From 8a50538275ff2afd329516eb871565bd54113a5a Mon Sep 17 00:00:00 2001
From: npeter83 <npeter83+claude@gmail.com>
Date: Sun, 13 Sep 2026 01:45:28 +0200
Subject: [PATCH] fix(ui): a write that fails says what failed and why,
 wherever it fails

`req` opens its blocking dialog for 400/409/422/5xx only. A 403/404/429 was left to the caller and a
502-504 or lost connection got just the self-clearing connection notice, so 34 mutations without a
reporting onError and 15 direct api writes with a bare or missing catch failed in silence: a hidden
channel that snapped back, a share link that never revoked, a confirmed account delete that ended
with nothing said.

- lib/queryClient: one MutationCache reporter. react-query awaits it BEFORE the mutation's own
  onError, so it waits a task and skips a failure the site already reported; the toast title comes
  from `meta.failTitle`, `meta.errorShownInline` opts out.
- notifyActionError: title names WHAT failed, the line under it says WHY (`failureReason`: a
  structured localized detail, else the status). It marks the error reported (`markReported`), so
  nothing reports one failure twice; `notifyKnownFailure` for a site's own specific sentence.
- Every silent site named (45 new HU/EN titles, 4 reasons). Stale "the global dialog shows it"
  comments corrected on the job actions, channel block, Plex sync, cookies, download enqueue,
  account delete and usePref.
- Channels passed the tag hook's channel id to the reporter as the error; the tag and priority hooks
  now report themselves, with the real error.
- A contract test keeps every useMutation naming its failure; CLAUDE.md gains the pattern row.
- history.dom.test: the "navigates only once the entries are back" assertion raced an async act's
  macrotask flush (1 in 5 runs); it now asserts from a later popstate listener.
---
 CLAUDE.md                                     |   1 +
 frontend/src/App.tsx                          |  11 +-
 frontend/src/components/AccountPanel.tsx      |  14 +--
 .../src/components/AdminUsers.dom.test.tsx    |   3 +-
 frontend/src/components/AdminUsers.tsx        |  31 ++----
 frontend/src/components/ChannelPage.tsx       |  37 ++++---
 frontend/src/components/Channels.tsx          |   8 +-
 frontend/src/components/DownloadCenter.tsx    |   7 +-
 frontend/src/components/DownloadDialog.tsx    |   8 +-
 frontend/src/components/Feed.tsx              |  32 ++++--
 .../src/components/NotificationsPanel.tsx     |  24 +++-
 frontend/src/components/OnboardingWizard.tsx  |   2 +
 .../src/components/Playlists.dom.test.tsx     |   2 +-
 frontend/src/components/Playlists.tsx         |  18 +--
 frontend/src/components/PlaylistsRail.tsx     |   1 +
 frontend/src/components/PlexSidebar.tsx       |  12 +-
 frontend/src/components/ProfileEditor.tsx     |   2 +
 frontend/src/components/Scheduler.tsx         |   4 +
 frontend/src/components/SettingsPanel.tsx     |  18 ++-
 frontend/src/components/ShareDialog.tsx       |   4 +
 frontend/src/components/Stats.tsx             |   5 +
 frontend/src/components/SyncStatus.tsx        |   6 +
 frontend/src/components/TagManager.tsx        |   1 +
 .../WriteFailureReporting.dom.test.tsx        |  24 +++-
 .../src/components/accountPanel.dom.test.tsx  |   6 +-
 frontend/src/components/downloadAdmin.tsx     |   2 +
 frontend/src/components/downloadModals.tsx    |   1 +
 .../usePlaylistActions.dom.test.tsx           |   9 +-
 frontend/src/i18n/locales/en/errors.json      |  52 +++++++++
 frontend/src/i18n/locales/hu/errors.json      |  52 +++++++++
 frontend/src/lib/actionErrors.test.ts         |  75 ++++++++++++-
 frontend/src/lib/actionErrors.ts              |  68 +++++++++---
 frontend/src/lib/api/core.ts                  |   8 ++
 .../src/lib/channelWriteFailures.dom.test.tsx |  83 ++++++++++++++
 frontend/src/lib/history.dom.test.ts          |  20 +++-
 .../src/lib/mutationFailures.contract.test.ts |  77 +++++++++++++
 frontend/src/lib/queryClient.dom.test.ts      | 104 ++++++++++++++++++
 frontend/src/lib/queryClient.ts               |  42 +++++++
 frontend/src/lib/useChannelPriority.ts        |   9 +-
 frontend/src/lib/useChannelTagActions.ts      |  17 ++-
 frontend/src/lib/usePref.dom.test.ts          |   7 +-
 .../src/lib/usePref.saveFailure.dom.test.ts   |  51 +++++++++
 frontend/src/lib/usePref.ts                   |  16 ++-
 frontend/src/lib/youtubeErrors.ts             |   2 +
 frontend/src/main.tsx                         |   7 +-
 45 files changed, 851 insertions(+), 132 deletions(-)
 create mode 100644 frontend/src/lib/channelWriteFailures.dom.test.tsx
 create mode 100644 frontend/src/lib/mutationFailures.contract.test.ts
 create mode 100644 frontend/src/lib/queryClient.dom.test.ts
 create mode 100644 frontend/src/lib/queryClient.ts
 create mode 100644 frontend/src/lib/usePref.saveFailure.dom.test.ts

diff --git a/CLAUDE.md b/CLAUDE.md
index caca1a13..af4f1665 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -91,6 +91,7 @@ CQRS, a repository/DI framework, a router library, a generic CRUD-router factory
 | FE keyboard shortcut    | a row in `lib/keymap.ts` + `dispatch()`                      | a private `keydown` listener with its own key literals |
 | FE control              | the chip/segmented primitives (`Select` only for >6 options) | a native `<select>`                                    |
 | FE loading/empty/error  | `QueryState` / `StateMessage`                                | ad-hoc conditionals                                    |
+| FE failed write         | `meta.failTitle`, else `notifyActionError(err, title)`       | a bare `catch {}` or `notify()` with no error attached |
 
 ## House rules that bite (learned from real bugs)
 
diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index 869d7533..75061a50 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -3,6 +3,7 @@ import { qk } from "./lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { api, getActiveAccount, setActiveAccount, setUnauthorizedHandler } from "./lib/api";
+import { notifyActionError } from "./lib/actionErrors";
 import { setLanguage, isSupported, type LangCode } from "./i18n";
 import { clearParams } from "./lib/urlState";
 import type { Page } from "./lib/urlState";
@@ -97,7 +98,9 @@ export default function App() {
   function setPanelLayout(panel: PanelId, next: PanelLayout) {
     setPanelLayouts((prev) => ({ ...prev, [panel]: next }));
     saveLayoutLocal(panel, next);
-    api.savePrefs({ [PREF_KEY[panel]]: next }).catch(() => {});
+    api
+      .savePrefs({ [PREF_KEY[panel]]: next })
+      .catch((e: unknown) => notifyActionError(e, t("errors.writeFailed.prefs")));
   }
 
   // Collapse state for the two full-height panels (left nav + filter sidebar). Persisted to the
@@ -287,7 +290,11 @@ export default function App() {
   // Language is edited outside the Settings page (sidebar/login), so it stays instant.
   function changeLanguage(code: LangCode) {
     setLanguage(code);
-    api.savePrefs({ language: code }).catch(() => {});
+    // Also from the signed-out login page, where the save 401s — `req` marks that as handled, so
+    // the reporter stays quiet there.
+    api
+      .savePrefs({ language: code })
+      .catch((e: unknown) => notifyActionError(e, t("errors.writeFailed.prefs")));
   }
 
   // First-run: a fresh instance is in setup mode (all other routes are locked) → show the wizard.
diff --git a/frontend/src/components/AccountPanel.tsx b/frontend/src/components/AccountPanel.tsx
index 4b1377ca..42298122 100644
--- a/frontend/src/components/AccountPanel.tsx
+++ b/frontend/src/components/AccountPanel.tsx
@@ -7,6 +7,7 @@ import { LoadingState, StateMessage } from "./ui/QueryState";
 import { api } from "../lib/api";
 import { qk } from "../lib/queryKeys";
 import { notify } from "../lib/notifications";
+import { notifyActionError, notifyKnownFailure } from "../lib/actionErrors";
 import { copyText } from "../lib/clipboard";
 import { invalidateUserAccess, isEmailOff } from "../lib/adminUsers";
 import type { AdminUserAccess, AdminUserRow } from "../lib/api/admin";
@@ -66,25 +67,22 @@ export default function AccountPanel({
   const issue = useMutation({
     mutationFn: () => api.createSetPasswordLink(user.id),
     onSuccess: (r) => setLink({ url: r.url, expires_at: r.expires_at }),
-    onError: () => notify({ level: "error", message: t("users.account.linkFailed") }),
+    onError: (err) => notifyActionError(err, t("users.account.linkFailed")),
   });
 
   const resend = useMutation({
     mutationFn: () => api.resendUserVerification(user.id),
     onSuccess: () => notify({ level: "success", message: t("users.account.resendSent") }),
     onError: (err) =>
-      notify({
-        level: "error",
-        message: isEmailOff(err)
-          ? t("users.pendingAccess.resendEmailOff")
-          : t("users.pendingAccess.resendFailed"),
-      }),
+      isEmailOff(err)
+        ? notifyKnownFailure(err, t("users.pendingAccess.resendEmailOff"))
+        : notifyActionError(err, t("users.pendingAccess.resendFailed")),
   });
 
   const activate = useMutation({
     mutationFn: () => api.activateUser(user.id),
     onSuccess: () => invalidateUserAccess(qc, user.id),
-    onError: () => notify({ level: "error", message: t("users.pendingAccess.activateFailed") }),
+    onError: (err) => notifyActionError(err, t("users.pendingAccess.activateFailed")),
   });
 
   const copy = async () => {
diff --git a/frontend/src/components/AdminUsers.dom.test.tsx b/frontend/src/components/AdminUsers.dom.test.tsx
index d94ad5e8..bf582486 100644
--- a/frontend/src/components/AdminUsers.dom.test.tsx
+++ b/frontend/src/components/AdminUsers.dom.test.tsx
@@ -86,7 +86,8 @@ describe("a user action that fails", () => {
     await waitFor(() =>
       expect(notify).toHaveBeenCalledWith({
         level: "error",
-        message: i18n.t("errors.notFound.user"), // the request layer localizes a structured detail
+        title: "users.roles.failed", // what failed…
+        message: i18n.t("errors.notFound.user"), // …and why: the request layer localizes a structured detail
       }),
     );
     await waitFor(() => expect(users).toHaveBeenCalledTimes(2));
diff --git a/frontend/src/components/AdminUsers.tsx b/frontend/src/components/AdminUsers.tsx
index 90ab5f27..2bd5a7d2 100644
--- a/frontend/src/components/AdminUsers.tsx
+++ b/frontend/src/components/AdminUsers.tsx
@@ -38,7 +38,7 @@ import { useConfirm } from "./ConfirmProvider";
 import Tabs, { usePersistedTab } from "./ui/Tabs";
 import { PageToolbar } from "./ui/PageShell";
 import { useMe } from "../lib/useMe";
-import { notifyActionError } from "../lib/actionErrors";
+import { notifyActionError, notifyKnownFailure } from "../lib/actionErrors";
 
