diff --git a/web-player/src/lib/player/listenTracker.test.ts b/web-player/src/lib/player/listenTracker.test.ts
new file mode 100644
index 0000000..7449436
--- /dev/null
+++ b/web-player/src/lib/player/listenTracker.test.ts
@@ -0,0 +1,110 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { ListenTracker } from './listenTracker';
+
+let fetchMock: ReturnType<typeof vi.fn>;
+
+function sentSeconds(call: number): number {
+	const init = fetchMock.mock.calls[call][1] as RequestInit;
+	return JSON.parse(init.body as string).playedSeconds;
+}
+
+beforeEach(() => {
+	vi.useFakeTimers();
+	fetchMock = vi.fn().mockResolvedValue({ ok: true });
+	vi.stubGlobal('fetch', fetchMock);
+});
+
+afterEach(() => {
+	vi.useRealTimers();
+	vi.unstubAllGlobals();
+});
+
+function startedTracker(id = 'l1') {
+	const tracker = new ListenTracker();
+	tracker.begin(id);
+	tracker.tick(0);
+	return tracker;
+}
+
+describe('ListenTracker', () => {
+	it('sums small forward advances', () => {
+		const tracker = startedTracker();
+		tracker.tick(1);
+		tracker.tick(2);
+		tracker.flush();
+		expect(fetchMock).toHaveBeenCalledTimes(1);
+		expect(sentSeconds(0)).toBe(2);
+	});
+
+	it('ignores seeks forward and backwards', () => {
+		const tracker = startedTracker();
+		tracker.tick(1);
+		tracker.tick(60);
+		tracker.tick(10);
+		tracker.tick(11);
+		tracker.flush();
+		expect(sentSeconds(0)).toBe(2);
+	});
+
+	it('does not count the tick after interrupt()', () => {
+		const tracker = startedTracker();
+		tracker.tick(1);
+		tracker.interrupt();
+		tracker.tick(2);
+		tracker.tick(3);
+		tracker.flush();
+		expect(sentSeconds(0)).toBe(2);
+	});
+
+	it('sends after 30 seconds of wall time', () => {
+		const tracker = startedTracker();
+		tracker.tick(1);
+		expect(fetchMock).not.toHaveBeenCalled();
+		vi.advanceTimersByTime(30_000);
+		tracker.tick(2);
+		expect(fetchMock).toHaveBeenCalledTimes(1);
+		expect(sentSeconds(0)).toBe(2);
+	});
+
+	it('does not resend without new progress', () => {
+		const tracker = startedTracker();
+		tracker.tick(1);
+		tracker.flush();
+		tracker.flush();
+		expect(fetchMock).toHaveBeenCalledTimes(1);
+		tracker.tick(2);
+		tracker.flush();
+		expect(fetchMock).toHaveBeenCalledTimes(2);
+		expect(sentSeconds(1)).toBe(2);
+	});
+
+	it('sends nothing when there is no listen id', () => {
+		const tracker = new ListenTracker();
+		tracker.begin(null);
+		tracker.tick(0);
+		tracker.tick(1);
+		tracker.flush();
+		expect(fetchMock).not.toHaveBeenCalled();
+	});
+
+	it('resends the total after a failed request', async () => {
+		fetchMock.mockRejectedValueOnce(new Error('offline'));
+		const tracker = startedTracker();
+		tracker.tick(1);
+		tracker.flush();
+		await vi.runAllTimersAsync();
+		tracker.flush();
+		expect(fetchMock).toHaveBeenCalledTimes(2);
+		expect(sentSeconds(1)).toBe(1);
+	});
+
+	it('flushes the previous listen when a new one begins', () => {
+		const tracker = startedTracker('old');
+		tracker.tick(1);
+		tracker.begin('new');
+		expect(fetchMock).toHaveBeenCalledTimes(1);
+		expect(fetchMock.mock.calls[0][0]).toBe('/api/listens/old/progress');
+		tracker.flush();
+		expect(fetchMock).toHaveBeenCalledTimes(1);
+	});
+});
diff --git a/web-player/src/lib/state/liked.test.ts b/web-player/src/lib/state/liked.test.ts
new file mode 100644
index 0000000..273c4bc
--- /dev/null
+++ b/web-player/src/lib/state/liked.test.ts
@@ -0,0 +1,80 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import type { LikedTrack, Track } from '$lib/types';
+
+function track(id: string): Track {
+	return {
+		id,
+		title: id,
+		artist: 'Artist',
