diff --git a/.claude/skills/hassio-addon-workflow/references/traps.md b/.claude/skills/hassio-addon-workflow/references/traps.md
index aade569a08..bcadcd6c11 100644
--- a/.claude/skills/hassio-addon-workflow/references/traps.md
+++ b/.claude/skills/hassio-addon-workflow/references/traps.md
@@ -220,6 +220,14 @@ Assistant **ignores** the add-on's `shm_size`, so the real size varies per insta
 on one host. Detect at runtime rather than assuming either way; keep the flag when the size
 cannot be determined, because the crash it prevents is worse than its overhead.
 
+**Not every Electron app accepts arbitrary Chromium switches.** Joplin parses its own argv
+against an allow-list and aborts on anything else: `--disable-dev-shm-usage` produced
+`An error occurred: Unknown flag: --disable-dev-shm-usage` and no app window
+(`packages/lib/utils/processStartFlags.ts`; it accepts `--no-sandbox`, `--ozone-platform=`,
+`--disable-gpu` and a few others). The build-time `grep` that "verified" the flag only proved
+the text was in the wrapper, not that the app tolerated it. Check the app's argument parser before
+passing a Chromium flag to anything that is not Chromium itself (joplin_desktop, PR #3112, fixed in the follow-up PR).
+
 ## CI and review bots
 
 **What CI actually gates** — checked against the workflows, because assuming costs a cycle.
diff --git a/joplin_desktop/CHANGELOG.md b/joplin_desktop/CHANGELOG.md
index fa60707684..3b9e4d0dae 100644
--- a/joplin_desktop/CHANGELOG.md
+++ b/joplin_desktop/CHANGELOG.md
@@ -1,4 +1,7 @@
 
+## 3.7.21.1 (2026-10-11)
+- Fix Joplin not opening ("An error occurred: Unknown flag: --disable-dev-shm-usage"): Joplin only accepts its own list of command-line flags, so the extra Chromium flag added in 3.7.21 is removed
+
 ## 3.7.21 (2026-10-11)
 - Initial release: Joplin desktop streamed to the browser with Selkies, based on linuxserver/docker-joplin (amd64 only, as Joplin publishes no Linux arm64 build)
 - Shares the Selkies startup scripts with the webtop and claude_desktop add-ons (data location, GPU permissions, ingress, SSL)
diff --git a/joplin_desktop/Dockerfile b/joplin_desktop/Dockerfile
index 7d296660a1..ac2065ccff 100644
--- a/joplin_desktop/Dockerfile
+++ b/joplin_desktop/Dockerfile
@@ -57,13 +57,6 @@ RUN chmod 744 /ha_lsio.sh && if grep -qr "lsio" /etc; then /ha_lsio.sh "$CONFIGL
 COPY rootfs/ /
 RUN find . -type f \( -name "*.sh" -o -name "run" -o -name "finish" \) -print -exec chmod +x {} \;
 
-# Docker's default /dev/shm is 64 MB and Home Assistant ignores an add-on's shm_size, while
-# LinuxServer documents 1 GB as needed for Electron apps. Make Chromium keep its shared memory
-# out of /dev/shm, as claude_desktop does when /dev/shm is small. The grep fails the build if a
-# future upstream wrapper no longer has the --no-sandbox anchor, instead of shipping without it.
-RUN sed -i 's|--no-sandbox|--no-sandbox --disable-dev-shm-usage|' /usr/bin/joplin && \
-    grep -q -- '--disable-dev-shm-usage' /usr/bin/joplin
-
 # Uses /bin for compatibility purposes
 # hadolint ignore=DL4005
 RUN if [ ! -f /bin/sh ] && [ -f /usr/bin/sh ]; then ln -s /usr/bin/sh /bin/sh; fi && \
diff --git a/joplin_desktop/config.yaml b/joplin_desktop/config.yaml
index 546c18feaa..38c82a81c5 100644
--- a/joplin_desktop/config.yaml
+++ b/joplin_desktop/config.yaml
@@ -103,5 +103,5 @@ schema:
 slug: joplin_desktop
 udev: true
 url: https://github.com/alexbelgium/hassio-addons/tree/master/joplin_desktop
-version: "3.7.21"
+version: "3.7.21.1"
 video: true