 // Admin-only user & access management: roles, access requests (the Invite whitelist), and the
 // demo whitelist + reset. Each section is its own tab (persisted across reloads); the Access tab
@@ -351,7 +351,7 @@ function PendingAccess() {
       refresh();
       notify({ level: "success", message: t("users.pendingAccess.activated", { email: u.email }) });
     },
-    onError: () => notify({ level: "error", message: t("users.pendingAccess.activateFailed") }),
+    onError: (err) => notifyActionError(err, t("users.pendingAccess.activateFailed")),
   });
 
   const resend = useMutation({
@@ -363,12 +363,9 @@ function PendingAccess() {
     // The public resend is deliberately silent; this one is not, so say WHY it failed — an instance
     // with no working SMTP would otherwise show a success toast for a mail that was never sent.
     onError: (err) =>
-      notify({
-        level: "error",
-        message: isEmailOff(err)
-          ? t("users.pendingAccess.resendEmailOff")
-          : t("users.pendingAccess.resendFailed"),
-      }),
+      isEmailOff(err)
+        ? notifyKnownFailure(err, t("users.pendingAccess.resendEmailOff"))
+        : notifyActionError(err, t("users.pendingAccess.resendFailed")),
   });
 
   const list = rows.data ?? [];
@@ -563,12 +560,8 @@ function AdminInvites() {
       // A 409 means the list was stale — someone registered on that address since it loaded. Say
       // what to do instead of a generic failure, and refetch so the row re-renders as read-only.
       const conflict = e instanceof HttpError && e.status === 409;
-      notify({
-        level: "error",
-        message: t(
-          conflict ? "settings.invites.deleteHasAccount" : "settings.invites.deleteFailed",
-        ),
-      });
+      if (conflict) notifyKnownFailure(e, t("settings.invites.deleteHasAccount"));
+      else notifyActionError(e, t("settings.invites.deleteFailed"));
       if (conflict) refresh();
     },
   });
@@ -599,13 +592,9 @@ function AdminInvites() {
       notify({ level: "success", message: t("settings.invites.invited", { email: v.inv.email }) });
     },
     onError: (err) =>
-      notify({
-        level: "error",
-        message:
-          err instanceof HttpError && inviteEmailOff(err)
-            ? t("settings.invites.inviteEmailOff")
-            : t("settings.invites.inviteFailed"),
-      }),
+      err instanceof HttpError && inviteEmailOff(err)
+        ? notifyKnownFailure(err, t("settings.invites.inviteEmailOff"))
+        : notifyActionError(err, t("settings.invites.inviteFailed")),
   });
   // The one recipient the app has no language signal about: the admin typed this address in, so
   // nobody ever told us how to write to its owner. The dialog is where that gets decided — and it
diff --git a/frontend/src/components/ChannelPage.tsx b/frontend/src/components/ChannelPage.tsx
index 9098cdd8..fc7bca47 100644
--- a/frontend/src/components/ChannelPage.tsx
+++ b/frontend/src/components/ChannelPage.tsx
@@ -29,6 +29,7 @@ import { useFeedFilters } from "./FeedFiltersProvider";
 import { useConfirm } from "./ConfirmProvider";
 import { notify } from "../lib/notifications";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
+import { notifyActionError } from "../lib/actionErrors";
 import { api, DEFAULT_TAG_MODES, type FeedFilters, type Me, type ChannelDetail } from "../lib/api";
 import type { FeedView } from "../lib/feedView";
 import { channelYouTubeUrl, formatViews } from "../lib/format";
@@ -106,18 +107,23 @@ export default function ChannelPage({
   const runExplore = useCallback(
     (token?: string | null) => {
       setExploring(true);
-      return api
-        .exploreChannel(channelId, token ?? undefined)
-        .then((r) => {
-          setNextToken(r.next_page_token);
-          qc.invalidateQueries({ queryKey: qk.feed() });
-          // Refetch the detail so `explored` flips true → the "Exploring" badge shows on the
-          // first visit (the GET that drove this render predated the explore that just ran).
-          qc.invalidateQueries({ queryKey: qk.channel(channelId) });
-        })
-        .finally(() => setExploring(false));
+      return (
+        api
+          .exploreChannel(channelId, token ?? undefined)
+          .then((r) => {
+            setNextToken(r.next_page_token);
+            qc.invalidateQueries({ queryKey: qk.feed() });
+            // Refetch the detail so `explored` flips true → the "Exploring" badge shows on the
+            // first visit (the GET that drove this render predated the explore that just ran).
+            qc.invalidateQueries({ queryKey: qk.channel(channelId) });
+          })
+          // Both the automatic first-visit explore and "load more" used to fail in silence: an empty
+          // channel page that looked like a channel with no videos (E11 S2b).
+          .catch((err: unknown) => notifyActionError(err, t("errors.writeFailed.exploreChannel")))
+          .finally(() => setExploring(false))
+      );
     },
-    [channelId, qc],
+    [channelId, qc, t],
   );
 
   // First visit of an un-subscribed channel → ingest its recent uploads. Skipped for the demo
@@ -129,7 +135,7 @@ export default function ChannelPage({
     if (ch.subscribed || ch.blocked) return;
     if (ch.explored && ch.stored_videos > 0) return;
     autoTried.current = channelId;
-    runExplore().catch(() => {});
+    void runExplore();
   }, [ch, channelId, me.is_demo, runExplore]);
 
   const block = useMutation({
@@ -139,15 +145,16 @@ export default function ChannelPage({
       qc.invalidateQueries({ queryKey: qk.feed() });
       qc.invalidateQueries({ queryKey: qk.blockedChannels() });
     },
-    // No onError: block/unblock is a local DB op (not a YouTube write, so no 403 "connect" case),
-    // and its 400/409/422/500 are already surfaced by the global error modal (see api.ts req) —
-    // a caller toast here would just double it.
+    // No onError: a local DB op (not a YouTube write, so no 403 "connect" case). The app-wide
+    // reporter names the failure the error dialog does not cover (a 404 for a channel gone).
+    meta: { failTitle: "errors.writeFailed.blockChannel" },
   });
 
   // "Hide from feed": the same per-subscription flag the manager toggles (updateChannel {hidden}).
   // Local DB op — keeps the subscription, only drops the channel's videos from the feed/explore.
   const toggleHidden = useMutation({
     mutationFn: () => api.updateChannel(channelId, { hidden: !ch?.hidden }),
+    meta: { failTitle: "errors.writeFailed.hideChannel" },
     onSuccess: () => {
       qc.invalidateQueries({ queryKey: qk.channel(channelId) });
       qc.invalidateQueries({ queryKey: qk.feed() });
diff --git a/frontend/src/components/Channels.tsx b/frontend/src/components/Channels.tsx
index 909c7874..f35a436d 100644
--- a/frontend/src/components/Channels.tsx
+++ b/frontend/src/components/Channels.tsx
@@ -6,7 +6,6 @@ import { Ban, Eye, EyeOff, History, LayoutGrid, Pencil, RefreshCw, Table, X } fr
 import ExcludedChannelList from "./ExcludedChannelList";
 import { api, NO_TAG_SENTINEL, type ManagedChannel } from "../lib/api";
 import { Sparkles } from "lucide-react";
-import { notifyActionError } from "../lib/actionErrors";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
 import { accountKey, LS } from "../lib/storage";
 import { buildChannelColumns, applySyncFilter } from "./channelColumns";
@@ -110,6 +109,7 @@ export default function Channels() {
   const [tagManagerOpen, setTagManagerOpen] = useState(false);
   const unblock = useMutation({
     mutationFn: (id: string) => api.unblockChannel(id),
+    meta: { failTitle: "errors.writeFailed.unblockChannel" },
     onSuccess: () => {
       qc.invalidateQueries({ queryKey: qk.blockedChannels() });
       qc.invalidateQueries({ queryKey: qk.feed() });
@@ -133,6 +133,7 @@ export default function Channels() {
       id: string;
       body: { priority?: number; hidden?: boolean; deep_requested?: boolean };
     }) => api.updateChannel(v.id, v.body),
+    meta: { failTitle: "errors.writeFailed.channelSettings" },
     // Requesting full history may have just pulled in a channel's recent uploads, so
     // refresh the per-user status and feed too — not only the channel list.
     onSuccess: () => {
@@ -182,11 +183,10 @@ export default function Channels() {
     // A failed attach/detach reverts the optimistic change by refetching the list. Also refetch
     // suggestions: acceptSuggestion optimistically dropped the channel from that cache, and on
     // failure the tag never actually attached — so the server still suggests it and the ghost chips
-    // must come back rather than leaving an empty cell (honest states). Surface the write failure too.
-    onError: (e) => {
+    // must come back rather than leaving an empty cell (honest states). The hook reports the failure.
+    onError: () => {
       qc.invalidateQueries({ queryKey: qk.channels() });
       qc.invalidateQueries({ queryKey: qk.tagSuggestions() });
-      notifyActionError(e, t("channels.tags.updateFailed"));
     },
   });
   // --- Suggested tags (E8·S4): learned from the user's own manual-tag pattern, shown as accept/dismiss
diff --git a/frontend/src/components/DownloadCenter.tsx b/frontend/src/components/DownloadCenter.tsx
index a4fa1787..335a4e0f 100644
--- a/frontend/src/components/DownloadCenter.tsx
+++ b/frontend/src/components/DownloadCenter.tsx
@@ -34,6 +34,7 @@ import { useConfirm } from "./ConfirmProvider";
 import { useLiveQuery } from "../lib/useLiveQuery";
 import { useMe } from "../lib/useMe";
 import { api, type DownloadJob } from "../lib/api";
+import { notifyActionError } from "../lib/actionErrors";
 import { invalidateDownloads, quickShareLink } from "../lib/downloads";
 import { libraryRowActions, sharedRowActions } from "../lib/downloadActions";
 import { inputCls } from "./ui/form";
@@ -95,14 +96,16 @@ export default function DownloadCenter() {
               ? api.redownloadDownload(id)
               : api.deleteDownload(id),
     onSuccess: invalidate,
-    // No onError: a failed job action returns 400/409/422/500, already surfaced by the global
-    // error modal (api.ts req) with the backend's specific message — a caller toast would double it.
+    // A 400/409/422/500 opens the global error dialog with the backend's own message (and is then
+    // skipped here); a job that is gone (404) or a lost connection is said here, naming the action.
+    onError: (e, v) => notifyActionError(e, t(`errors.writeFailed.job.${v.op}`)),
   });
 
   // Remove a shared-with-me item from your list (only your grant; the owner's file is untouched).
   const removeShared = useMutation({
     mutationFn: (id: number) => api.removeSharedDownload(id),
     onSuccess: () => qc.invalidateQueries({ queryKey: qk.downloadsShared() }),
+    meta: { failTitle: "errors.writeFailed.removeShared" },
   });
 
   const confirmThen = async (title: string, body: string, fn: () => void) => {
diff --git a/frontend/src/components/DownloadDialog.tsx b/frontend/src/components/DownloadDialog.tsx
index 0ad44a56..dac5c792 100644
--- a/frontend/src/components/DownloadDialog.tsx
+++ b/frontend/src/components/DownloadDialog.tsx
@@ -10,6 +10,7 @@ import { invalidateDownloads } from "../lib/downloads";
 // Lazy: the full profile editor only opens from the dialog's "manage presets" affordance.
 const ProfileEditor = lazy(() => import("./ProfileEditor"));
 import { notify } from "../lib/notifications";
+import { notifyActionError } from "../lib/actionErrors";
 import { navigateTo } from "../lib/nav";
 import { inputCls } from "./ui/form";
 
