diff --git a/frontend/src/routes/api/[...path]/+server.spec.ts b/frontend/src/routes/api/[...path]/+server.spec.ts
new file mode 100644
index 00000000..310fa632
--- /dev/null
+++ b/frontend/src/routes/api/[...path]/+server.spec.ts
@@ -0,0 +1,156 @@
+import { describe, it, expect, vi } from 'vitest';
+
+process.env.API_INTERNAL_URL = 'http://backend:8080';
+
+const { GET, PUT, POST } = await import('./+server');
+
+function makeEvent(
+	path: string,
+	method = 'GET',
+	mockFetch = vi
+		.fn()
+		.mockResolvedValue(
+			new Response('{}', { status: 200, headers: { 'Content-Type': 'application/json' } })
+		)
+) {
+	return {
+		params: { path },
+		request: new Request(`http://localhost/api/${path}`, { method }),
+		url: new URL(`http://localhost/api/${path}`),
+		fetch: mockFetch
+	};
+}
+
+describe('catch-all API proxy — security guards', () => {
+	it('returns 400 for path traversal with double-dot', async () => {
+		const event = makeEvent('documents/../../../etc/passwd');
+		const response = await GET(event as never);
+		expect(response.status).toBe(400);
+	});
+
+	it('returns 400 for URL-encoded dot traversal (%2e)', async () => {
+		const event = makeEvent('documents/%2e%2e/sensitive');
+		const response = await GET(event as never);
+		expect(response.status).toBe(400);
+	});
+
+	it('returns 400 for mixed-case encoded traversal (%2E)', async () => {
+		const event = makeEvent('documents/%2E%2E/sensitive');
+		const response = await GET(event as never);
+		expect(response.status).toBe(400);
+	});
+
+	it('returns 404 for actuator paths', async () => {
+		const event = makeEvent('actuator/heapdump');
+		const response = await GET(event as never);
+		expect(response.status).toBe(404);
+	});
+
+	it('returns 404 for actuator/env', async () => {
+		const event = makeEvent('actuator/env');
+		const response = await GET(event as never);
+		expect(response.status).toBe(404);
+	});
+});
+
+describe('catch-all API proxy — body size limit', () => {
+	it('returns 413 when Content-Length exceeds 1 MB', async () => {
+		const mockFetch = vi.fn();
+		const event = makeEvent('documents', 'POST', mockFetch);
+		(event.request as Request) = new Request('http://localhost/api/documents', {
+			method: 'POST',
+			headers: { 'Content-Type': 'application/json', 'Content-Length': '1048577' },
+			body: JSON.stringify({})
+		});
+
+		const response = await POST(event as never);
+
+		expect(response.status).toBe(413);
+		expect(mockFetch).not.toHaveBeenCalled();
+	});
+
+	it('does not reject request with Content-Length exactly at 1 MB', async () => {
+		const mockFetch = vi.fn().mockResolvedValue(new Response('{}', { status: 200 }));
+		const event = makeEvent('documents', 'POST', mockFetch);
+		(event.request as Request) = new Request('http://localhost/api/documents', {
+			method: 'POST',
+			headers: { 'Content-Type': 'application/json', 'Content-Length': '1048576' },
+			body: JSON.stringify({})
+		});
+
+		const response = await POST(event as never);
+
+		expect(response.status).toBe(200);
+	});
+});
+
+describe('catch-all API proxy — forwarding', () => {
+	it('forwards PUT request to backend with correct URL', async () => {
+		const mockFetch = vi.fn().mockResolvedValue(
+			new Response('{"id":"block-1"}', {
+				status: 200,
+				headers: { 'Content-Type': 'application/json' }
+			})
+		);
+		const event = makeEvent('documents/doc-1/transcription-blocks/block-1', 'PUT', mockFetch);
+		(event.request as Request) = new Request(
+			'http://localhost/api/documents/doc-1/transcription-blocks/block-1',
+			{
+				method: 'PUT',
+				headers: { 'Content-Type': 'application/json' },
+				body: JSON.stringify({ text: 'hello' })
+			}
+		);
+
+		const response = await PUT(event as never);
+
+		expect(mockFetch).toHaveBeenCalledWith(
+			'http://backend:8080/api/documents/doc-1/transcription-blocks/block-1',
+			expect.objectContaining({ method: 'PUT' })
+		);
+		expect(response.status).toBe(200);
+	});
+
+	it('passes through the backend response status code', async () => {
+		const mockFetch = vi.fn().mockResolvedValue(new Response(null, { status: 404 }));
+		const event = makeEvent('documents/nonexistent', 'GET', mockFetch);
+
+		const response = await GET(event as never);
+
+		expect(response.status).toBe(404);
+	});
+
+	it('forwards Content-Disposition response header', async () => {
+		const mockFetch = vi.fn().mockResolvedValue(
+			new Response('data', {
+				status: 200,
+				headers: {
+					'Content-Type': 'application/pdf',
+					'Content-Disposition': 'attachment; filename="document.pdf"'
+				}
+			})
+		);
+		const event = makeEvent('documents/doc-1/export', 'GET', mockFetch);
+
+		const response = await GET(event as never);
+
+		expect(response.headers.get('Content-Disposition')).toBe('attachment; filename="document.pdf"');
+	});
+
+	it('does not forward hop-by-hop headers like transfer-encoding', async () => {
+		const mockFetch = vi.fn().mockResolvedValue(
+			new Response('data', {
+				status: 200,
+				headers: {
+					'Content-Type': 'application/json',
+					'Transfer-Encoding': 'chunked'
+				}
+			})
+		);
+		const event = makeEvent('documents', 'GET', mockFetch);
+
+		const response = await GET(event as never);
+
+		expect(response.headers.get('Transfer-Encoding')).toBeNull();
+	});
+});
diff --git a/frontend/src/routes/api/[...path]/+server.ts b/frontend/src/routes/api/[...path]/+server.ts
index dab2308c..9a66699d 100644
--- a/frontend/src/routes/api/[...path]/+server.ts
+++ b/frontend/src/routes/api/[...path]/+server.ts
@@ -3,22 +3,65 @@ import { env } from 'process';
 
