diff --git a/.mergify.yml b/.mergify.yml
index d60209f0..396d37a0 100644
--- a/.mergify.yml
+++ b/.mergify.yml
@@ -1,9 +1,46 @@
 # Mergify: policy lives in systemfsoftware/.github (.mergify.yml). This file only names this repo's CI checks.
 extends: .github
 queue_rules:
+    # Checked on the PR itself before it enters the queue.
+  - name: subtree
+    merge_method: merge
+    queue_conditions:
+      # Recognised from the commits themselves: both anchored git-subtree trailer
+      # lines must be present (ruling 9, SYS-1246). Same checks as default.
+      - "commits[*].commit_message ~= (?m)^git-subtree-dir: \\S+$"
+      - "commits[*].commit_message ~= (?m)^git-subtree-split: [0-9a-f]{40}$"
+      - or:
+          - check-success = review-gate
+          - approved-reviews-by = kiro-systemf[bot]
+          - and:
+              - author = dependabot[bot]
+              - dependabot-update-type != version-update:semver-major
+              - or:
+                  - dependabot-update-type = version-update:semver-patch
+                  - dependabot-update-type = version-update:semver-minor
+                  - "title ~= (?i)\\bthe [a-z0-9@/._-]*(minor|patch)[a-z0-9@/._-]* group\\b"
+      - check-success = checks / the gate
+      - or:
+          - check-success = check / a publishable-package change needs an intent
+          - check-skipped = check / a publishable-package change needs an intent
+      - check-success = Same bytes on every host
+    # Checked on Mergify's queue draft PR (mergify/merge-queue/*), i.e. CI on the PR rebased with main and any batch.
+    merge_conditions:
+      - check-success = checks / the gate
+      - check-success = Same bytes on every host
+
   - name: default
     # Checked on the PR itself before it enters the queue.
     queue_conditions:
+      # Ruling 9 (SYS-1246): subtree PRs merge via the subtree queue above; a subtree
+      # claim without trailers matches no queue and cannot merge at all.
+      - "-commits[*].commit_message ~= (?m)^git-subtree-dir: \\S+$"
+      - "-title ~= (?i)\\b(add|pull|update|sync|vendor|refresh|import)\\b[^.]*\\bsubtrees?\\b|\\bsubtrees?\\b[^.]*(\\badd\\b|\\bpull\\b|\\bupdate\\b|\\bsync\\b|\\bvendor\\b|\\brefresh\\b|\\bimport\\b)"
+      # Vendored subtree prefixes in this repo: hand edits without trailers cannot merge.
+      - not:
+          or:
+            - "added-files ~= ^(repos/alchemy|repos/effect)/"
+            - "modified-files ~= ^(repos/alchemy|repos/effect)/"
       - or:
           - check-success = review-gate
           - approved-reviews-by = kiro-systemf[bot]