diff --git a/integrations/calendar/app/controller.ts b/integrations/calendar/app/controller.ts
index 966bcba7..d2077566 100644
--- a/integrations/calendar/app/controller.ts
+++ b/integrations/calendar/app/controller.ts
@@ -34,6 +34,7 @@ import {
   type Outcome,
   type PendingEdit,
 } from './sync.js';
+import { errorStack, report } from './report.js';
 
 /** What went wrong, in the terms the banners use (DESIGN.md §5.12). */
 export interface Problem {
@@ -412,7 +413,13 @@ export function createController(
     }
   }
 
-  const fail = (error: unknown, outcomes?: Outcome[]) =>
+  const fail = (error: unknown, outcomes?: Outcome[]) => {
+    const problem = classify(error);
+    report('error', message(error), {
+      problem: problem.kind,
+      ...(problem.status ? { status: problem.status } : {}),
+      stack: errorStack(error),
+    });
     set({
       kind: 'error',
       message: message(error),
@@ -420,10 +427,11 @@ export function createController(
         connections.length === 0 ||
         (error as { reconnect?: boolean }).reconnect === true ||
         spent(error),
-      problem: classify(error),
+      problem,
       ...(outcomes ? { outcomes } : {}),
       ...(last ? { summary: last.summary, at: last.at } : {}),
     });
+  };
 
   const reload = async () => {
     if (!meta) return;
@@ -590,6 +598,19 @@ export function createController(
         );
         stale = false;
         last = { summary, at: new Date() };
+        report(summary.unreadable.length ? 'warn' : 'info', 'Sync finished', {
+          total: summary.total,
+          added: summary.added,
+          updated: summary.updated,
+          unchanged: summary.unchanged,
+          skipped: summary.skipped,
+          conflicts: summary.conflicts.length,
+          review: summary.review.length,
+          unreadable: summary.unreadable.map(({ id, reason }) => ({
+            id,
+            reason,
+          })),
+        });
         await reload();
         set({ kind: 'ready', at: last.at, summary, outcomes: [] });
       } catch (error) {
diff --git a/integrations/calendar/app/main.ts b/integrations/calendar/app/main.ts
index fb301c72..1d12ebd7 100644
--- a/integrations/calendar/app/main.ts
+++ b/integrations/calendar/app/main.ts
@@ -29,6 +29,7 @@ import { detail, editor } from './drawer.js';
 import { busyDays, nextEvent, type CalEvent } from './events.js';
 import { firstRun, importing, noRelay, picker } from './screens.js';
 import { conflicts, review, shortcuts } from './sheets.js';
+import { reportUncaught } from './report.js';
 import { anySkipped, notShown, sidebar } from './sidebar.js';
 import type { ViewArgs } from './store.js';
 import type { Choice, Conflict, ImportSummary } from './sync.js';
@@ -96,6 +97,7 @@ const EMPTY_SUMMARY: ImportSummary = {
 };
 
 export async function view({ root, store }: ViewArgs): Promise<void> {
+  reportUncaught(root.ownerDocument.defaultView ?? window);
   const doc = root.ownerDocument;
   const win = doc.defaultView!;
   installTheme(root, `${PLUGIN_CSS}\n${CALENDAR_CSS}`, store);
diff --git a/integrations/calendar/app/report.test.ts b/integrations/calendar/app/report.test.ts
new file mode 100644
index 00000000..767c5105
--- /dev/null
+++ b/integrations/calendar/app/report.test.ts
@@ -0,0 +1,31 @@
+// @wc-ignore-file
+import { describe, expect, it, vi } from 'vitest';
+import { report } from './report.js';
+
+describe('report', () => {
+  it('does nothing without a hook, as in every published build', () => {
+    expect(() => report('error', 'boom')).not.toThrow();
+    expect(() => report('error', 'boom', {}, 'not a function')).not.toThrow();
+  });
+
+  it('hands one entry to the hook a test build installs', () => {
+    const hook = vi.fn();
+
+    report('warn', 'Sync finished', { total: 3 }, hook);
+
+    expect(hook).toHaveBeenCalledWith({
+      source: 'calendar',
+      level: 'warn',
+      message: 'Sync finished',
+      total: 3,
+    });
+  });
+
+  it('never throws when the hook does', () => {
+    const hook = () => {
+      throw new Error('collector down');
+    };
+
+    expect(() => report('error', 'boom', {}, hook)).not.toThrow();
+  });
+});
diff --git a/integrations/calendar/app/report.ts b/integrations/calendar/app/report.ts
new file mode 100644
index 00000000..6261fe6a
--- /dev/null
+++ b/integrations/calendar/app/report.ts
@@ -0,0 +1,62 @@
+// @wc-ignore-file
+/**
+ * Hands this app's errors, and a line per finished sync, to a user-testing
+ * log hook, when one is installed.
+ *
+ * The app runs in a sandboxed frame, and the host shows an app's errors there
+ * without reporting them anywhere, so without this nobody but the person at
+ * the screen sees them. The app itself still makes no network request
+ * (build.test.ts checks the bundle for `fetch(`): the test catalog's build
+ * (ontola/atomic-plugins `usertest/catalog.mjs`) prepends a small prelude
+ * that defines `globalThis.__USERTEST_REPORT__` and posts to the collector.
+ * A published build has no prelude, so `report` does nothing there.
+ *
+ * Handed over: the error message and stack, the banner's problem kind and
+ * status, and per sync the counts plus the ids and reasons of unreadable
+ * events. Never: event titles, descriptions or other row content.
+ */
+
+export type Level = 'error' | 'warn' | 'info';
+
+type Hook = (entry: Record<string, unknown>) => void;
+
+declare global {
+  var __USERTEST_REPORT__: Hook | undefined;
+}
+
+export function report(
+  level: Level,
+  message: string,
+  data: Record<string, unknown> = {},
+  hook: unknown = globalThis.__USERTEST_REPORT__,
+): void {
+  if (typeof hook !== 'function') return;
+
+  try {
+    (hook as Hook)({ source: 'calendar', level, message, ...data });
+  } catch {
+    // A broken hook never affects the app.
+  }
+}
+
+export const errorStack = (error: unknown) =>
+  error instanceof Error ? error.stack : undefined;
+
+/** Reports what escapes every handler. */
+export function reportUncaught(target: Window): void {
+  target.addEventListener('error', event =>
+    report('error', event.message, {
+      kind: 'uncaught',
+      stack: errorStack(event.error),
+    }),
+  );
+  target.addEventListener('unhandledrejection', event =>
+    report(
+      'error',
+      event.reason instanceof Error
+        ? event.reason.message
+        : String(event.reason),
+      { kind: 'unhandled-rejection', stack: errorStack(event.reason) },
+    ),
+  );
+}