+		duration: 100,
+		explicit: false,
+		listensCount: 0
+	} as unknown as Track;
+}
+
+function likedTrack(id: string): LikedTrack {
+	return { ...track(id), likedAt: 1 } as LikedTrack;
+}
+
+async function freshStore() {
+	vi.resetModules();
+	return (await import('./liked.svelte')).liked;
+}
+
+afterEach(() => {
+	vi.unstubAllGlobals();
+});
+
+describe('liked store', () => {
+	it('hydrates only once', async () => {
+		const liked = await freshStore();
+		liked.hydrate([likedTrack('a')]);
+		liked.hydrate([likedTrack('b')]);
+		expect(liked.isLiked('a')).toBe(true);
+		expect(liked.isLiked('b')).toBe(false);
+		expect(liked.count).toBe(1);
+	});
+
+	it('likes and unlikes a track', async () => {
+		vi.stubGlobal('fetch', vi.fn().mockResolvedValue({ ok: true }));
+		const liked = await freshStore();
+		await liked.toggle(track('a'));
+		expect(liked.isLiked('a')).toBe(true);
+		await liked.toggle(track('a'));
+		expect(liked.isLiked('a')).toBe(false);
+		expect(fetch).toHaveBeenCalledTimes(2);
+	});
+
+	it('reverts a failed like', async () => {
+		vi.stubGlobal('fetch', vi.fn().mockResolvedValue({ ok: false }));
+		const liked = await freshStore();
+		await liked.toggle(track('a'));
+		expect(liked.isLiked('a')).toBe(false);
+	});
+
+	it('reverts a failed unlike', async () => {
+		const liked = await freshStore();
+		liked.hydrate([likedTrack('a')]);
+		vi.stubGlobal('fetch', vi.fn().mockRejectedValue(new Error('offline')));
+		await liked.toggle(track('a'));
+		expect(liked.isLiked('a')).toBe(true);
+	});
+
+	it('keeps changes made to other tracks while a request fails', async () => {
+		const liked = await freshStore();
+		let rejectFirst: (reason: Error) => void = () => {};
+		const fetchMock = vi
+			.fn()
+			.mockImplementationOnce(() => new Promise((_, reject) => (rejectFirst = reject)))
+			.mockResolvedValueOnce({ ok: true });
+		vi.stubGlobal('fetch', fetchMock);
+
+		const first = liked.toggle(track('a'));
+		await liked.toggle(track('b'));
+		rejectFirst(new Error('offline'));
+		await first;
+
+		expect(liked.isLiked('a')).toBe(false);
+		expect(liked.isLiked('b')).toBe(true);
+	});
+});
diff --git a/web-player/src/lib/state/pagedList.test.ts b/web-player/src/lib/state/pagedList.test.ts
new file mode 100644
index 0000000..ccbc4a2
--- /dev/null
+++ b/web-player/src/lib/state/pagedList.test.ts
@@ -0,0 +1,73 @@
+import { describe, expect, it, vi } from 'vitest';
+import type { Paged } from '$lib/types';
+import { createPagedList } from './pagedList.svelte';
+
+interface Item {
+	id: number;
+}
+
+function page(ids: number[], pageNumber: number, hasNextPage: boolean): Paged<Item> {
+	return { items: ids.map((id) => ({ id })), pageNumber, hasNextPage, totalItemCount: 99 };
+}
+
+describe('createPagedList', () => {
+	it('appends the next page without duplicates and updates hasMore', async () => {
+		const fetchPage = vi.fn().mockResolvedValue(page([2, 3], 2, false));
+		const list = createPagedList<Item>(fetchPage, (item) => item.id);
+		list.reset(page([1, 2], 1, true));
+		expect(list.hasMore).toBe(true);
+
+		await list.loadMore();
+
+		expect(fetchPage).toHaveBeenCalledWith(2);
+		expect(list.items.map((item) => item.id)).toEqual([1, 2, 3]);
+		expect(list.hasMore).toBe(false);
+	});
+
+	it('ignores loadMore while another is in flight', async () => {
+		let resolve: (value: Paged<Item>) => void = () => {};
+		const fetchPage = vi.fn().mockImplementation(
+			() =>
+				new Promise<Paged<Item>>((r) => {
+					resolve = r;
+				})
+		);
+		const list = createPagedList<Item>(fetchPage, (item) => item.id);
+		list.reset(page([1], 1, true));
+