@@ -51,8 +52,11 @@ export default function DownloadDialog({
         action: { label: t("downloads.dialog.view"), onClick: () => navigateTo("downloads") },
       });
       onClose();
-    } catch {
-      // Non-quiet request: the global error dialog already surfaced the reason (e.g. quota).
+    } catch (e) {
+      // A quota or validation refusal (422/409) is already in the global error dialog, which the
+      // reporter skips; a 403/404/429 or a lost connection is not, and used to leave the dialog
+      // open with nothing said (E11 S2b).
+      notifyActionError(e, t("errors.writeFailed.enqueueDownload"));
       setBusy(false);
     }
   };
diff --git a/frontend/src/components/Feed.tsx b/frontend/src/components/Feed.tsx
index 182fb3ca..e4a8d84c 100644
--- a/frontend/src/components/Feed.tsx
+++ b/frontend/src/components/Feed.tsx
@@ -23,6 +23,7 @@ import {
 } from "lucide-react";
 import { api, HttpError, type FeedFilters, type Video, type VideoStatus } from "../lib/api";
 import i18n from "../i18n";
+import { notifyActionError } from "../lib/actionErrors";
 import { notify, resolveVideo } from "../lib/notifications";
 import { useDebounced } from "../lib/useDebounced";
 import {
@@ -308,15 +309,17 @@ export default function Feed({
             resolveVideo(id);
           }
         })
-        .catch(() =>
+        .catch((e: unknown) => {
           // Server rejected the change — drop the optimistic override so the card
-          // reverts to the real (unchanged) state instead of staying phantom-hidden.
+          // reverts to the real (unchanged) state instead of staying phantom-hidden, and say so:
+          // a card that silently snaps back reads as a misclick.
+          notifyActionError(e, i18n.t("errors.writeFailed.videoState"));
           setOverrides((o) => {
             const next = { ...o };
             delete next[id];
             return next;
-          }),
-        );
+          });
+        });
     },
     [qc],
   );
@@ -330,13 +333,14 @@ export default function Feed({
         .clearState(id)
         // Stale-mark only (see setState above): the optimistic override already reset the card.
         .then(() => qc.invalidateQueries({ queryKey: qk.feed(), refetchType: "none" }))
-        .catch(() =>
+        .catch((e: unknown) => {
+          notifyActionError(e, i18n.t("errors.writeFailed.videoReset"));
           setOverrides((o) => {
             const next = { ...o };
             delete next[id];
             return next;
-          }),
-        );
+          });
+        });
     },
     [qc],
   );
@@ -349,7 +353,15 @@ export default function Feed({
           qc.invalidateQueries({ queryKey: qk.playlists() });
           qc.invalidateQueries({ queryKey: qk.playlist() });
         })
-        .catch(() => setSavedOverrides((o) => ({ ...o, [id]: !saved })));
+        .catch((e: unknown) => {
+          notifyActionError(
+            e,
+            i18n.t(
+              saved ? "errors.writeFailed.watchLaterAdd" : "errors.writeFailed.watchLaterRemove",
+            ),
+          );
+          setSavedOverrides((o) => ({ ...o, [id]: !saved }));
+        });
     },
     [qc],
   );
@@ -466,7 +478,9 @@ export default function Feed({
                     .then((r) =>
                       notify({ message: i18n.t("feed.yt.cleared", { count: r.removed }) }),
                     )
-                    .catch(() => {});
+                    .catch((e: unknown) =>
+                      notifyActionError(e, i18n.t("errors.writeFailed.clearSearch")),
+                    );
                   setFilters({ ...filters, librarySource: "organic", sort: "newest" });
                   onExitYtSearch();
                   dropFeedCaches();
diff --git a/frontend/src/components/NotificationsPanel.tsx b/frontend/src/components/NotificationsPanel.tsx
index b03805ae..6abb4f10 100644
--- a/frontend/src/components/NotificationsPanel.tsx
+++ b/frontend/src/components/NotificationsPanel.tsx
@@ -79,10 +79,26 @@ export default function NotificationsPanel() {
     qc.invalidateQueries({ queryKey: qk.notifications() });
     qc.invalidateQueries({ queryKey: qk.notifUnread() });
   };
-  const readMut = useMutation({ mutationFn: api.markNotificationRead, onSuccess: refresh });
-  const readAllMut = useMutation({ mutationFn: api.markAllNotificationsRead, onSuccess: refresh });
-  const dismissMut = useMutation({ mutationFn: api.dismissNotification, onSuccess: refresh });
-  const clearMut = useMutation({ mutationFn: api.clearNotifications, onSuccess: refresh });
+  const readMut = useMutation({
+    mutationFn: api.markNotificationRead,
+    onSuccess: refresh,
+    meta: { failTitle: "errors.writeFailed.notifRead" },
+  });
+  const readAllMut = useMutation({
+    mutationFn: api.markAllNotificationsRead,
+    onSuccess: refresh,
+    meta: { failTitle: "errors.writeFailed.notifReadAll" },
+  });
+  const dismissMut = useMutation({
+    mutationFn: api.dismissNotification,
+    onSuccess: refresh,
+    meta: { failTitle: "errors.writeFailed.notifDismiss" },
+  });
+  const clearMut = useMutation({
+    mutationFn: api.clearNotifications,
+    onSuccess: refresh,
+    meta: { failTitle: "errors.writeFailed.notifClear" },
+  });
 
   const serverUnread = serverItems.some((n) => !n.read);
   const hasAny = serverItems.length > 0 || clientItems.length > 0;
diff --git a/frontend/src/components/OnboardingWizard.tsx b/frontend/src/components/OnboardingWizard.tsx
index 863b11d8..ab2db044 100644
--- a/frontend/src/components/OnboardingWizard.tsx
+++ b/frontend/src/components/OnboardingWizard.tsx
@@ -82,6 +82,8 @@ export default function OnboardingWizard({ me, onClose }: { me: Me; onClose: ()
       qc.invalidateQueries({ queryKey: qk.channels() });
       qc.invalidateQueries({ queryKey: qk.myStatus() });
     },
+    // The step itself says the import failed and offers a retry button — no toast on top.
+    meta: { errorShownInline: true },
   });
 
   // First time we see read access with zero imported channels, pull the user's
diff --git a/frontend/src/components/Playlists.dom.test.tsx b/frontend/src/components/Playlists.dom.test.tsx
index 38ec9004..d786233d 100644
--- a/frontend/src/components/Playlists.dom.test.tsx
+++ b/frontend/src/components/Playlists.dom.test.tsx
@@ -380,7 +380,7 @@ describe("Playlists — a change the server refused is taken back on screen", ()
     await act(async () => void (await new Promise((r) => setTimeout(r, 20))));
     expect(rowsOnScreen()).toEqual(["Video V", "Video W"]);
     expect(h.notify).toHaveBeenCalledWith(
-      expect.objectContaining({ level: "error", message: "playlists.removeFailed" }),
+      expect.objectContaining({ level: "error", title: "playlists.removeFailed" }),
     );
   });
 });
diff --git a/frontend/src/components/Playlists.tsx b/frontend/src/components/Playlists.tsx
index f30f7d98..252cd2cc 100644
--- a/frontend/src/components/Playlists.tsx
+++ b/frontend/src/components/Playlists.tsx
@@ -676,13 +676,17 @@ export default function Playlists() {
   }
 
   function playerState(id: string, status: string) {
-    api.setState(id, status).then(() => {
-      // Every cached list, not `qk.playlist(selectedId)`: the player may have been launched from a
-      // list that is no longer the selected one (see `playing`), and that list holds the row whose
-      // status just changed. Only lists actually on screen refetch.
-      qc.invalidateQueries({ queryKey: qk.playlist() });
-      qc.invalidateQueries({ queryKey: qk.feed() });
-    });
+    api
+      .setState(id, status)
+      .then(() => {
+        // Every cached list, not `qk.playlist(selectedId)`: the player may have been launched from a
+        // list that is no longer the selected one (see `playing`), and that list holds the row whose
+        // status just changed. Only lists actually on screen refetch.
+        qc.invalidateQueries({ queryKey: qk.playlist() });
+        qc.invalidateQueries({ queryKey: qk.feed() });
+      })
+      // No catch used to mean an unhandled rejection and nothing said (E11 S2b).
+      .catch((e: unknown) => notifyActionError(e, t("errors.writeFailed.videoState")));
   }
 
   // Stable identity: PlayerModal's keydown/scroll-lock effect depends on it, so a fresh closure
diff --git a/frontend/src/components/PlaylistsRail.tsx b/frontend/src/components/PlaylistsRail.tsx
index ec8053cf..471c4d08 100644
--- a/frontend/src/components/PlaylistsRail.tsx
+++ b/frontend/src/components/PlaylistsRail.tsx
@@ -160,6 +160,7 @@ export default function PlaylistsRail({
 
   const createMut = useMutation({
     mutationFn: (name: string) => api.createPlaylist(name),
+    meta: { failTitle: "errors.writeFailed.createPlaylist" },
     onSuccess: (pl: Playlist) => {
       setNewName("");
       qc.invalidateQueries({ queryKey: qk.playlists() });
diff --git a/frontend/src/components/PlexSidebar.tsx b/frontend/src/components/PlexSidebar.tsx
index cda82882..b6e742de 100644
--- a/frontend/src/components/PlexSidebar.tsx
+++ b/frontend/src/components/PlexSidebar.tsx
@@ -6,6 +6,7 @@ import { plexLang } from "../i18n";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { ListMusic, Plus, SlidersHorizontal, X } from "lucide-react";
 import { api, plexFilterCount, type PlexFilters } from "../lib/api";
+import { notifyActionError } from "../lib/actionErrors";
 import { type PanelLayout } from "../lib/panelLayout";
 import { type MediaSourceKind } from "../lib/mediaSource";
 import { useDebounced } from "../lib/useDebounced";
@@ -377,9 +378,14 @@ function PlaylistPicker() {
   ];
 
   async function createPlaylist(title: string) {
-    const pl = await api.plexCreatePlaylist(title);
-    qc.invalidateQueries({ queryKey: qk.plexPlaylists() });
-    onOpenPlaylist(pl.id);
+    try {
+      const pl = await api.plexCreatePlaylist(title);
+      qc.invalidateQueries({ queryKey: qk.plexPlaylists() });
+      onOpenPlaylist(pl.id);
+    } catch (e) {
+      // Called without an await from the picker, so a refusal was an unhandled rejection (E11 S2b).
+      notifyActionError(e, t("plex.playlistAdd.createFailed"));
+    }
   }
 
   return (
diff --git a/frontend/src/components/ProfileEditor.tsx b/frontend/src/components/ProfileEditor.tsx
index 199b2689..8e43ba54 100644
--- a/frontend/src/components/ProfileEditor.tsx
+++ b/frontend/src/components/ProfileEditor.tsx
@@ -77,10 +77,12 @@ export default function ProfileEditor({ onClose }: { onClose: () => void }) {
       invalidate();
       setFormOpen(false);
     },
+    meta: { failTitle: "errors.writeFailed.downloadProfileSave" },
   });
   const del = useMutation({
     mutationFn: (id: number) => api.deleteDownloadProfile(id),
     onSuccess: invalidate,
+    meta: { failTitle: "errors.writeFailed.downloadProfileDelete" },
   });
 
   const onDelete = async (p: DownloadProfile) => {
diff --git a/frontend/src/components/Scheduler.tsx b/frontend/src/components/Scheduler.tsx
index b1d779c0..5f0e62be 100644
--- a/frontend/src/components/Scheduler.tsx
+++ b/frontend/src/components/Scheduler.tsx
@@ -390,6 +390,7 @@ export default function Scheduler() {
   // live poll surfaces the "running" state within a tick, so we just nudge a refetch.
   const runMut = useMutation({
     mutationFn: (jobId: string) => api.runSchedulerJob(jobId),
+    meta: { failTitle: "errors.writeFailed.schedulerRun" },
     onSuccess: (_d, jobId) => {
       notify({
         level: "success",
@@ -400,6 +401,7 @@ export default function Scheduler() {
   });
   const runAllMut = useMutation({
     mutationFn: () => api.runAllSchedulerJobs(),
+    meta: { failTitle: "errors.writeFailed.schedulerRunAll" },
     onSuccess: (res) => {
       notify({
         level: "success",
@@ -410,10 +412,12 @@ export default function Scheduler() {
   });
   const batchMut = useMutation({
     mutationFn: (v: number) => api.updateMaintenanceBatch(v),
+    meta: { failTitle: "errors.writeFailed.maintenanceBatch" },
     onSuccess: () => qc.invalidateQueries({ queryKey: qk.scheduler() }),
   });
   const purgeMut = useMutation({
     mutationFn: () => api.purgeDiscovery(),
+    meta: { failTitle: "errors.writeFailed.purgeDiscovery" },
     onSuccess: (res) => {
       const removed =
         (res.search_videos_deleted ?? 0) + (res.videos_deleted ?? 0) + (res.channels_deleted ?? 0);
diff --git a/frontend/src/components/SettingsPanel.tsx b/frontend/src/components/SettingsPanel.tsx
index e32c270c..1798a33f 100644
--- a/frontend/src/components/SettingsPanel.tsx
+++ b/frontend/src/components/SettingsPanel.tsx
@@ -6,6 +6,7 @@ import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { Bell, Check, Loader2, Monitor, RefreshCw, Trash2, User } from "lucide-react";
 import { SCHEMES, type Scheme, type ThemePrefs } from "../lib/theme";
 import { api, HttpError, type Me } from "../lib/api";
+import { notifyActionError } from "../lib/actionErrors";
 import { LS, useAccountPersistedState } from "../lib/storage";
 import Avatar from "./ui/Avatar";
 import { notify, type NotifSettings } from "../lib/notifications";
@@ -553,10 +554,11 @@ function PlexWatchSync() {
   };
 
   // No onError on these: they're Plex, not YouTube (notifyYouTubeActionError would wrongly say
-  // "connect YouTube"), and their 409/500 are already surfaced by the global error modal — a caller
-  // toast would double it. Actual import failures come back as 200 {error}, handled by announce.
+  // "connect YouTube"). Their 409/500 open the global error dialog and the rest go to the app-wide
+  // reporter (lib/queryClient). Actual import failures come back as 200 {error}, handled by announce.
   const toggle = useMutation({
     mutationFn: (next: boolean) => api.plexWatchSetLink(next),
+    meta: { failTitle: "errors.writeFailed.plexWatchLink" },
     onSuccess: (res) => {
       qc.setQueryData(qk.plexWatchLink(), res);
       announce(res.import);
@@ -564,6 +566,7 @@ function PlexWatchSync() {
   });
   const reimport = useMutation({
     mutationFn: () => api.plexWatchImport(),
+    meta: { failTitle: "errors.writeFailed.plexWatchImport" },
     onSuccess: (res) => {
       qc.setQueryData(qk.plexWatchLink(), res);
       announce(res.import);
@@ -637,10 +640,13 @@ export function DownloadAuth() {
       notify({ level: "success", message: t("settings.downloadAuth.saved") });
     },
     // Validation errors (empty / not-a-cookies.txt) surface via the global error modal, which
-    // localizes the structured { code: "cookies" } detail (see lib/api core localizeDetail).
+    // localizes the structured { code: "cookies" } detail (see lib/api core localizeDetail); the
+    // statuses it does not cover go to the app-wide reporter.
+    meta: { failTitle: "settings.downloadAuth.failed" },
   });
   const remove = useMutation({
     mutationFn: (host: string) => api.deleteDownloadCredential(host),
+    meta: { failTitle: "errors.writeFailed.downloadCookiesRemove" },
     onSuccess: (res) => {
       qc.setQueryData(qk.downloadCredentials(), res);
       notify({ level: "success", message: t("settings.downloadAuth.removed") });
@@ -781,8 +787,10 @@ function Account({ me, onOpenWizard }: { me: Me; onOpenWizard: () => void }) {
       await api.deleteAccount();
       // Session cleared server-side → /api/me 401s → Welcome. The flag shows the confirmation banner.
       window.location.href = "/?deleted=1";
-    } catch {
-      /* the global error dialog surfaces the reason (e.g. last admin) */
+    } catch (e) {
+      // A refusal with a reason (the last admin, a 409) is already in the global error dialog, which
+      // the reporter skips; anything else used to end the confirmed delete with nothing said.
+      notifyActionError(e, t("errors.writeFailed.deleteAccount"));
     }
   };
   return (
diff --git a/frontend/src/components/ShareDialog.tsx b/frontend/src/components/ShareDialog.tsx
index 67ec04b5..bae70fc6 100644
--- a/frontend/src/components/ShareDialog.tsx
+++ b/frontend/src/components/ShareDialog.tsx
@@ -106,15 +106,18 @@ function LinkRow({ link, onChanged }: { link: ShareLink; onChanged: () => void }
   const toggle = useMutation({
     mutationFn: () => api.updateDownloadLink(link.id, { allow_download: !link.allow_download }),
     onSuccess: onChanged,
+    meta: { failTitle: "errors.writeFailed.shareLinkDownload" },
   });
   const revoke = useMutation({
     mutationFn: () => api.revokeDownloadLink(link.id),
     onSuccess: onChanged,
+    meta: { failTitle: "errors.writeFailed.shareLinkRevoke" },
   });
   // Set / change / clear the link password. Rotating (or clearing) it bumps the link's
   // password_version server-side, which invalidates any watch grant issued under the old password.
   const savePass = useMutation({
     mutationFn: (pw: string) => api.updateDownloadLink(link.id, { password: pw }),
+    meta: { failTitle: "errors.writeFailed.shareLinkPassword" },
     onSuccess: () => {
       setEditPass(false);
       setNewPass("");
@@ -282,6 +285,7 @@ function LinkShare({ job }: { job: DownloadJob }) {
       setExpiryDays(null);
       setPassword("");
     },
+    meta: { failTitle: "downloads.share.linkFailed" },
   });
 
   // Open with a link already generated: sharing is almost always the goal, so mint a default
diff --git a/frontend/src/components/Stats.tsx b/frontend/src/components/Stats.tsx
index d07d62c4..21513e9d 100644
--- a/frontend/src/components/Stats.tsx
+++ b/frontend/src/components/Stats.tsx
@@ -210,6 +210,11 @@ function SystemStats() {
   const pauseResume = useMutation({
     mutationFn: (paused: boolean) => (paused ? api.resumeSync() : api.pauseSync()),
     onSuccess: () => qc.invalidateQueries({ queryKey: qk.myStatus() }),
+    onError: (e, paused) =>
+      notifyActionError(
+        e,
+        t(paused ? "errors.writeFailed.syncResume" : "errors.writeFailed.syncPause"),
+      ),
   });
   const data = q.data;
   const maxDaily = Math.max(1, ...(data?.daily ?? []).map((d) => d.total));
diff --git a/frontend/src/components/SyncStatus.tsx b/frontend/src/components/SyncStatus.tsx
index 5d04fa71..4a09c159 100644
--- a/frontend/src/components/SyncStatus.tsx
+++ b/frontend/src/components/SyncStatus.tsx
@@ -4,6 +4,7 @@ import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { CheckCircle2, Clock, Database, History, Loader2, Pause, Play } from "lucide-react";
 import { api, type MyStatus } from "../lib/api";
+import { notifyActionError } from "../lib/actionErrors";
 import { formatViews } from "../lib/format";
 import { PopoverSurface, useAnchoredPopover } from "./ui/AnchoredPopover";
 
@@ -53,6 +54,11 @@ export default function SyncStatus({
   const toggle = useMutation({
     mutationFn: () => (data?.paused ? api.resumeSync() : api.pauseSync()),
     onSuccess: () => qc.invalidateQueries({ queryKey: qk.myStatus() }),
+    onError: (e) =>
+      notifyActionError(
+        e,
+        t(data?.paused ? "errors.writeFailed.syncResume" : "errors.writeFailed.syncPause"),
+      ),
   });
 
   if (!data) return null;
diff --git a/frontend/src/components/TagManager.tsx b/frontend/src/components/TagManager.tsx
index a377abd4..4d0dfbb9 100644
--- a/frontend/src/components/TagManager.tsx
+++ b/frontend/src/components/TagManager.tsx
@@ -129,6 +129,7 @@ export default function TagManager({
   };
   const create = useMutation({
     mutationFn: (name: string) => api.createTag({ name, category: "other" }),
+    meta: { failTitle: "channels.tags.createFailed" },
     onSuccess: () => {
       setNewName("");
       invalidate();
diff --git a/frontend/src/components/WriteFailureReporting.dom.test.tsx b/frontend/src/components/WriteFailureReporting.dom.test.tsx
index b76e1555..f94b1a9f 100644
--- a/frontend/src/components/WriteFailureReporting.dom.test.tsx
+++ b/frontend/src/components/WriteFailureReporting.dom.test.tsx
@@ -19,7 +19,7 @@ vi.mock("react-i18next", async (importOriginal) => {
 });
 vi.mock("./ConfirmProvider", () => ({ useConfirm: () => async () => true }));
 const { notify } = vi.hoisted(() => ({
-  notify: vi.fn((_n: { level: string; message: string }) => 0),
+  notify: vi.fn((_n: { level: string; title?: string; message: string }) => 0),
 }));
 vi.mock("../lib/notifications", async (importOriginal) => ({
   ...(await importOriginal<typeof import("../lib/notifications")>()),
@@ -28,6 +28,7 @@ vi.mock("../lib/notifications", async (importOriginal) => ({
 
 import PlexCollectionEditor from "./PlexCollectionEditor";
 import UploadDialog from "./UploadDialog";
+import DownloadDialog from "./DownloadDialog";
 import { api, HttpError } from "../lib/api";
 
 beforeAll(() => {
@@ -48,7 +49,9 @@ afterEach(() => {
   vi.restoreAllMocks();
 });
 
-const errors = () => notify.mock.calls.filter(([n]) => n.level === "error").map(([n]) => n.message);
+// What failed, as the toast names it: the title (E11 S2b — the title says WHAT, the line under it WHY).
+const errors = () =>
+  notify.mock.calls.filter(([n]) => n.level === "error").map(([n]) => n.title ?? n.message);
 const client = () => new QueryClient({ defaultOptions: { queries: { retry: false } } });
 
 describe("the Plex collection editor", () => {
@@ -144,3 +147,20 @@ describe("the upload dialog", () => {
     expect(errors()).toEqual([]);
   });
 });
+
+describe("the download dialog", () => {
+  it("names a queueing the server refused with no dialog of its own", async () => {
+    // Its catch said "the global error dialog already surfaced the reason" — true for a quota 422,
+    // not for a 403/404/429 or a lost connection, where the dialog just stayed open (E11 S2b).
+    vi.spyOn(api, "downloadProfiles").mockResolvedValue([{ id: 1, name: "Best" }] as never);
+    vi.spyOn(api, "enqueueDownload").mockRejectedValue(new HttpError(403, "nope"));
+    render(
+      <QueryClientProvider client={client()}>
+        <DownloadDialog source="https://example.com/v" onClose={() => {}} />
+      </QueryClientProvider>,
+    );
+    await screen.findByText("Best");
+    fireEvent.click(screen.getByText("downloads.dialog.submit"));
+    await waitFor(() => expect(errors()).toEqual(["errors.writeFailed.enqueueDownload"]));
+  });
+});
diff --git a/frontend/src/components/accountPanel.dom.test.tsx b/frontend/src/components/accountPanel.dom.test.tsx
index 3de8c62e..21ff9949 100644
--- a/frontend/src/components/accountPanel.dom.test.tsx
+++ b/frontend/src/components/accountPanel.dom.test.tsx
@@ -87,11 +87,13 @@ beforeEach(() => {
 });
 afterEach(cleanup);
 
+// What each error toast names: its title when it has one (a failure reported with its reason under
+// it — E11 S2b), else the specific sentence (the no-mail case).
 const errorMessages = () =>
   notifyMock.mock.calls
-    .map(([a]) => a as { level?: string; message?: string })
+    .map(([a]) => a as { level?: string; title?: string; message?: string })
     .filter((a) => a.level === "error")
-    .map((a) => a.message);
+    .map((a) => a.title ?? a.message);
 
 describe("the account panel reports a refused recovery action", () => {
   it("says the link could not be minted instead of leaving the button inert", async () => {
diff --git a/frontend/src/components/downloadAdmin.tsx b/frontend/src/components/downloadAdmin.tsx
index 1a4a6870..4b5ee7ad 100644
--- a/frontend/src/components/downloadAdmin.tsx
+++ b/frontend/src/components/downloadAdmin.tsx
@@ -62,10 +62,12 @@ function QuotaModal({
         unlimited: state!.unlimited,
       }),
     onSuccess: done,
+    meta: { failTitle: "errors.writeFailed.downloadQuota" },
   });
   const reset = useMutation({
     mutationFn: () => api.adminResetDownloadQuota(userId),
     onSuccess: done,
+    meta: { failTitle: "errors.writeFailed.downloadQuotaReset" },
   });
   if (!state) return null;
   const upd = (p: Partial<typeof state>) => setForm({ ...state, ...p });
diff --git a/frontend/src/components/downloadModals.tsx b/frontend/src/components/downloadModals.tsx
index 0cf918e8..7c54a069 100644
--- a/frontend/src/components/downloadModals.tsx
+++ b/frontend/src/components/downloadModals.tsx
@@ -41,6 +41,7 @@ export function MetaEditModal({ job, onClose }: { job: DownloadJob; onClose: ()
       qc.invalidateQueries({ queryKey: qk.downloads() });
       onClose();
     },
+    meta: { failTitle: "errors.writeFailed.downloadDetails" },
   });
 