 const NO_BODY_METHODS = new Set(['GET', 'HEAD', 'DELETE']);
 
-async function proxy(event: Parameters<RequestHandler>[0]): Promise<Response> {
-	const apiUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
-	const backendUrl = `${apiUrl}/api/${event.params.path}${event.url.search}`;
+// Hop-by-hop headers must not be forwarded to the client — they are connection-scoped.
+const HOP_BY_HOP_HEADERS = new Set([
+	'transfer-encoding',
+	'connection',
+	'keep-alive',
+	'upgrade',
+	'proxy-authenticate',
+	'proxy-authorization',
+	'te',
+	'trailer'
+]);
+
+async function proxy(event: Parameters<RequestHandler>[0]): Promise<Response> {
+	const rawPath = event.params.path;
+
+	// Block path traversal attempts — SvelteKit decodes %2F before routing,
+	// so encoded dots could produce ../../../ sequences after normalization.
+	if (rawPath.includes('..') || /%(2e)/i.test(rawPath)) {
+		return new Response('Bad Request', { status: 400 });
+	}
+
+	// Block Actuator endpoints — heapdump, env, configprops expose credentials
+	// and heap memory and must never be reachable through the SvelteKit proxy.
+	if (rawPath.startsWith('actuator')) {
+		return new Response('Not Found', { status: 404 });
+	}
+
+	const apiUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
+	const backendUrl = `${apiUrl}/api/${rawPath}${event.url.search}`;
 
-	const contentType = event.request.headers.get('Content-Type');
 	const hasBody = !NO_BODY_METHODS.has(event.request.method);
 
+	const contentLength = event.request.headers.get('Content-Length');
+	if (contentLength && parseInt(contentLength, 10) > 1_048_576) {
+		return new Response('Payload Too Large', { status: 413 });
+	}
+
+	// Only forward Content-Type from the browser request — other headers (Cookie,
+	// Authorization, Accept-Encoding, etc.) must not be forwarded directly.
+	// Authentication is injected by handleFetch in hooks.server.ts from the
+	// server-side auth_token cookie.
+	const contentType = event.request.headers.get('Content-Type');
+	const requestHeaders: Record<string, string> = {};
+	if (contentType) requestHeaders['Content-Type'] = contentType;
+
 	const response = await event.fetch(backendUrl, {
 		method: event.request.method,
-		headers: contentType ? { 'Content-Type': contentType } : {},
+		headers: requestHeaders,
 		body: hasBody ? await event.request.arrayBuffer() : undefined
 	});
 
+	// Forward all response headers except hop-by-hop so that Content-Disposition,
+	// ETag, Cache-Control, Location (redirects), etc. reach the browser.
 	const responseHeaders: Record<string, string> = {};
-	const responseContentType = response.headers.get('Content-Type');
-	if (responseContentType) responseHeaders['Content-Type'] = responseContentType;
+	response.headers.forEach((value, key) => {
+		if (!HOP_BY_HOP_HEADERS.has(key.toLowerCase())) {
+			responseHeaders[key] = value;
+		}
+	});
 
 	return new Response(response.body, {
 		status: response.status,