+		const first = list.loadMore();
+		expect(list.loading).toBe(true);
+		await list.loadMore();
+		expect(fetchPage).toHaveBeenCalledTimes(1);
+
+		resolve(page([2], 2, false));
+		await first;
+		expect(list.loading).toBe(false);
+	});
+
+	it('flags an error and keeps the items when the fetch fails', async () => {
+		const fetchPage = vi.fn().mockRejectedValue(new Error('boom'));
+		const list = createPagedList<Item>(fetchPage, (item) => item.id);
+		list.reset(page([1, 2], 1, true));
+
+		await list.loadMore();
+
+		expect(list.error).toBe(true);
+		expect(list.loading).toBe(false);
+		expect(list.items).toHaveLength(2);
+	});
+
+	it('reset clears the error', async () => {
+		const fetchPage = vi.fn().mockRejectedValue(new Error('boom'));
+		const list = createPagedList<Item>(fetchPage, (item) => item.id);
+		list.reset(page([1], 1, true));
+		await list.loadMore();
+		expect(list.error).toBe(true);
+
+		list.reset(page([5], 1, false));
+
+		expect(list.error).toBe(false);
+		expect(list.items).toEqual([{ id: 5 }]);
+		expect(list.hasMore).toBe(false);
+	});
+});
diff --git a/web-player/src/routes/api/likes/server.test.ts b/web-player/src/routes/api/likes/server.test.ts
new file mode 100644
index 0000000..d5e50fd
--- /dev/null
+++ b/web-player/src/routes/api/likes/server.test.ts
@@ -0,0 +1,57 @@
+import { describe, expect, it, vi } from 'vitest';
+
+vi.mock('$lib/server/config', () => ({ apiConfig: { baseUrl: 'http://api.test' } }));
+
+import { POST } from './+server';
+
+type PostEvent = Parameters<typeof POST>[0];
+
+function event(body: unknown, options: { token?: string | null; api?: Response } = {}) {
+	const { token = 'token', api = Response.json(true) } = options;
+	const fetchFn = vi.fn().mockResolvedValue(api);
+	const request = new Request('http://x/api/likes', {
+		method: 'POST',
+		headers: { 'content-type': 'application/json' },
+		body: typeof body === 'string' ? body : JSON.stringify(body)
+	});
+	return {
+		event: {
+			request,
+			locals: { accessToken: token, user: null },
+			fetch: fetchFn
+		} as unknown as PostEvent,
+		fetchFn
+	};
+}
+
+describe('POST /api/likes', () => {
+	it('returns 401 without a token', async () => {
+		const { event: e, fetchFn } = event({ trackId: 't1' }, { token: null });
+		expect((await POST(e)).status).toBe(401);
+		expect(fetchFn).not.toHaveBeenCalled();
+	});
+
+	it.each([[{}], [{ trackId: '' }], [{ trackId: 5 }], ['not json']])(
+		'returns 400 for an invalid body %j',
+		async (body) => {
+			const { event: e, fetchFn } = event(body);
+			expect((await POST(e)).status).toBe(400);
+			expect(fetchFn).not.toHaveBeenCalled();
+		}
+	);
+
+	it('returns the liked state from the API', async () => {
+		const { event: e } = event({ trackId: 't1' });
+		const res = await POST(e);
+		expect(res.status).toBe(200);
+		expect(await res.json()).toEqual({ liked: true });
+	});
+
+	it('returns 502 when the API fails', async () => {
+		const { event: e } = event(
+			{ trackId: 't1' },
+			{ api: Response.json({ title: 'boom' }, { status: 500 }) }
+		);
+		expect((await POST(e)).status).toBe(502);
+	});
+});
diff --git a/web-player/src/routes/api/listens/[id]/progress/server.test.ts b/web-player/src/routes/api/listens/[id]/progress/server.test.ts
new file mode 100644
index 0000000..7ae6cfb
--- /dev/null
+++ b/web-player/src/routes/api/listens/[id]/progress/server.test.ts
@@ -0,0 +1,61 @@
+import { describe, expect, it, vi } from 'vitest';
+
+vi.mock('$lib/server/config', () => ({ apiConfig: { baseUrl: 'http://api.test' } }));
+
+import { PUT } from './+server';
+
+type PutEvent = Parameters<typeof PUT>[0];
