diff --git a/app/api/calendar-agenda/route.ts b/app/api/calendar-agenda/route.ts
new file mode 100644
index 00000000..4f39e455
--- /dev/null
+++ b/app/api/calendar-agenda/route.ts
@@ -0,0 +1,334 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { logger } from '@/lib/logger';
+import { getStalwartCredentials } from '@/lib/stalwart/credentials';
+import { normalizeCalendarEventLike } from '@/lib/calendar-event-normalization';
+import { expandRecurringEvents } from '@/lib/recurrence-expansion';
+import { parseISO } from 'date-fns';
+import type { CalendarEvent } from '@/lib/jmap/types';
+
+/**
+ * POST /api/calendar-agenda
+ *
+ * Sidecar for the "Calendar Agenda" plugin. Resolves the caller's calendar
+ * account from the stored Stalwart auth context, queries upcoming
+ * CalendarEvents over JMAP, expands recurring series server-side, and returns
+ * a slim, structured-cloneable agenda the plugin can render directly.
+ *
+ * Credentials never leave the server — the sandboxed plugin only ever sees
+ * the resulting agenda DTOs.
+ *
+ * Body: { days?: number (1-90, default 7), limit?: number (1-200, default 50) }
+ */
+
+const CALENDAR_CAP = 'urn:ietf:params:jmap:calendars';
+const PRINCIPALS_CAP = 'urn:ietf:params:jmap:principals';
+
+// Mirror of lib/jmap/client.ts CALENDAR_EVENT_PROPERTIES, trimmed to what the
+// agenda actually needs (start/recurrence/display fields).
+const EVENT_PROPERTIES = [
+  'id', '@type', 'uid', 'calendarIds', 'title', 'start', 'duration', 'timeZone',
+  'showWithoutTime', 'utcStart', 'utcEnd', 'status', 'freeBusyStatus', 'color',
+  'locations', 'recurrenceId', 'recurrenceIdTimeZone', 'recurrenceRule',
+  'recurrenceOverrides', 'excludedRecurrenceRule',
+] as const;
+
+interface JmapSession {
+  apiUrl?: string;
+  primaryAccounts?: Record<string, string>;
+  capabilities?: Record<string, unknown>;
+}
+
+interface AgendaEvent {
+  id: string;
+  uid: string | null;
+  title: string;
+  start: string;
+  end: string;
+  allDay: boolean;
+  status: string | null;
+  color: string | null;
+  location: string | null;
+  calendarId: string | null;
+}
+
+// Pure, server-safe equivalents of lib/calendar-utils' getEventStartDate /
+// getEventEndDate. We can't import that module here because it transitively
+// pulls in a "use client" calendar component (which throws at load on the
+// server). Logic mirrors the originals.
+function parseDurationMinutes(duration: string | undefined): number {
+  if (!duration) return 0;
+  let total = 0;
+  const week = duration.match(/(\d+)W/);
+  const day = duration.match(/(\d+)D/);
+  const hour = duration.match(/(\d+)H/);
+  const min = duration.match(/(\d+)M/);
+  if (week) total += parseInt(week[1], 10) * 7 * 24 * 60;
+  if (day) total += parseInt(day[1], 10) * 24 * 60;
+  if (hour) total += parseInt(hour[1], 10) * 60;
+  if (min) total += parseInt(min[1], 10);
+  return total;
+}
+
+function eventStart(event: Partial<CalendarEvent>): Date {
+  if (!event.showWithoutTime && event.utcStart) {
+    const utc = parseISO(event.utcStart);
+    if (!isNaN(utc.getTime())) return utc;
+  }
+  return parseISO(event.start as string);
+}
+
+function eventEnd(event: Partial<CalendarEvent>): Date {
+  if (!event.showWithoutTime && event.utcEnd) {
+    const utc = parseISO(event.utcEnd);
+    if (!isNaN(utc.getTime())) return utc;
+  }
+  const start = eventStart(event);
+  if (!event.duration) return start;
+  return new Date(start.getTime() + parseDurationMinutes(event.duration) * 60000);
+}
+
+function firstLocationName(event: Partial<CalendarEvent>): string | null {
+  const locations = event.locations;
+  if (!locations || typeof locations !== 'object') return null;
+  for (const loc of Object.values(locations)) {
+    const name = (loc as { name?: unknown })?.name;
+    if (typeof name === 'string' && name.trim()) return name.trim();
+  }
+  return null;
+}
+
+function firstCalendarId(event: Partial<CalendarEvent>): string | null {
+  const ids = event.calendarIds;
+  if (!ids || typeof ids !== 'object') return null;
+  const keys = Object.keys(ids);
+  return keys.length > 0 ? keys[0] : null;
+}
+
+export async function POST(request: NextRequest) {
+  try {
+    const creds = await getStalwartCredentials(request);
+    if (!creds) {
+      return NextResponse.json({ error: 'Not authenticated' }, { status: 401 });
+    }
+
+    let body: { days?: unknown; limit?: unknown } = {};
+    try {
+      body = await request.json();
+    } catch {
+      /* empty body is fine */
+    }
+    const days = clampInt(body.days, 1, 90, 7);
+    const limit = clampInt(body.limit, 1, 200, 50);
+
+    // ── Resolve the calendar account from the JMAP session ──
+    // Hit Stalwart's canonical session endpoint on the SAME host as serverUrl.
+    // We deliberately avoid /.well-known/jmap: it 301s to the server's
+    // configured public hostname, which the host process may not be able to
+    // resolve (the browser client rewrites those URLs back to the origin for
+    // the same reason). Fall back to /.well-known/jmap for non-Stalwart servers.
+    const session = await fetchJmapSession(creds.serverUrl, creds.authHeader);
+    if (!session) {
+      return NextResponse.json({ error: 'JMAP session fetch failed' }, { status: 502 });
+    }
+    const accountId = session.primaryAccounts?.[CALENDAR_CAP];
+    if (!accountId) {
+      // No calendar account for this user — return an empty agenda, not an error.
+      return NextResponse.json({ events: [], generatedAt: new Date().toISOString() });
+    }
+
+    const using = ['urn:ietf:params:jmap:core', CALENDAR_CAP];
+    if (session.capabilities && PRINCIPALS_CAP in session.capabilities) {
+      using.push('urn:ietf:params:jmap:principals:owner');
+    }
+
+    // Send method calls to the same-origin JMAP endpoint the app's passthrough
+    // uses — never to session.apiUrl's (possibly unreachable) public host.
+    const apiUrl = `${creds.serverUrl}/jmap/`;
+
+    const now = new Date();
+    const horizon = new Date(now.getTime() + days * 24 * 60 * 60 * 1000);
+    // Expand from the start of today so all-day / already-running events still
+    // show in the agenda.
+    const windowStart = new Date(now);
+    windowStart.setHours(0, 0, 0, 0);
+
+    // ── 1) Query event IDs in range + load calendars (colours) ──
+    const queryReq = {
+      using,
+      methodCalls: [
+        [
+          'CalendarEvent/query',
+          {
+            accountId,
+            // Mirror the app's calendar store: an { after, before } window lets
+            // Stalwart evaluate recurrence so masters with occurrences in range
+            // are returned (a `before`-only filter can drop unbounded series).
+            filter: { after: windowStart.toISOString(), before: horizon.toISOString() },
+            limit: 1000,
+          },
+          '0',
+        ],
+        [
+          'Calendar/get',
+          { accountId, ids: null, properties: ['id', 'name', 'color'] },
+          'c',
+        ],
+      ],
+    };
+
+    const queryRes = await jmapPost(apiUrl, creds.authHeader, queryReq);
+    const queryResp = findResponse(queryRes, 'CalendarEvent/query', '0');
+    if (!queryResp) {
+      const err = findResponse(queryRes, 'error', '0');
+      return NextResponse.json(
+        { error: (err?.description as string) || 'CalendarEvent/query failed' },
+        { status: 502 },
+      );
+    }
+    const ids = (queryResp.ids as string[]) || [];
+
+    const calColors = new Map<string, { name: string; color: string | null }>();
+    const calResp = findResponse(queryRes, 'Calendar/get', 'c');
+    for (const cal of ((calResp?.list as Array<Record<string, unknown>>) || [])) {
+      if (typeof cal.id === 'string') {
+        calColors.set(cal.id, {
+          name: typeof cal.name === 'string' ? cal.name : '',
+          color: typeof cal.color === 'string' ? cal.color : null,
+        });
+      }
+    }
+
+    if (ids.length === 0) {
+      return NextResponse.json({ events: [], generatedAt: now.toISOString() });
+    }
+
+    // ── 2) Fetch full event objects (batched) ──
+    const raw: Array<Record<string, unknown>> = [];
+    const BATCH = 100;
+    for (let i = 0; i < ids.length; i += BATCH) {
+      const batch = ids.slice(i, i + BATCH);
+      const getRes = await jmapPost(apiUrl, creds.authHeader, {
+        using,
+        methodCalls: [
+          ['CalendarEvent/get', { accountId, ids: batch, properties: EVENT_PROPERTIES }, '0'],
+        ],
+      });
+      const getResp = findResponse(getRes, 'CalendarEvent/get', '0');
+      if (getResp?.list) raw.push(...(getResp.list as Array<Record<string, unknown>>));
+    }
+
+    // ── 3) Normalize + expand recurrences server-side ──
+    const normalized = raw
+      .map((e) => normalizeCalendarEventLike(e as Partial<CalendarEvent>))
+      .filter((e) => (e['@type'] ?? 'Event') === 'Event')
+      // Drop malformed events without a parseable start (would crash format()/
+      // parseISO downstream) — mirrors the calendar store guard (#316).
+      .filter((e) => typeof e.start === 'string' && e.start && !isNaN(parseISO(e.start).getTime())) as CalendarEvent[];
+
+    const expanded = expandRecurringEvents(
+      normalized,
+      windowStart.toISOString(),
+      horizon.toISOString(),
+    );
+
+    // ── 4) Keep ongoing/upcoming, sort, slice, map to DTOs ──
+    const agenda: AgendaEvent[] = expanded
+      .filter((e) => eventEnd(e).getTime() >= now.getTime())
+      .sort((a, b) => eventStart(a).getTime() - eventStart(b).getTime())
+      .slice(0, limit)
+      .map((e) => {
+        const calId = firstCalendarId(e);
+        const cal = calId ? calColors.get(calId) : undefined;
+        return {
+          id: String(e.id ?? ''),
+          uid: e.uid ?? null,
+          title: (e.title ?? '').trim() || '(no title)',
+          start: eventStart(e).toISOString(),
+          end: eventEnd(e).toISOString(),
+          allDay: !!e.showWithoutTime,
+          status: e.status ?? null,
+          color: e.color || cal?.color || null,
+          location: firstLocationName(e),
+          calendarId: calId,
+        };
+      });
+
+    return NextResponse.json({ events: agenda, generatedAt: now.toISOString() });
+  } catch (error) {
+    // `fetch failed` from undici is too generic to debug — the real reason
+    // (ENOTFOUND, ECONNREFUSED, self-signed TLS, …) lives on `error.cause`.
+    const err = error as Error & { cause?: { code?: string; message?: string } };
+    logger.error('Calendar agenda error', {
+      error: err?.message ?? 'Unknown',
+      causeCode: err?.cause?.code,
+      causeMessage: err?.cause?.message,
+    });
+    return NextResponse.json({ error: 'Internal server error' }, { status: 500 });
+  }
+}
+
+function clampInt(value: unknown, min: number, max: number, fallback: number): number {
+  const n = typeof value === 'number' ? value : Number(value);
+  if (!Number.isFinite(n)) return fallback;
+  return Math.min(max, Math.max(min, Math.round(n)));
+}
+
+/**
+ * Fetch the JMAP session from the same host as `serverUrl`. Tries Stalwart's
+ * canonical /jmap/session first (no redirect), then /.well-known/jmap as a
+ * fallback for other servers. Returns null if neither yields a usable session.
+ */
+async function fetchJmapSession(
+  serverUrl: string,
+  authHeader: string,
+): Promise<JmapSession | null> {
+  const candidates = [`${serverUrl}/jmap/session`, `${serverUrl}/.well-known/jmap`];
+  for (const url of candidates) {
+    try {
+      const res = await fetch(url, {
+        method: 'GET',
+        headers: { Authorization: authHeader },
+        redirect: 'follow',
+      });
+      if (!res.ok) continue;
+      const session = (await res.json()) as JmapSession;
+      if (session && typeof session === 'object' && session.primaryAccounts) {
+        return session;
+      }
+    } catch {
+      // Try the next candidate (e.g. canonical path 404s on a non-Stalwart server).
+    }
+  }
+  return null;
+}
+
+async function jmapPost(
+  apiUrl: string,
+  authHeader: string,
+  payload: unknown,
+): Promise<unknown> {
+  const res = await fetch(apiUrl, {
+    method: 'POST',
+    headers: { Authorization: authHeader, 'Content-Type': 'application/json' },
+    body: JSON.stringify(payload),
+  });
+  if (!res.ok) {
+    throw new Error(`JMAP request failed (${res.status})`);
+  }
+  return res.json();
+}
+
+function findResponse(
+  res: unknown,
+  name: string,
+  callId: string,
+): Record<string, unknown> | null {
+  const responses = (res as { methodResponses?: unknown[] })?.methodResponses;
+  if (!Array.isArray(responses)) return null;
+  for (const entry of responses) {
+    if (Array.isArray(entry) && entry[0] === name && entry[2] === callId) {
+      return entry[1] as Record<string, unknown>;
+    }
+  }
+  return null;
+}
diff --git a/components/email/email-viewer.tsx b/components/email/email-viewer.tsx
index 75dafef2..3a1fc8c4 100644
--- a/components/email/email-viewer.tsx
+++ b/components/email/email-viewer.tsx
@@ -1034,12 +1034,29 @@ export function EmailViewer({
   const [embeddedEmailAttachments, setEmbeddedEmailAttachments] = useState<PostalMimeAttachment[]>([]);
   const [embeddedEmailUnwrapped, setEmbeddedEmailUnwrapped] = useState(false);
 
-  // Plugin detail sidebar state
+  // Plugin detail sidebar state. Collapsed/width persist across opens and
+  // sessions so the panel reopens the way the user last left it.
   const detailSlots = usePluginSlotOffers('email-detail-sidebar');
   const hasDetailSidebar = detailSlots.length > 0;
-  const [detailSidebarCollapsed, setDetailSidebarCollapsed] = useState(false);
-  const [detailSidebarWidth, setDetailSidebarWidth] = useState(280);
-  const detailSidebarWidthRef = useRef(280);
+  const [detailSidebarCollapsed, setDetailSidebarCollapsed] = useState<boolean>(() => {
+    if (typeof window === 'undefined') return false;
+    try { return localStorage.getItem('emailDetailSidebarCollapsed') === '1'; } catch { return false; }
+  });
+  const [detailSidebarWidth, setDetailSidebarWidth] = useState<number>(() => {
+    if (typeof window === 'undefined') return 280;
+    try {
+      const n = parseInt(localStorage.getItem('emailDetailSidebarWidth') ?? '', 10);
+      return Number.isFinite(n) ? Math.max(200, Math.min(500, n)) : 280;
+    } catch { return 280; }
+  });
+  const detailSidebarWidthRef = useRef(detailSidebarWidth);
+
+  useEffect(() => {
+    try { localStorage.setItem('emailDetailSidebarCollapsed', detailSidebarCollapsed ? '1' : '0'); } catch { /* ignore */ }
+  }, [detailSidebarCollapsed]);
+  useEffect(() => {
+    try { localStorage.setItem('emailDetailSidebarWidth', String(detailSidebarWidth)); } catch { /* ignore */ }
+  }, [detailSidebarWidth]);
 
   // Ensure S/MIME key records are loaded from IndexedDB
   useLayoutEffect(() => {