   return (
diff --git a/frontend/src/components/usePlaylistActions.dom.test.tsx b/frontend/src/components/usePlaylistActions.dom.test.tsx
index 5ab31b77..a132d70a 100644
--- a/frontend/src/components/usePlaylistActions.dom.test.tsx
+++ b/frontend/src/components/usePlaylistActions.dom.test.tsx
@@ -61,13 +61,16 @@ describe("usePlaylistActions — deleting a list", () => {
   });
 
   it("does say one it was left to handle — in the reader's language", async () => {
-    // A plain-string `detail` is whatever English the route hand-wrote; the caller's own translated
-    // fallback is the one a Hungarian reader can read (deep-review round 5).
+    // A plain-string `detail` is whatever English the route hand-wrote, so it is never shown: the
+    // caller's translated title says what failed, and the status says why (deep-review round 5, S2b).
     vi.spyOn(api, "deletePlaylist").mockRejectedValue(new HttpError(404, "Unknown playlist"));
     const { result } = mount();
     await act(() => result.current.remove(linked));
     expect(h.notify).toHaveBeenCalledWith(
-      expect.objectContaining({ message: "playlists.deleteYtFailed" }),
+      expect.objectContaining({ title: "playlists.deleteYtFailed" }),
+    );
+    expect(h.notify).not.toHaveBeenCalledWith(
+      expect.objectContaining({ message: "Unknown playlist" }),
     );
   });
 
diff --git a/frontend/src/i18n/locales/en/errors.json b/frontend/src/i18n/locales/en/errors.json
index f7420be4..47a56ce9 100644
--- a/frontend/src/i18n/locales/en/errors.json
+++ b/frontend/src/i18n/locales/en/errors.json
@@ -20,6 +20,58 @@
     "recipient": "There's no user with that email address.",
     "job": "This job doesn't exist any more."
   },
+  "reason": {
+    "unreachable": "The server couldn't be reached, so the change didn't go through. Try again.",
+    "forbidden": "You don't have permission to do that.",
+    "rateLimited": "Too many requests at once — wait a moment and try again.",
+    "unexpected": "Something went wrong in the app. Try again."
+  },
+  "writeFailed": {
+    "hideChannel": "Couldn't change whether the channel is hidden",
+    "blockChannel": "Couldn't change the channel's block",
+    "unblockChannel": "Couldn't unblock the channel",
+    "channelSettings": "Couldn't save the channel's settings",
+    "channelPriority": "Couldn't save the channel's priority",
+    "exploreChannel": "Couldn't load the channel's videos",
+    "downloadQuota": "Couldn't save the download quota",
+    "downloadQuotaReset": "Couldn't reset the download quota",
+    "job": {
+      "pause": "Couldn't pause the download",
+      "resume": "Couldn't resume the download",
+      "cancel": "Couldn't cancel the download",
+      "delete": "Couldn't delete the download",
+      "redownload": "Couldn't start the download again"
+    },
+    "removeShared": "Couldn't remove the shared item",
+    "downloadDetails": "Couldn't save the download's details",
+    "enqueueDownload": "Couldn't queue the download",
+    "downloadProfileSave": "Couldn't save the download profile",
+    "downloadProfileDelete": "Couldn't delete the download profile",
+    "downloadCookiesRemove": "Couldn't remove those cookies",
+    "notifRead": "Couldn't mark the notification as read",
+    "notifReadAll": "Couldn't mark the notifications as read",
+    "notifDismiss": "Couldn't dismiss the notification",
+    "notifClear": "Couldn't clear the notifications",
+    "createPlaylist": "Couldn't create the playlist",
+    "schedulerRun": "Couldn't start the job",
+    "schedulerRunAll": "Couldn't start the jobs",
+    "maintenanceBatch": "Couldn't save the maintenance batch size",
+    "purgeDiscovery": "Couldn't clear the discovery data",
+    "plexWatchLink": "Couldn't change the Plex watch-state sync",
+    "plexWatchImport": "Couldn't import the watch state from Plex",
+    "shareLinkDownload": "Couldn't change whether the link allows downloading",
+    "shareLinkRevoke": "Couldn't revoke the link",
+    "shareLinkPassword": "Couldn't change the link's password",
+    "syncPause": "Couldn't pause syncing",
+    "syncResume": "Couldn't resume syncing",
+    "prefs": "Couldn't save your settings",
+    "deleteAccount": "Couldn't delete your account",
+    "videoState": "Couldn't change the video's state",
+    "videoReset": "Couldn't reset the video",
+    "watchLaterAdd": "Couldn't add it to Watch later",
+    "watchLaterRemove": "Couldn't remove it from Watch later",
+    "clearSearch": "Couldn't clear the search results"
+  },
   "ok": "OK",
   "offline": {
     "title": "Connection lost",
diff --git a/frontend/src/i18n/locales/hu/errors.json b/frontend/src/i18n/locales/hu/errors.json
index f6e73b46..7c45b93f 100644
--- a/frontend/src/i18n/locales/hu/errors.json
+++ b/frontend/src/i18n/locales/hu/errors.json
@@ -20,6 +20,58 @@
     "recipient": "Nincs felhasználó ezzel az e-mail címmel.",
     "job": "Ez a feladat már nem létezik."
   },
+  "reason": {
+    "unreachable": "A művelet nem ment végbe: a szerver nem volt elérhető. Próbáld újra.",
+    "forbidden": "Ehhez nincs jogosultságod.",
+    "rateLimited": "Túl sok kérés egyszerre — várj egy kicsit, és próbáld újra.",
+    "unexpected": "Váratlan hiba történt az alkalmazásban. Próbáld újra."
+  },
+  "writeFailed": {
+    "hideChannel": "Nem sikerült módosítani a csatorna elrejtését",
+    "blockChannel": "Nem sikerült módosítani a csatorna tiltását",
+    "unblockChannel": "Nem sikerült feloldani a csatorna tiltását",
+    "channelSettings": "Nem sikerült menteni a csatorna beállításait",
+    "channelPriority": "Nem sikerült menteni a csatorna prioritását",
+    "exploreChannel": "Nem sikerült betölteni a csatorna videóit",
+    "downloadQuota": "Nem sikerült menteni a letöltési kvótát",
+    "downloadQuotaReset": "Nem sikerült visszaállítani a letöltési kvótát",
+    "job": {
+      "pause": "Nem sikerült szüneteltetni a letöltést",
+      "resume": "Nem sikerült folytatni a letöltést",
+      "cancel": "Nem sikerült megszakítani a letöltést",
+      "delete": "Nem sikerült törölni a letöltést",
+      "redownload": "Nem sikerült újraindítani a letöltést"
+    },
+    "removeShared": "Nem sikerült eltávolítani a megosztott elemet",
+    "downloadDetails": "Nem sikerült menteni a letöltés adatait",
+    "enqueueDownload": "Nem sikerült sorba állítani a letöltést",
+    "downloadProfileSave": "Nem sikerült menteni a letöltési profilt",
+    "downloadProfileDelete": "Nem sikerült törölni a letöltési profilt",
+    "downloadCookiesRemove": "Nem sikerült eltávolítani ezeket a sütiket",
+    "notifRead": "Nem sikerült olvasottnak jelölni az értesítést",
+    "notifReadAll": "Nem sikerült olvasottnak jelölni az értesítéseket",
+    "notifDismiss": "Nem sikerült elvetni az értesítést",
+    "notifClear": "Nem sikerült törölni az értesítéseket",
+    "createPlaylist": "Nem sikerült létrehozni a lejátszási listát",
+    "schedulerRun": "Nem sikerült elindítani a feladatot",
+    "schedulerRunAll": "Nem sikerült elindítani a feladatokat",
+    "maintenanceBatch": "Nem sikerült menteni a karbantartási köteg méretét",
+    "purgeDiscovery": "Nem sikerült törölni a felfedezési adatokat",
+    "plexWatchLink": "Nem sikerült módosítani a Plex megtekintésiállapot-szinkront",
+    "plexWatchImport": "Nem sikerült importálni a megtekintési állapotot a Plexből",
+    "shareLinkDownload": "Nem sikerült módosítani, hogy a link engedi-e a letöltést",
+    "shareLinkRevoke": "Nem sikerült visszavonni a linket",
+    "shareLinkPassword": "Nem sikerült módosítani a link jelszavát",
+    "syncPause": "Nem sikerült szüneteltetni a szinkronizálást",
+    "syncResume": "Nem sikerült folytatni a szinkronizálást",
+    "prefs": "Nem sikerült menteni a beállításaidat",
+    "deleteAccount": "Nem sikerült törölni a fiókodat",
+    "videoState": "Nem sikerült módosítani a videó állapotát",
+    "videoReset": "Nem sikerült alaphelyzetbe állítani a videót",
+    "watchLaterAdd": "Nem sikerült hozzáadni a Megnézem később listához",
+    "watchLaterRemove": "Nem sikerült eltávolítani a Megnézem később listából",
+    "clearSearch": "Nem sikerült törölni a keresési találatokat"
+  },
   "ok": "OK",
   "offline": {
     "title": "Megszakadt a kapcsolat",
diff --git a/frontend/src/lib/actionErrors.test.ts b/frontend/src/lib/actionErrors.test.ts
index 1e5f5b22..83c25542 100644
--- a/frontend/src/lib/actionErrors.test.ts
+++ b/frontend/src/lib/actionErrors.test.ts
@@ -8,9 +8,10 @@ vi.mock("./notifications", () => ({ notify: vi.fn(() => 0) }));
 vi.mock("./errorDialog", () => ({ reportError: vi.fn() }));
 
 import { HttpError, api } from "./api";
+import { alreadyReported } from "./api/core";
 import { notify } from "./notifications";
 import { reportError } from "./errorDialog";
-import { notifyActionError } from "./actionErrors";
+import { failureReason, notifyActionError, notifyKnownFailure } from "./actionErrors";
 import i18n from "../i18n";
 
 const notifyMock = vi.mocked(notify);
@@ -33,7 +34,7 @@ describe("a Plex collection name that is already taken", () => {
     notifyMock.mockClear();
     notifyActionError(err, "EDIT_FAILED");
     expect(notifyMock.mock.calls).toEqual([
-      [{ level: "error", message: i18n.t("plex.collection.nameTaken") }],
+      [{ level: "error", title: "EDIT_FAILED", message: i18n.t("plex.collection.nameTaken") }],
     ]);
     expect(reportError).not.toHaveBeenCalled();
   });
@@ -60,12 +61,30 @@ describe("notifyActionError", () => {
     expect(notifyMock).toHaveBeenCalled();
   });
 
+  it("reports one failure once, however many callers try", () => {
+    // What keeps the app-wide mutation reporter quiet for a mutation whose own onError already spoke.
+    const err = new HttpError(404, "gone");
+    notifyActionError(err, "FIRST");
+    notifyActionError(err, "SECOND");
+    expect(notifyMock).toHaveBeenCalledTimes(1);
+    expect(alreadyReported(err)).toBe(true);
+  });
+
+  it("says nothing about an upload the user cancelled", () => {
+    notifyActionError(new HttpError(0, "aborted"), "FAILED");
+    expect(notifyMock).not.toHaveBeenCalled();
+  });
+
   it("says a bare-English server reason in the reader's language, not the server's", () => {
     // `reportFailure` leaves 403/404/429 to the caller, so for exactly those statuses whatever
     // English a route hand-wrote went straight into the toast — "No user with that email." in an
     // otherwise Hungarian UI, while the caller's translated fallback sat unused (round 5).
     notifyActionError(new HttpError(404, "No user with that email."), "TRANSLATED");
-    expect(notifyMock).toHaveBeenCalledWith({ level: "error", message: "TRANSLATED" });
+    expect(notifyMock).toHaveBeenCalledWith({
+      level: "error",
+      title: "TRANSLATED",
+      message: i18n.t("errors.notFound.generic"),
+    });
   });
 
   it("tells a demo user that the demo account is the reason", async () => {
@@ -84,13 +103,14 @@ describe("notifyActionError", () => {
     notifyActionError(err, "GENERIC FALLBACK");
     expect(notifyMock).toHaveBeenCalledWith({
       level: "error",
+      title: "GENERIC FALLBACK",
       message: i18n.t("errors.demo"),
     });
   });
 
   it("still prefers a STRUCTURED reason, which the request layer has already localized", () => {
     // `req` runs `{code, reason}` through localizeDetail, so `detail` is in the reader's language
-    // AND more specific than the caller's generic fallback — quota numbers, which row is gone.
+    // AND more specific than anything a status can say — quota numbers, which row is gone.
     const err = new HttpError(404, "Ez a csatorna már nincs a könyvtárban.", {
       code: "not_found",
       reason: "channel",
@@ -98,7 +118,54 @@ describe("notifyActionError", () => {
     notifyActionError(err, "TRANSLATED FALLBACK");
     expect(notifyMock).toHaveBeenCalledWith({
       level: "error",
+      title: "TRANSLATED FALLBACK",
       message: "Ez a csatorna már nincs a könyvtárban.",
     });
   });
 });
+
+describe("failureReason — why a write failed, when the server did not say", () => {
+  it.each([
+    [403, "errors.reason.forbidden"],
+    [404, "errors.notFound.generic"],
+    [429, "errors.reason.rateLimited"],
+    [502, "errors.reason.unreachable"],
+    [503, "errors.reason.unreachable"],
+    [504, "errors.reason.unreachable"],
+    [0, "errors.reason.unreachable"], // uploadReq's network failure
+    [500, "errors.generic"],
+  ])("a %s reads %s", (status, key) => {
+    expect(failureReason(new HttpError(status, "hand-written English"))).toBe(i18n.t(key));
+  });
+
+  it("a request that never reached the server reads as unreachable", () => {
+    expect(failureReason(new TypeError("Failed to fetch"))).toBe(
+      i18n.t("errors.reason.unreachable"),
+    );
+  });
+
+  it("anything else is the app's own fault, not the server's", () => {
+    expect(failureReason(new Error("key import failed"))).toBe(i18n.t("errors.reason.unexpected"));
+  });
+
+  it("every reason exists in both languages", () => {
+    for (const lng of ["en", "hu"])
+      for (const key of [
+        "errors.reason.forbidden",
+        "errors.reason.rateLimited",
+        "errors.reason.unreachable",
+        "errors.reason.unexpected",
+      ])
+        expect(i18n.exists(key, { lng }), `${lng}:${key}`).toBe(true);
+  });
+});
+
+describe("notifyKnownFailure", () => {
+  it("shows the caller's specific sentence and marks the failure reported", () => {
+    const err = new HttpError(409, "x", { code: "resend", reason: "email_off" });
+    notifyKnownFailure(err, "EMAIL IS OFF");
+    expect(notifyMock).toHaveBeenCalledWith({ level: "error", message: "EMAIL IS OFF" });
+    notifyActionError(err, "GENERIC");
+    expect(notifyMock).toHaveBeenCalledTimes(1);
+  });
+});
diff --git a/frontend/src/lib/actionErrors.ts b/frontend/src/lib/actionErrors.ts
index 5a8949f2..c4e7d10a 100644
--- a/frontend/src/lib/actionErrors.ts
+++ b/frontend/src/lib/actionErrors.ts
@@ -1,28 +1,62 @@
-// From the core, not the `./api` barrel: these two are pure (no request), and a test that mocks the
+// From the core, not the `./api` barrel: these are pure (no request), and a test that mocks the
 // barrel wholesale to stub its requests must not take the error path's own plumbing with it.
-import { alreadyReported, HttpError } from "./api/core";
+import { alreadyReported, HttpError, isAborted, markReported } from "./api/core";
 import { notify } from "./notifications";
+import i18n from "../i18n";
+
+const GATEWAY = new Set([502, 503, 504]);
 
 /**
- * Surface a failed write action as an error toast instead of the click silently doing nothing.
- * Prefers the server's own reason over the caller's already-translated generic fallback — but only
- * when the server sent a STRUCTURED detail (`{code, reason}`), which `req` has already run through
- * `localizeDetail`. A plain-string `detail` is whatever English the route hand-wrote, and
- * `reportFailure` deliberately leaves 403/404/429 to the caller, so exactly those statuses used to
- * put the route's English sentence into an otherwise Hungarian UI — "No user with that email.",
- * "Not subscribed to this channel" — where the caller's own translated fallback was right there
- * (deep-review round 5). Domain-agnostic: used by the Plex watched-toggles and the saved-view ops,
- * and delegated to by `notifyYouTubeActionError` for its non-403 case.
+ * WHY a write failed, in the reader's language — the line under the toast's title.
+ *
+ * A STRUCTURED server detail (`{code, reason}`) wins: `req` has already run it through
+ * `localizeDetail`, and it is the most specific thing there is to say (quota numbers, which row is
+ * gone). A plain-string `detail` never does — it is whatever English the route hand-wrote, and
+ * `reportFailure` leaves exactly 403/404/429 to the caller, so those used to put "No user with that
+ * email." into a Hungarian UI (deep-review round 5). Otherwise the status says it.
+ */
+export function failureReason(err: unknown): string {
+  const t = i18n.t.bind(i18n);
+  if (err instanceof HttpError) {
+    if (err.detailData != null && err.detail) return err.detail;
+    // 0: `uploadReq`'s network failure. 502–504: the gateway could not reach the app. Either way the
+    // change did not go through, which the connection notice alone does not say.
+    if (err.status === 0 || GATEWAY.has(err.status)) return t("errors.reason.unreachable");
+    if (err.status === 403) return t("errors.reason.forbidden");
+    if (err.status === 404) return t("errors.notFound.generic");
+    if (err.status === 429) return t("errors.reason.rateLimited");
+    return t("errors.generic");
+  }
+  // `fetch` rejects with a TypeError when the request never reached the server.
+  if (err instanceof TypeError) return t("errors.reason.unreachable");
+  return t("errors.reason.unexpected");
+}
+
+/**
+ * Surface a failed write as an error toast instead of the click silently doing nothing: the title
+ * names WHAT failed (the caller's translated "Couldn't hide the channel"), the line under it says
+ * WHY (`failureReason`). Domain-agnostic — the one way every action failure is reported, including
+ * the app-wide mutation reporter in lib/queryClient.
  */
 export function notifyActionError(
   err: unknown,
-  fallbackMessage: string,
+  title: string,
   level: "error" | "warning" = "error",
 ): void {
-  // Already on screen (`req` opened the error dialog, or the tab is being signed out): a toast on
-  // top reported one fault twice. Playlists filtered this by status by hand; the watched toggles,
-  // the saved views and a playlist delete did not.
+  // Already on screen (`req` opened the error dialog, the tab is being signed out, or a caller's own
+  // `onError` got there first): a second toast reported one fault twice. A cancel is not a failure.
+  if (alreadyReported(err) || isAborted(err)) return;
+  notify({ level, title, message: failureReason(err) });
+  markReported(err);
+}
+
+/**
+ * A failure the caller has a SPECIFIC sentence for, because it read the error's own code ("this
+ * instance cannot send email", "someone registered on that address") — shown as it is, and marked,
+ * so the app-wide mutation reporter does not put a second, generic toast next to it.
+ */
+export function notifyKnownFailure(err: unknown, message: string): void {
   if (alreadyReported(err)) return;
-  const localized = err instanceof HttpError && err.detailData != null ? err.detail : null;
-  notify({ level, message: localized || fallbackMessage });
+  notify({ level: "error", message });
+  markReported(err);
 }
diff --git a/frontend/src/lib/api/core.ts b/frontend/src/lib/api/core.ts
index 45858f3f..7397e54f 100644
--- a/frontend/src/lib/api/core.ts
+++ b/frontend/src/lib/api/core.ts
@@ -23,10 +23,18 @@ export const uiLang = (): string => i18n.language.split("-")[0] || "en";
 // removes itself on the next answer — which every caller's own resync refetch fetches at once. Marked
 // as reported, a delete that hit a keepalive reset (a 502) flashed the notice for a moment and was
 // then reported nowhere at all, the list still there (deep-review round 4).
+//
+// A caller that reports a failure itself marks it too (`notifyActionError` does), which is what lets
+// the ONE app-wide mutation reporter (lib/queryClient) stay quiet for a mutation whose own `onError`
+// already said something, and speak for every one that did not.
 const reported = new WeakSet<object>();
 export function alreadyReported(err: unknown): boolean {
   return typeof err === "object" && err !== null && reported.has(err);
 }
+/** Record that this failure is on screen now, so nothing reports it a second time. */
+export function markReported(err: unknown): void {
+  if (typeof err === "object" && err !== null) reported.add(err);
+}
 
 /** What an upload the USER cancelled rejects with. A cancel is not a failure to report, and the
  *  only other rejection carrying status 0 is a genuine network drop, which IS worth reporting — so
diff --git a/frontend/src/lib/channelWriteFailures.dom.test.tsx b/frontend/src/lib/channelWriteFailures.dom.test.tsx
new file mode 100644
index 00000000..6d90be4f
--- /dev/null
+++ b/frontend/src/lib/channelWriteFailures.dom.test.tsx
@@ -0,0 +1,83 @@
+// @vitest-environment jsdom
+//
+// The two shared channel write flows report their own failures, with the real error (E11 S2b).
+// Before: the channel detail page's tag toggle and both pages' priority stepper only refetched, so a
+// refused write just snapped back; and the manager reported a failed tag toggle by handing the
+// CHANNEL ID to the reporter as though it were the error.
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import type { ReactNode } from "react";
+import { act, renderHook, waitFor } from "@testing-library/react";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+
+vi.mock("./notifications", () => ({ notify: vi.fn(() => 0) }));
+
+import { api, HttpError } from "./api";
+import { notify } from "./notifications";
+import { useChannelTagActions } from "./useChannelTagActions";
+import { useChannelPriority } from "./useChannelPriority";
+import i18n from "../i18n";
+
+const notifyMock = vi.mocked(notify);
+beforeEach(() => notifyMock.mockClear());
+afterEach(() => vi.restoreAllMocks());
+
+const wrapper = ({ children }: { children: ReactNode }) => (
+  <QueryClientProvider client={new QueryClient()}>{children}</QueryClientProvider>
+);
+
+describe("useChannelTagActions", () => {
+  it("names a refused tag toggle with its real reason, then lets the caller revert", async () => {
+    vi.spyOn(api, "attachChannelTag").mockRejectedValue(new HttpError(403, "nope"));
+    const onError = vi.fn();
+    const { result } = renderHook(
+      () => useChannelTagActions({ optimisticToggle: () => {}, onError }),
+      { wrapper },
+    );
+    act(() => result.current.toggleTag("UC1", 7, false));
+    await waitFor(() => expect(onError).toHaveBeenCalledWith("UC1"));
+    expect(notifyMock).toHaveBeenCalledWith({
+      level: "error",
+      title: i18n.t("channels.tags.updateFailed"),
+      message: i18n.t("errors.reason.forbidden"),
+    });
+  });
+
+  it("names a refused create-and-attach", async () => {
+    vi.spyOn(api, "createTag").mockRejectedValue(new HttpError(429, "slow"));
+    const { result } = renderHook(
+      () => useChannelTagActions({ optimisticToggle: () => {}, onError: () => {} }),
+      { wrapper },
+    );
+    act(() => result.current.createAndAttachTag("UC1", "Music"));
+    await waitFor(() =>
+      expect(notifyMock).toHaveBeenCalledWith({
+        level: "error",
+        title: i18n.t("channels.tags.createFailed"),
+        message: i18n.t("errors.reason.rateLimited"),
+      }),
+    );
+  });
+});
+
+describe("useChannelPriority", () => {
+  it("names a refused priority save, then lets the caller revert", async () => {
+    vi.spyOn(api, "updateChannel").mockRejectedValue(new TypeError("Failed to fetch"));
+    const onError = vi.fn();
+    let priority = 1;
+    const { result } = renderHook(() =>
+      useChannelPriority({
+        read: () => priority,
+        optimistic: (_id, next) => void (priority = next),
+        onError,
+        delay: 0,
+      }),
+    );
+    act(() => result.current.bumpPriority("UC1", 1));
+    await waitFor(() => expect(onError).toHaveBeenCalledWith("UC1"));
+    expect(notifyMock).toHaveBeenCalledWith({
+      level: "error",
+      title: i18n.t("errors.writeFailed.channelPriority"),
+      message: i18n.t("errors.reason.unreachable"),
+    });
+  });
+});
diff --git a/frontend/src/lib/history.dom.test.ts b/frontend/src/lib/history.dom.test.ts
index db358af9..f34f9907 100644
--- a/frontend/src/lib/history.dom.test.ts
+++ b/frontend/src/lib/history.dom.test.ts
@@ -139,13 +139,21 @@ describe("closeOverlaysThenNavigate", () => {
       expect(closed).toEqual(["palette", "player"]);
       expect(navigate).not.toHaveBeenCalled();
 
-      await settle(); // the reconcile reclaims both entries in one go
-      expect(back).toHaveBeenCalledTimes(2);
       // Not inside the popstate dispatch that settled it — a later listener would still apply the
-      // OLD entry's page over the push.
-      expect(navigate).not.toHaveBeenCalled();
-
-      await tick();
+      // OLD entry's page over the push. Asserted FROM such a later listener (registered after the
+      // helper's own), not after `settle()`: that async act can flush the unmounts on a macrotask,
+      // and the navigate task sometimes ran first — a 1-in-5 false failure (E11 S2b gate).
+      const navigatedByPopstate: number[] = [];
+      const laterListener = () => navigatedByPopstate.push(navigate.mock.calls.length);
+      window.addEventListener("popstate", laterListener);
+      try {
+        await settle(); // the reconcile reclaims both entries in one go
+        expect(back).toHaveBeenCalledTimes(2);
+        await tick();
+      } finally {
+        window.removeEventListener("popstate", laterListener);
+      }
+      expect(navigatedByPopstate).toEqual([0, 0]);
       expect(navigate).toHaveBeenCalledTimes(1);
       await tick();
       expect(back).toHaveBeenCalledTimes(2); // nothing reclaims over the entry navigate pushed
diff --git a/frontend/src/lib/mutationFailures.contract.test.ts b/frontend/src/lib/mutationFailures.contract.test.ts
new file mode 100644
index 00000000..257abb55
--- /dev/null
+++ b/frontend/src/lib/mutationFailures.contract.test.ts
@@ -0,0 +1,77 @@
+import { describe, expect, it } from "vitest";
+
+// Every `useMutation` NAMES what failed (E11 S2b, owner decision: the toast's title says what, the
+// line under it why). The app-wide reporter (lib/queryClient) reports a mutation that names nothing
+// too — so nothing is silent any more — but under the generic "Couldn't complete that", which is the
+// very toast that says nothing about what it is about when it lands a few seconds after the click.
+//
+// A mutation passes when it carries one of:
+//   - `meta: { failTitle }`        the reporter's title for it,
+//   - `meta: { errorShownInline }` the page itself renders the failure,
+//   - an `onError`                 which reports (or rolls back — the reporter then speaks for it,
+//                                  under the title from `meta` if there is one).
+//
+// The assertion pins the SET of unnamed mutations, never a count, and checks the glob matched at
+// all: a guard whose glob stops matching is green over nothing (E10 review round 4, round 7).
+const NAMES = /\bfailTitle\b|\berrorShownInline\b|\bonError\b/;
+
+/** The source of each `useMutation(...)` call, found by paren balance with comments stripped —
+ *  a comment with a stray "(" would otherwise run one block into the next one's `failTitle`. */
+export function mutationCalls(source: string): string[] {
+  const code = source.replace(/\/\*[\s\S]*?\*\//g, "").replace(/(^|[^:"'`])\/\/.*$/gm, "$1");
+  const calls: string[] = [];
+  let from = 0;
+  for (;;) {
+    const at = code.indexOf("useMutation(", from);
+    if (at < 0) return calls;
+    let depth = 0;
+    let i = at + "useMutation".length;
+    for (; i < code.length; i++) {
+      if (code[i] === "(") depth++;
+      else if (code[i] === ")" && --depth === 0) break;
+    }
+    calls.push(code.slice(at, i + 1));
+    from = i + 1;
+  }
+}
+
+function unnamed(): { hits: string[]; mutations: number } {
+  const files = import.meta.glob("../**/*.{ts,tsx}", {
+    query: "?raw",
+    import: "default",
+    eager: true,
+  });
+  const hits: string[] = [];
+  let mutations = 0;
+  for (const [rel, source] of Object.entries(files)) {
+    const path = rel.startsWith("./") ? `lib/${rel.slice(2)}` : rel.replace(/^\.\.\//, "");
+    if (/\.test\.tsx?$|\.d\.ts$/.test(path)) continue;
+    mutationCalls(String(source)).forEach((call, n) => {
+      mutations++;
+      if (!NAMES.test(call)) hits.push(`${path}#${n + 1}`);
+    });
+  }
+  return { hits: hits.sort(), mutations };
+}
+
+describe("the failed-write contract", () => {
+  it("has no mutation that leaves its failure unnamed", () => {
+    const { hits, mutations } = unnamed();
+    expect(mutations).toBeGreaterThan(50);
+    expect(hits).toEqual([]);
+  });
+
+  it("reads a mutation call to its end, whatever its body holds", () => {
+    const src = `
+      const a = useMutation({
+        mutationFn: () => api.x(fn(1), (2)),
+        // a stray "(" in a comment must not swallow the next call
+      });
+      const b = useMutation({ mutationFn: api.y, meta: { failTitle: "k" } });
+    `;
+    const calls = mutationCalls(src);
+    expect(calls).toHaveLength(2);
+    expect(NAMES.test(calls[0] ?? "")).toBe(false);
+    expect(NAMES.test(calls[1] ?? "")).toBe(true);
+  });
+});
diff --git a/frontend/src/lib/queryClient.dom.test.ts b/frontend/src/lib/queryClient.dom.test.ts
new file mode 100644
index 00000000..d0140000
--- /dev/null
+++ b/frontend/src/lib/queryClient.dom.test.ts
@@ -0,0 +1,104 @@
+// @vitest-environment jsdom
+//
+// The ONE app-wide reporter for a failed mutation (E11 S2b). `req` shows its blocking dialog for
+// 400/409/422/5xx only, so before this every mutation without a reporting `onError` failed in
+// silence on a 403/404/429 or a lost connection — 34 of them.
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("./notifications", () => ({ notify: vi.fn(() => 0) }));
+vi.mock("./errorDialog", () => ({ reportError: vi.fn() }));
+
+import { MutationObserver, type MutationObserverOptions } from "@tanstack/react-query";
+import { createQueryClient } from "./queryClient";
+import { HttpError, req } from "./api/core";
+import { notify } from "./notifications";
+import { reportError } from "./errorDialog";
+import { notifyActionError } from "./actionErrors";
+import i18n from "../i18n";
+
+const notifyMock = vi.mocked(notify);
+beforeEach(() => {
+  notifyMock.mockClear();
+  vi.mocked(reportError).mockClear();
+});
+afterEach(() => vi.restoreAllMocks());
+
+async function fail(options: MutationObserverOptions<unknown, unknown, void>): Promise<void> {
+  const observer = new MutationObserver(createQueryClient(), options);
+  await observer.mutate().catch(() => {});
+  // The reporter waits a task for the mutation's own onError; give it that task.
+  await new Promise((r) => setTimeout(r, 5));
+}
+
+const TITLE = "errors.writeFailed.hideChannel";
+
+describe("a failed mutation", () => {
+  it("is named, with the reason under it, when the mutation did not report it itself", async () => {
+    await fail({
+      mutationFn: () => Promise.reject(new HttpError(404, "Not subscribed")),
+      meta: { failTitle: TITLE },
+    });
+    expect(notifyMock.mock.calls).toEqual([
+      [{ level: "error", title: i18n.t(TITLE), message: i18n.t("errors.notFound.generic") }],
+    ]);
+  });
+
+  it("is still reported under the generic title when the mutation names nothing", async () => {
+    await fail({ mutationFn: () => Promise.reject(new HttpError(429, "slow down")) });
+    expect(notifyMock).toHaveBeenCalledWith({
+      level: "error",
+      title: i18n.t("errors.title"),
+      message: i18n.t("errors.reason.rateLimited"),
+    });
+  });
+
+  it("says a lost connection is why the change did not go through", async () => {
+    await fail({
+      mutationFn: () => Promise.reject(new TypeError("Failed to fetch")),
+      meta: { failTitle: TITLE },
+    });
+    expect(notifyMock).toHaveBeenCalledWith(
+      expect.objectContaining({ message: i18n.t("errors.reason.unreachable") }),
+    );
+  });
+
+  it("gets ONE toast when the mutation's own onError reported it — react-query runs that one second", async () => {
+    await fail({
+      mutationFn: () => Promise.reject(new HttpError(404, "gone")),
+      meta: { failTitle: TITLE },
+      onError: (e) => notifyActionError(e, "OWN TITLE"),
+    });
+    expect(notifyMock.mock.calls.map(([n]) => n.title)).toEqual(["OWN TITLE"]);
+  });
+
+  it("is still reported when the mutation's onError only rolls back", async () => {
+    const rollback = vi.fn();
+    await fail({
+      mutationFn: () => Promise.reject(new HttpError(403, "nope")),
+      meta: { failTitle: TITLE },
+      onError: rollback,
+    });
+    expect(rollback).toHaveBeenCalled();
+    expect(notifyMock.mock.calls.map(([n]) => n.title)).toEqual([i18n.t(TITLE)]);
+  });
+
+  it("gets no toast when the page shows the failure itself", async () => {
+    await fail({
+      mutationFn: () => Promise.reject(new HttpError(403, "nope")),
+      meta: { errorShownInline: true },
+    });
+    expect(notifyMock).not.toHaveBeenCalled();
+  });
+
+  it("gets no toast on top of the error dialog the request layer already opened", async () => {
+    vi.spyOn(globalThis, "fetch").mockResolvedValue(
+      new Response(JSON.stringify({ detail: "conflict" }), {
+        status: 409,
+        headers: { "content-type": "application/json" },
+      }),
+    );
+    await fail({ mutationFn: () => req("/api/x", { method: "POST" }), meta: { failTitle: TITLE } });
+    expect(reportError).toHaveBeenCalled();
+    expect(notifyMock).not.toHaveBeenCalled();
+  });
+});
diff --git a/frontend/src/lib/queryClient.ts b/frontend/src/lib/queryClient.ts
new file mode 100644
index 00000000..4309a785
--- /dev/null
+++ b/frontend/src/lib/queryClient.ts
@@ -0,0 +1,42 @@
+import { MutationCache, QueryClient } from "@tanstack/react-query";
+import i18n from "../i18n";
+import { notifyActionError } from "./actionErrors";
+
+declare module "@tanstack/react-query" {
+  interface Register {
+    mutationMeta: {
+      /** i18n key of the toast title that names what failed ("Couldn't hide the channel"). Without
+       *  one the failure is still reported, under the generic "Couldn't complete that". */
+      failTitle?: string;
+      /** The component renders this mutation's failure on the page itself — no toast. */
+      errorShownInline?: boolean;
+    };
+  }
+}
+
+type FailMeta = { failTitle?: string; errorShownInline?: boolean } | undefined;
+
+/**
+ * The ONE reporter for a failed mutation. `req` shows its blocking dialog for 400/409/422/5xx only;
+ * a 403/404/429 was left to the caller and a 502–504 or network failure got just the connection
+ * notice — so every `useMutation` without a reporting `onError` failed in silence: a channel hide
+ * that snapped back, a share link that never revoked, a scheduler run that never started (E11 S2b
+ * counted 34 such mutations). Reporting centrally means a new mutation cannot be silent.
+ *
+ * react-query awaits this BEFORE the mutation's own `onError`, so it waits a task: a site that
+ * reported the failure itself (through `notifyActionError`, which marks it) has had its turn, and
+ * `notifyActionError` skips what is already on screen.
+ */
+function reportMutationFailure(err: unknown, meta: FailMeta): void {
+  if (meta?.errorShownInline) return;
+  setTimeout(() => notifyActionError(err, i18n.t(meta?.failTitle ?? "errors.title")), 0);
+}
+
+export function createQueryClient(): QueryClient {
+  return new QueryClient({
+    mutationCache: new MutationCache({
+      onError: (err, _vars, _onMutateResult, mutation) => reportMutationFailure(err, mutation.meta),
+    }),
+    defaultOptions: { queries: { retry: false, staleTime: 30_000, refetchOnWindowFocus: false } },
+  });
+}
diff --git a/frontend/src/lib/useChannelPriority.ts b/frontend/src/lib/useChannelPriority.ts
index 53db8b38..81d781c7 100644
--- a/frontend/src/lib/useChannelPriority.ts
+++ b/frontend/src/lib/useChannelPriority.ts
@@ -1,5 +1,7 @@
 import { useCallback, useEffect, useRef } from "react";
 import { api } from "./api";
+import { notifyActionError } from "./actionErrors";
+import i18n from "../i18n";
 