+
+function event(body: string, api: Response = new Response(null, { status: 204 })) {
+	const fetchFn = vi.fn().mockResolvedValue(api);
+	const request = new Request('http://x/api/listens/l1/progress', {
+		method: 'PUT',
+		headers: { 'content-type': 'application/json' },
+		body
+	});
+	return {
+		event: {
+			request,
+			params: { id: 'l1' },
+			locals: { accessToken: 'token', user: null },
+			fetch: fetchFn
+		} as unknown as PutEvent,
+		fetchFn
+	};
+}
+
+describe('PUT /api/listens/[id]/progress', () => {
+	it('returns 401 without a token', async () => {
+		const { event: e } = event('{"playedSeconds":5}');
+		(e.locals as { accessToken: string | null }).accessToken = null;
+		expect((await PUT(e)).status).toBe(401);
+	});
+
+	it.each([
+		['invalid JSON', 'nope'],
+		['negative seconds', '{"playedSeconds":-1}'],
+		['string seconds', '{"playedSeconds":"5"}'],
+		['missing seconds', '{}'],
+		['null seconds', '{"playedSeconds":null}']
+	])('returns 400 for %s', async (_name, body) => {
+		const { event: e, fetchFn } = event(body);
+		expect((await PUT(e)).status).toBe(400);
+		expect(fetchFn).not.toHaveBeenCalled();
+	});
+
+	it('returns 204 when the API accepts the progress', async () => {
+		const { event: e, fetchFn } = event('{"playedSeconds":12.5}');
+		expect((await PUT(e)).status).toBe(204);
+		expect(fetchFn).toHaveBeenCalledTimes(1);
+	});
+
+	it('returns 404 when the API returns 404', async () => {
+		const { event: e } = event('{"playedSeconds":5}', Response.json({}, { status: 404 }));
+		expect((await PUT(e)).status).toBe(404);
+	});
+
+	it('returns 502 for other API failures', async () => {
+		const { event: e } = event('{"playedSeconds":5}', Response.json({}, { status: 500 }));
+		expect((await PUT(e)).status).toBe(502);
+	});
+});
diff --git a/web-player/src/routes/api/tracks/server.test.ts b/web-player/src/routes/api/tracks/server.test.ts
new file mode 100644
index 0000000..5635e0f
--- /dev/null
+++ b/web-player/src/routes/api/tracks/server.test.ts
@@ -0,0 +1,42 @@
+import { describe, expect, it, vi } from 'vitest';
+import { EXPLORE_PAGE_SIZE, MAX_PAGE_SIZE } from '$lib/config';
+
+vi.mock('$lib/server/config', () => ({ apiConfig: { baseUrl: 'http://api.test' } }));
+
+import { GET } from './+server';
+
+type GetEvent = Parameters<typeof GET>[0];
+
+async function requestedQuery(search: string) {
+	const fetchFn = vi
+		.fn()
+		.mockResolvedValue(
+			Response.json({ items: [], pageNumber: 1, hasNextPage: false, totalItemCount: 0 })
+		);
+	const res = await GET({
+		url: new URL(`http://x/api/tracks${search}`),
+		locals: { accessToken: null, user: null },
+		fetch: fetchFn
+	} as unknown as GetEvent);
+	expect(res.status).toBe(200);
+	const request = fetchFn.mock.calls[0][0] as Request;
+	return new URL(request.url).searchParams;
+}
+
+describe('GET /api/tracks', () => {
+	it('caps pageSize at MAX_PAGE_SIZE', async () => {
+		const query = await requestedQuery('?pageSize=100000');
+		expect(query.get('pageSize')).toBe(String(MAX_PAGE_SIZE));
+	});
+
+	it('uses the default page size for invalid values', async () => {
+		expect((await requestedQuery('?pageSize=abc')).get('pageSize')).toBe(String(EXPLORE_PAGE_SIZE));
+		expect((await requestedQuery('')).get('pageSize')).toBe(String(EXPLORE_PAGE_SIZE));
+	});
+
+	it('resets an invalid pageNumber to 1', async () => {
+		expect((await requestedQuery('?pageNumber=-3')).get('pageNumber')).toBe('1');
+		expect((await requestedQuery('?pageNumber=abc')).get('pageNumber')).toBe('1');
+		expect((await requestedQuery('?pageNumber=4')).get('pageNumber')).toBe('4');
+	});
+});