/HHMMZ.md` before
+  the tick stops, so the next session sees the prior tick's
+  context by reading the repo.
+- **PR queue is on the host.** Open PRs are visible via
+  `gh pr list` without session state; the next tick can
+  poll the gate (real-dependency-wait discipline) without
+  remembering which PRs are in flight.
+- **Substrate precedes narration.** Decisions, corrections,
+  and framings land as memory files / backlog rows / docs
+  in committed substrate BEFORE chat narration. Per
+  substrate-or-it-didn't-happen (Otto-363) +
+  non-durable-means-does-not-exist (the human maintainer,
+  2026-04-30), if it isn't on `main`, the next tick
+  won't see it.
+- **Cron is the cadence engine, not session memory.** The
+  every-minute heartbeat fires whenever a session is active
+  to receive it; `CronList` is checked end-of-tick so the
+  [last-check → next-fire] window is minimized; the next
+  tick fires without depending on the prior tick's
+  process state. Sessions that resume via `--resume` /
+  `--continue` inherit the registered cron when it's still
+  within its 7-day expiry; brand-new sessions re-arm via
+  the every-tick checklist (per the "Session-restart
+  recovery" section below).
+
+Future revisions of this file may add, drop, or restructure
+the checklist below — provided the rediscoverable-from-`main`
+invariant is preserved or strengthened. Treat the four
+properties above as the test surface.
+
+> The human maintainer, 2026-04-30 (paraphrased — the original
+> framing was "Six-step tick close has a satisfying property
+> that should be an invariant for all future changes to this
+> file" followed by a sharpening "not six step the
+> rediscoverable on main part"; the bracketed gloss reflects
+> the sharpening): *"[the rediscoverable-on-main part]
+> should be an invariant for all future changes to this
+> file."* The meta-property is what makes the discipline
+> portable across sessions, agents, and forks of the
+> factory.
+
 ## The every-tick checklist
 
 This is the load-bearing bit. **Every tick**, in order: