diff --git a/e2e/loadtest/upload-integrity-check.mjs b/e2e/loadtest/upload-integrity-check.mjs
new file mode 100644
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+++ b/e2e/loadtest/upload-integrity-check.mjs
@@ -0,0 +1,150 @@
+#!/usr/bin/env node
+/**
+ * Drives a REAL upload through the composer's file picker and proves the bytes survived.
+ *
+ * Written during the v0.18.3–v0.18.6 run of hotfixes, which were all one failure wearing
+ * different masks: the browser handing the queue something that was not the file. First an
+ * empty body (WebKit stores a picked `File` as a reference to an OS file iOS then deletes),
+ * then — once the bytes were copied — the risk of a SHORT one, because that copy is a
+ * multi-second chunked loop for a video and the same purge can land partway through it.
+ *
+ * Every check that missed those bugs shared one shortcut: it asserted the upload was
+ * ACCEPTED. A truncated file is accepted. It passes the magic-byte sniff, the size cap and the
+ * decode budget, is stored, gets a preview, and shows in the gallery — and is still not the
+ * guest's video. So this asserts the only thing that actually settles it: fetch the stored
+ * original back and compare its SHA-256 to the file on disk.
+ *
+ * Two paths matter and they are not the same code:
+ *   * at or below MATERIALISE_CHUNK_BYTES (4 MB) the copy is a single `arrayBuffer()`
+ *   * above it, a chunked loop — the one that can produce a short blob
+ * Pass at least one file of each, or the run proves half of what it claims.
+ *
+ * Requires the sim stack (e2e/docker-compose.sim.yml) on :3102.
+ *
+ *     node e2e/loadtest/upload-integrity-check.mjs photo.jpg big-video.mp4
+ */
+import { chromium, devices } from '@playwright/test';
+import { readFile, stat } from 'node:fs/promises';
+import { createHash } from 'node:crypto';
+import { basename } from 'node:path';
+
+const BASE = process.env.SIM_BASE ?? 'http://localhost:3102';
+const API = `${BASE}/api/v1`;
+/** Mirrors MATERIALISE_CHUNK_BYTES in frontend/src/lib/upload-queue.ts. */
+const CHUNK_BYTES = 4 * 1024 * 1024;
+
+const sha = (buf) => createHash('sha256').update(buf).digest('hex');
+
+const files = process.argv.slice(2);
+if (!files.length) {
+	console.error('usage: upload-integrity-check.mjs <file> [file...]  (include one >4 MB)');
+	process.exit(2);
+}
+
+async function main() {
+	const browser = await chromium.launch();
+	const context = await browser.newContext({ ...devices['Pixel 7'] });
+	// The guide is a modal over the composer; leaving it up means the picker is never reached.
+	await context.addInitScript(() => localStorage.setItem('eventsnap_guide_seen', '1'));
+	const page = await context.newPage();
+	const pageErrors = [];
+	page.on('pageerror', (e) => pageErrors.push(String(e).slice(0, 140)));
+
+	await page.goto(`${BASE}/join`, { waitUntil: 'load' });
+	await page.waitForTimeout(1200);
+	await page.fill('input[type=text]', `Integrity ${Date.now() % 100000}`);
+	await page.locator('button[type=submit]').first().click();
+	await page.waitForTimeout(4500);
+	for (const label of ['Weiter', 'Verstanden', 'Los geht']) {
+		const btn = page.getByRole('button', { name: new RegExp(label, 'i') });
+		if ((await btn.count()) && (await btn.first().isVisible())) {
+			await btn.first().click();
+			await page.waitForTimeout(700);
+			break;
+		}
+	}
+	const token = await page.evaluate(() => localStorage.getItem('eventsnap_jwt'));
+
+	// Report which upload entries the sheet offers, so a run also records whether
+	// PUBLIC_CAMERA_ENABLED was in effect — the composer path differs with it.
+	await page.locator('button[aria-label="Hochladen"]').last().click();
+	await page.waitForTimeout(900);
+	const sheet = await page.locator('body').innerText();
+	console.log(
+		`[sheet] Galerie=${sheet.includes('Foto oder Video wählen')} ` +
+			`Kamera=${sheet.includes('Jetzt aufnehmen')}\n`
+	);
+	// Close it again. The FAB TOGGLES, so leaving the sheet open here makes the loop's first
+	// "open the sheet" click close it instead — and the file chooser then never fires.
+	await page.getByRole('button', { name: /Abbrechen/i }).first().click();
+	await page.waitForTimeout(600);
+
+	const results = [];
+	for (const path of files) {
+		const name = basename(path);
+		const local = await readFile(path);
+		const size = (await stat(path)).size;
+		const caption = `integrity ${name} ${Date.now()}`;
+		const chunked = size > CHUNK_BYTES;
+
+		// Open the sheet fresh for every file — submitting returns to the feed, and the
+		// invariant this relies on is simply that the sheet is CLOSED at the top of each pass.
+		await page.locator('button[aria-label="Hochladen"]').last().click();
+		await page.waitForTimeout(900);
+		const chooser = page.waitForEvent('filechooser');
+		await page.getByText('Foto oder Video wählen').click();
+		(await chooser).setFiles(path);
+		await page.waitForTimeout(2500);
+		await page.locator('textarea').fill(caption);
+		await page.getByRole('button', { name: /^Hochladen$/ }).first().click();
+		// Generous: the queue retries, and a large file on a throttled box takes its time.
+		await page.waitForTimeout(Math.max(9000, (size / 1e6) * 900));
+
+		// Find it server-side. The feed is the guest's own view, so this also proves the photo
+		// is actually visible rather than merely stored.
+		const feed = await fetch(`${API}/feed?limit=50`, {
+			headers: { Authorization: `Bearer ${token}` },
+		}).then((r) => r.json());
+		const row = feed.uploads?.find((u) => u.caption === caption);
+		if (!row) {
+			results.push({ name, size, chunked, ok: false, why: 'never appeared in the feed' });
+			continue;
+		}
+		// The whole point: compare the bytes that came BACK, not the status that went out.
+		const served = Buffer.from(
+			await fetch(`${API}/upload/${row.id}/original`).then((r) => r.arrayBuffer())
+		);
+		const ok = served.length === size && sha(served) === sha(local);
+		results.push({
+			name,
+			size,
+			chunked,
+			ok,
+			why: ok ? '' : `stored ${served.length} of ${size} bytes / hash mismatch`,
+		});
+	}
+
+	await browser.close();
+
+	console.log('═'.repeat(66));
+	console.log('UPLOAD INTEGRITY');
+	console.log('═'.repeat(66));
+	for (const r of results) {
+		console.log(
+			`  ${r.ok ? '✓' : '✗'} ${r.name.padEnd(24)} ${(r.size / 1e6).toFixed(1).padStart(6)} MB  ` +
+				`${r.chunked ? 'chunked copy' : 'single read '}  ${r.why}`
+		);
+	}
+	if (!results.some((r) => r.chunked))
+		console.log('\n  ⚠ no file above 4 MB — the chunked copy path was NOT exercised');
+	if (pageErrors.length) console.log(`\n  page errors: ${pageErrors[0]}`);
+	const failed = results.filter((r) => !r.ok).length;
+	console.log(`\n${failed ? `✗ ${failed} of ${results.length} corrupted` : `✓ all ${results.length} byte-identical`}`);
+	console.log('═'.repeat(66));
+	process.exit(failed ? 1 : 0);
+}
+
+main().catch((e) => {
+	console.error('integrity check failed:', e);
+	process.exit(1);
+});