diff --git a/deploy/README.md b/deploy/README.md
index d4042f3..5651b20 100644
--- a/deploy/README.md
+++ b/deploy/README.md
@@ -252,6 +252,26 @@ sudo find /mnt/cephfs/media/wp-content/uploads -type d \
 The `d:` (default) entry is what makes each new `YYYY/MM` directory inherit the
 grant, so this does not need repeating every month.
 
+**But a later chmod silently disables it.** A chmod on an ACL'd directory sets
+the **ACL mask** from the mode's group bits, and the mask caps every named
+entry: `chmod 755` leaves `user:10001:rwx` printed but `#effective:r-x`. An
+rsync of the corpus run with `--chmod=D755` does this to every directory it
+touches (see `docs/HANDOVER.md` — that command now uses `D775`).
+
+Nothing notices at the time. Uploads keep working until the 1st of the next
+month, when the handler first has to create a new `YYYY/MM`, and then every
+upload 500s. Check the mask, not the entry:
+
+```bash
+getfacl -pc /mnt/cephfs/media/wp-content/uploads/2026 | grep -E '10001|mask::'
+# want: user:10001:rwx  (no "#effective:") and mask::rwx
+setfacl -m m::rwx /mnt/cephfs/media/wp-content/uploads{,/2026}   # repair
+```
+
+Only those two levels matter: `uploads/` creates the year, `uploads/YYYY/`
+creates the month. `triangle-infrastructure`'s `delta_cms_host` role repairs
+both on every playbook run.
+
 Without the `acl` package, setgid does the same job in plain POSIX, at the cost
 of changing group ownership rather than adding a grant beside it:
 
diff --git a/docs/ETL-REBUILD.md b/docs/ETL-REBUILD.md
index d9690b7..66d6db7 100644
--- a/docs/ETL-REBUILD.md
+++ b/docs/ETL-REBUILD.md
@@ -229,6 +229,17 @@ times on .../2026/08" is one directory's mtime, not a data failure — ignore it
 Newly synced files can 404 publicly for up to 4 hours while the pre-sync edge
 404 ages out. `?v=<ts>` returns 200 immediately; that confirms the file is fine.
 
+**Upload ACL mask.** The sync can leave media *readable* and uploads broken. A
+chmod on an ACL'd directory rewrites the ACL mask, capping the backend's
+`user:10001:rwx` grant to `#effective:r-x`; the symptom is delayed until the 1st
+of the next month, when `POST /v1/media` starts returning 500 on every upload.
+Check the mask, not the entry:
+
+```bash
+getfacl -pc /mnt/cephfs/media/wp-content/uploads/2026 | grep -E '10001|mask::'
+# want: user:10001:rwx (no "#effective:") and mask::rwx
+```
+
 **Orphaned author links** — byline present in `articles.authors` but the join
 returns nothing, so the API emits `authors: null`:
 
diff --git a/docs/HANDOVER.md b/docs/HANDOVER.md
index 10dd8b3..ad1f2d4 100644
--- a/docs/HANDOVER.md
+++ b/docs/HANDOVER.md
@@ -225,12 +225,36 @@ added to WP after the last sync will 404. The rsync must be **pushed from**
 `10.248.40.141` — Delta holds no key to pull:
 
 ```bash
-ssh tadmin@10.248.40.141 'rsync -a --no-owner --no-group --chmod=D755,F644 \
+ssh tadmin@10.248.40.141 'rsync -a --no-owner --no-group --chmod=D775,F664 \
   --exclude="*.php" --exclude="*.exe" --exclude="*.sh" \
   /var/www/html/thetriangle.org/wp-content/uploads/2026/ \
   tadmin@10.248.40.168:/mnt/cephfs/media/wp-content/uploads/2026/'
 ```
 
+**`D775`, not `D755` — the group bit is load-bearing.** The CMS backend runs as
+uid 10001 and owns none of this tree, so its write access comes from a POSIX ACL
+(`user:10001:rwx` plus the `default:` entries new directories inherit). A chmod
+on an ACL'd directory sets the **ACL mask** from the mode's group bits, so
+`D755` silently clamps that entry to `#effective:r-x`. `getfacl` still prints
+`user:10001:rwx` and `ls` shows only a `+`, so it reads as correct.
+
+Uploads then keep working until the 1st of the next month, when the handler
+first has to create a new `YYYY/MM` directory and can't — `POST /v1/media`
+returns 500 on **every** upload with `mkdir .../uploads/YYYY/MM: permission
+denied` in the backend log. That is exactly the 2026-09-01 outage.
+
+`D775` keeps the mask at `rwx` and matches the `0775` the CMS itself creates
+month directories with. Verify after any sync — the mask, not the entry, is
+what decides:
+
+```bash
+getfacl -pc /mnt/cephfs/media/wp-content/uploads/2026 | grep -E '10001|mask::'
+# want: user:10001:rwx  (no "#effective:") and mask::rwx
+```
+
+`ansible-playbook playbooks/delta-host.yml` in `triangle-infrastructure` repairs
+a clamped mask, but only on the next run — the fix belongs in the command above.
+
 It exits **23** with "failed to set times" on one directory. That is a
 directory mtime owned by another uid, not a data failure — ignore it.