 /** The shared feed-priority flow used by both the channel manager and the channel detail page:
  *  an optimistic bump/set with a debounced server flush (~400ms after the last notch) so a
@@ -34,7 +36,12 @@ export function useChannelPriority(opts: {
     api
       .updateChannel(channelId, { priority: latest })
       .then(() => optsRef.current.onSuccess?.(channelId))
-      .catch(() => optsRef.current.onError(channelId));
+      .catch((err: unknown) => {
+        // Said here, once, for both callers: their `onError` only refetches, so a refused priority
+        // used to just snap back — often after the stepper had already been let go (E11 S2b).
+        notifyActionError(err, i18n.t("errors.writeFailed.channelPriority"));
+        optsRef.current.onError(channelId);
+      });
   }, []);
 
   // Set an absolute priority (used by both the arrows — via bumpPriority — and the inline edit),
diff --git a/frontend/src/lib/useChannelTagActions.ts b/frontend/src/lib/useChannelTagActions.ts
index b847f137..cc64c92e 100644
--- a/frontend/src/lib/useChannelTagActions.ts
+++ b/frontend/src/lib/useChannelTagActions.ts
@@ -2,12 +2,16 @@ import { useQueryClient } from "@tanstack/react-query";
 import { useTranslation } from "react-i18next";
 import { api } from "./api";
 import { qk } from "./queryKeys";
-import { notify } from "./notifications";
+import { notifyActionError } from "./actionErrors";
 
 /** The shared channel-tag mutation flow used by both the channel manager and the channel detail
  *  page: optimistic attach/detach + a create-then-attach shortcut, with the cache-shape-specific
  *  optimistic update injected (the manager maps over a ManagedChannel[]; the detail patches one
- *  ChannelDetail). Keeps the two call sites from drifting. */
+ *  ChannelDetail). Keeps the two call sites from drifting.
+ *
+ *  A failure is REPORTED here, once, with the real error: the manager used to do it in its `onError`
+ *  and hand the channel id to the reporter as though it were the error, and the detail page did not
+ *  do it at all — a tag toggled there just snapped back (E11 S2b). */
 export function useChannelTagActions(opts: {
   /** Apply the optimistic tag change to whichever cache the caller owns. `attach` true = add. */
   optimisticToggle: (channelId: string, tagId: number, attach: boolean) => void;
@@ -24,7 +28,12 @@ export function useChannelTagActions(opts: {
     const call = hasTag
       ? api.detachChannelTag(channelId, tagId)
       : api.attachChannelTag(channelId, tagId);
-    call.then(() => opts.onSuccess?.(channelId)).catch(() => opts.onError(channelId));
+    call
+      .then(() => opts.onSuccess?.(channelId))
+      .catch((err: unknown) => {
+        notifyActionError(err, t("channels.tags.updateFailed"));
+        opts.onError(channelId);
+      });
   };
 
   // Create a brand-new personal tag and attach it in one step (so tagging doesn't require the
@@ -36,7 +45,7 @@ export function useChannelTagActions(opts: {
         qc.invalidateQueries({ queryKey: qk.tags() });
         toggleTag(channelId, tag.id, false);
       })
-      .catch(() => notify({ level: "error", message: t("channels.tags.createFailed") }));
+      .catch((err: unknown) => notifyActionError(err, t("channels.tags.createFailed")));
   };
 
   return { toggleTag, createAndAttachTag };
diff --git a/frontend/src/lib/usePref.dom.test.ts b/frontend/src/lib/usePref.dom.test.ts
index a9fb61aa..e382b524 100644
--- a/frontend/src/lib/usePref.dom.test.ts
+++ b/frontend/src/lib/usePref.dom.test.ts
@@ -14,7 +14,9 @@ const { store, savePrefs } = vi.hoisted(() => ({
   savePrefs: vi.fn(() => Promise.resolve({})),
 }));
 
-vi.mock("./storage", () => ({
+// Partial: i18n (which usePref now imports, to title a refused save) reads its key from `LS`.
+vi.mock("./storage", async (importOriginal) => ({
+  ...(await importOriginal<typeof import("./storage")>()),
   readAccount: (base: string, fallback: unknown) => {
     const raw = store.get(base);
     return raw == null ? fallback : JSON.parse(raw);
@@ -24,6 +26,9 @@ vi.mock("./storage", () => ({
   },
 }));
 vi.mock("./api", () => ({ api: { savePrefs, me: vi.fn() } }));
+// The failure path (usePref.saveFailure.dom.test.ts) is not this file's subject; stubbed so the
+// reporter's notifications → storage chain does not meet the storage mock above.
+vi.mock("./actionErrors", () => ({ notifyActionError: vi.fn() }));
 
 import { usePref, useSavePref } from "./usePref";
 import { qk } from "./queryKeys";
diff --git a/frontend/src/lib/usePref.saveFailure.dom.test.ts b/frontend/src/lib/usePref.saveFailure.dom.test.ts
new file mode 100644
index 00000000..5c8820b6
--- /dev/null
+++ b/frontend/src/lib/usePref.saveFailure.dom.test.ts
@@ -0,0 +1,51 @@
+// @vitest-environment jsdom
+//
+// A preference save the server refuses is REPORTED (E11 S2b). The local state and the `me` cache
+// already show the new value, so a swallowed failure was invisible until the next load quietly put
+// the old value back — and the comment above the swallow claimed `req` toasted it, which it does not
+// for a 403/404/429 or a lost connection.
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { act, renderHook, waitFor } from "@testing-library/react";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import { createElement, type ReactNode } from "react";
+
+const { savePrefs, notifyActionError } = vi.hoisted(() => ({
+  savePrefs: vi.fn(),
+  notifyActionError: vi.fn(),
+}));
+vi.mock("./api", () => ({ api: { savePrefs, me: vi.fn() } }));
+vi.mock("./actionErrors", () => ({ notifyActionError }));
+
+import { usePref, useSavePref } from "./usePref";
+import i18n from "../i18n";
+
+afterEach(() => {
+  savePrefs.mockReset();
+  notifyActionError.mockReset();
+});
+
+const wrapper = ({ children }: { children: ReactNode }) =>
+  createElement(QueryClientProvider, { client: new QueryClient() }, children);
+
+describe("a refused preference save", () => {
+  it("is reported from the usePref setter", async () => {
+    const refusal = new TypeError("Failed to fetch");
+    savePrefs.mockRejectedValue(refusal);
+    const { result } = renderHook(() => usePref<boolean>("hintsOn", true), { wrapper });
+    act(() => result.current[1](false));
+    await waitFor(() =>
+      expect(notifyActionError).toHaveBeenCalledWith(refusal, i18n.t("errors.writeFailed.prefs")),
+    );
+    expect(result.current[0]).toBe(false); // the local value stays; the toast says it did not save
+  });
+
+  it("is reported from the raw useSavePref persister", async () => {
+    const refusal = new Error("403");
+    savePrefs.mockRejectedValue(refusal);
+    const { result } = renderHook(() => useSavePref(), { wrapper });
+    act(() => result.current({ stripSources: { c1: false } }));
+    await waitFor(() =>
+      expect(notifyActionError).toHaveBeenCalledWith(refusal, i18n.t("errors.writeFailed.prefs")),
+    );
+  });
+});
diff --git a/frontend/src/lib/usePref.ts b/frontend/src/lib/usePref.ts
index 3a8b8a71..66b6ced2 100644
--- a/frontend/src/lib/usePref.ts
+++ b/frontend/src/lib/usePref.ts
@@ -4,6 +4,14 @@ import { qk } from "./queryKeys";
 import { api } from "./api";
 import type { Me } from "./api/account";
 import { readAccount, writeAccount } from "./storage";
+import { notifyActionError } from "./actionErrors";
+import i18n from "../i18n";
+
+// The local state + cache already show the new value, so a refused save is otherwise invisible until
+// the next load quietly puts the old one back. `req` reports none of 403/404/429 and only the
+// connection notice for a lost connection — say it (E11 S2b).
+const reportPrefSaveFailure = (err: unknown) =>
+  notifyActionError(err, i18n.t("errors.writeFailed.prefs"));
 
 // The ONE per-account + server preference hook. A preference (unlike an ephemeral filter) persists to
 // the account on the server (PUT /api/me/preferences, which MERGES the partial payload) AND is adopted
@@ -55,7 +63,7 @@ export function useSavePref(): (patch: Record<string, unknown>) => void {
   const qc = useQueryClient();
   return useCallback(
     (patch: Record<string, unknown>) => {
-      api.savePrefs(patch).catch(() => {});
+      api.savePrefs(patch).catch(reportPrefSaveFailure);
       mergePrefsCache(qc, patch);
     },
     [qc],
@@ -116,9 +124,9 @@ export function usePref<T>(
       // see the new value without a refetch. Merges into preferences, mirroring the server PUT.
       const payload = { [serverKey]: next };
       mergePrefsCache(qc, payload);
-      // savePrefs is fire-and-forget: api.req() already surfaces a failure toast, and the local state +
-      // cache already reflect the change (idempotent PUT retries a transient blip).
-      const flush = () => api.savePrefs(payload).catch(() => {});
+      // savePrefs is fire-and-forget (idempotent PUT, so `req` retries a transient blip once); a
+      // failure that survives that is reported, not swallowed.
+      const flush = () => api.savePrefs(payload).catch(reportPrefSaveFailure);
       if (debounceMs) {
         window.clearTimeout(saveTimer.current);
         pendingSave.current = flush;
diff --git a/frontend/src/lib/youtubeErrors.ts b/frontend/src/lib/youtubeErrors.ts
index 25c3773c..df369f98 100644
--- a/frontend/src/lib/youtubeErrors.ts
+++ b/frontend/src/lib/youtubeErrors.ts
@@ -1,4 +1,5 @@
 import { HttpError } from "./api";
+import { markReported } from "./api/core";
 import { notify } from "./notifications";
 import { notifyActionError } from "./actionErrors";
 import i18n from "../i18n";
@@ -22,6 +23,7 @@ export function notifyYouTubeActionError(
         ? { label: i18n.t("channels.notify.connect"), onClick: onConnect }
         : undefined,
     });
+    markReported(err);
     return;
   }
   // Any other error: prefer the server's own reason (e.g. "Not enough quota left today") over the
diff --git a/frontend/src/main.tsx b/frontend/src/main.tsx
index f0717f1c..925ec233 100644
--- a/frontend/src/main.tsx
+++ b/frontend/src/main.tsx
@@ -1,6 +1,7 @@
 import React, { lazy, Suspense } from "react";
 import { createRoot } from "react-dom/client";
-import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import { QueryClientProvider } from "@tanstack/react-query";
+import { createQueryClient } from "./lib/queryClient";
 import { FloatingTree } from "@floating-ui/react";
 import ErrorBoundary from "./components/ErrorBoundary";
 import { ConfirmProvider } from "./components/ConfirmProvider";
@@ -28,9 +29,7 @@ const PrivacyPolicy = lazy(() => import("./components/legal/PrivacyPolicy"));
 const Terms = lazy(() => import("./components/legal/Terms"));
 const WatchPage = lazy(() => import("./components/WatchPage"));
 
-const queryClient = new QueryClient({
-  defaultOptions: { queries: { retry: false, staleTime: 30_000, refetchOnWindowFocus: false } },
-});
+const queryClient = createQueryClient();
 
 // Baseline theme so the token CSS vars (--bg/--surface/--accent/…) resolve on first paint — including
 // the standalone public pages (/watch, /privacy, /terms) that render outside <App> and never call