diff --git a/docs/changelog-20260731-prefer-ipv6-research.md b/docs/changelog-20260731-prefer-ipv6-research.md
index ee1103d..477f423 100644
--- a/docs/changelog-20260731-prefer-ipv6-research.md
+++ b/docs/changelog-20260731-prefer-ipv6-research.md
@@ -554,3 +554,26 @@
 upper bound rather than a figure.
 
 **Revert.** Measurement only; nothing changed.
+
+## Item 18 -- BUILD the D-135 amendment: `jammy-backports` added at SOURCE, T10 re-pointed
+
+**`scripts/dc-mirror.sh`** -- the generated `<site>-mirror-sync` `--dist=` line gains
+`jammy-backports`, plus a comment block at the generator recording WHY it is not optional:
+the MAAS jammy image ships backports in its `sources.list` and juju's `apt-mirror`
+model-config rewrites every suite to this mirror, so a mirror without it 404s and every
+charm install hook fails. That cost 22 units on the 2026-07-31 dc0 deploy.
+
+**Fixed at SOURCE, not only on the live rack.** A live-only edit would have left the
+generator regenerating the old scope at the next `install` or at any future DC standup --
+the "ruled is not built" class inverted.
+
+**`tests/dc-mirror` T10 RE-POINTED, not deleted.** It asserted
+`--dist=jammy,jammy-updates,jammy-security` verbatim, which was correct for the old scope
+and had to MOVE when the scope changed -- the standing rule that a test asserting a
+literal finding gets re-pointed to the new invariant, never removed to go green. The
+reason is recorded in-file so a later reader does not "simplify" it back.
+
+**Mutation-proven:** reverting the generator's dist line to the old scope turns T10 RED
+(23/1); restored byte-identical, 24/24. Gauntlet ALL GREEN (93); repo-lint 0 fail.
+
+**Revert.** `git revert` -- generator and harness move together.
diff --git a/scripts/dc-mirror.sh b/scripts/dc-mirror.sh
index 1964221..6e2317c 100755
--- a/scripts/dc-mirror.sh
+++ b/scripts/dc-mirror.sh
@@ -170,13 +170,19 @@
 #!/usr/bin/env bash
 # ${SITE}-mirror-sync -- generated by scripts/dc-mirror.sh (D-135 item 1);
 # do not hand-edit. Jammy amd64 binary mirror + UCA caracal, GPG-verified.
+# jammy-backports ADDED by the D-135 AMENDMENT 2026-07-31 (operator: "Sync the
+# backports into the mirror"). It is NOT optional: the MAAS jammy image ships
+# jammy-backports in its sources.list, and juju's apt-mirror model-config rewrites
+# EVERY suite to this mirror -- so a mirror without it 404s and every charm's
+# install hook fails on 'does not have a Release file'. That cost 22 units on the
+# 2026-07-31 dc0 deploy. The mirror's suite list is a CONTRACT with the node image.
 set -uo pipefail
 STATUS="${MIRROR_ROOT}/last-sync.status"
 echo "RUNNING \$(date -u +%FT%TZ)" > "\$STATUS"
 mkdir -p ${MIRROR_ROOT}/ubuntu ${MIRROR_ROOT}/cloud-archive
 debmirror ${MIRROR_ROOT}/ubuntu \\
   --host=archive.ubuntu.com --root=ubuntu --method=http \\
-  --dist=jammy,jammy-updates,jammy-security \\
+  --dist=jammy,jammy-updates,jammy-security,jammy-backports \\
   --section=main,restricted,universe,multiverse \\
   --arch=amd64 --nosource --diff=none \\
   --keyring=${ARCHIVE_KEYRING} --progress
diff --git a/tests/dc-mirror/run-tests.sh b/tests/dc-mirror/run-tests.sh
index 9200171..5e3b612 100755
--- a/tests/dc-mirror/run-tests.sh
+++ b/tests/dc-mirror/run-tests.sh
@@ -50,10 +50,18 @@
   || no "T9 both mirrors --nosource (binary only)"
 
 # T10: the ruled dist set -- jammy triple + UCA caracal
-grep -q -- '--dist=jammy,jammy-updates,jammy-security' "$SCRIPT" \
+# T10 RE-POINTED 2026-07-31 (D-135 AMENDMENT, operator: "Sync the backports into the
+# mirror"). It previously asserted the exact string '--dist=jammy,jammy-updates,
+# jammy-security'. That assertion was CORRECT for the old scope and had to move, not be
+# deleted, when the scope changed -- the standing rule that a test asserting a literal
+# finding gets RE-POINTED to the new invariant. WHY the suite is mandatory: the MAAS jammy
+# image ships jammy-backports in its sources.list and juju's apt-mirror model-config
+# rewrites EVERY suite to this mirror, so a mirror without it 404s and every charm install
+# hook fails. That cost 22 units on the 2026-07-31 dc0 deploy.
+grep -q -- '--dist=jammy,jammy-updates,jammy-security,jammy-backports' "$SCRIPT" \
   && grep -q -- '--dist=jammy-updates/caracal' "$SCRIPT" \
-  && ok "T10 dists: jammy triple + UCA jammy-updates/caracal" \
-  || no "T10 dists: jammy triple + UCA jammy-updates/caracal"
+  && ok "T10 dists: jammy triple + backports + UCA jammy-updates/caracal" \
+  || no "T10 dists: jammy triple + backports + UCA jammy-updates/caracal"
 
 # T11: unknown site refuses with exit 2 and demands measured rows
 OUT="$(bash "$SCRIPT" check nosuchsite 2>&1)"; RC=$?