diff --git a/AGENTS.md b/AGENTS.md
index c27b0d58ed..d9d053739a 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -20,6 +20,17 @@ and do not file a question about it. This is a standing owner ruling.
 - **A test for code we wrote is fixed.** Fix a race by waiting for the event, not with a retry
 or a quarantine.
 
+## Syncing upstream into `rigel-release`
+
+1. Branch `harness/rig-5440-upstream-sync` from `rigel-release` and merge `main` into it. Keep the
+merge commit; this PR must not be squashed or rebased. Mutation reproof skips this exact branch name.
+2. Resolve conflicts in favor of our custody guarantees (custodian-run Linux pty seats, no
+numeric or process-group signals, retain or refuse an unproven reap). If an upstream test asserts
+the opposite, quarantine it unchanged.
+3. Quarantine every other upstream-red suite as above, then run `pnpm smoke:gate-inventory`.
+4. Add review fixes as new commits on the branch, never as a rebase. The PR body needs its own
+paragraph `Approved-at: <full 40-character head sha>` naming the final pushed head.
+
 ## What this is
 
 **Cotal** is a standard wire interface for software, especially AI agents, to coordinate in