diff --git a/src/align/jobs-pool.ts b/src/align/jobs-pool.ts
new file mode 100644
index 0000000..64ce5d1
--- /dev/null
+++ b/src/align/jobs-pool.ts
@@ -0,0 +1,64 @@
+// One worker thread (./jobs.ts) running jobs one at a time in FIFO order. Jobs whose AbortSignal fired while queued
+// are dropped without running (e.g. a preview render superseded by a newer edit). A crashed worker is replaced.
+import type { Job } from "./jobs.ts";
+
+interface Pending { job: Job; signal?: AbortSignal; resolve(v: unknown): void; reject(e: Error): void }
+
+export class JobWorker {
+  private w: Worker | null = null;
+  private queue: Pending[] = [];
+  private current: { id: number; p: Pending } | null = null;
+  private nextId = 1;
+  private closed = false;
+
+  run<T>(job: Job, signal?: AbortSignal): Promise<T> {
+    if (this.closed) return Promise.reject(new Error("job worker stopped"));
+    return new Promise<T>((resolve, reject) => {
+      this.queue.push({ job, signal, resolve: resolve as (v: unknown) => void, reject });
+      this.pump();
+    });
+  }
+
+  /** Jobs queued or running. */
+  get load(): number { return this.queue.length + (this.current ? 1 : 0); }
+
+  terminate(): void {
+    this.closed = true;
+    this.w?.terminate(); this.w = null;
+    const err = new Error("job worker stopped");
+    this.current?.p.reject(err); this.current = null;
+    for (const p of this.queue.splice(0)) p.reject(err);
+  }
+
+  private worker(): Worker {
+    if (this.w) return this.w;
+    const w = new Worker(new URL("./jobs.ts", import.meta.url).href);
+    (w as Worker & { unref?(): void }).unref?.(); // never keeps the process alive
+    w.onmessage = (e: MessageEvent<{ id: number; result?: unknown; error?: string }>) => {
+      const c = this.current;
+      if (!c || c.id !== e.data.id) return;
+      this.current = null;
+      if (e.data.error !== undefined) c.p.reject(new Error(e.data.error)); else c.p.resolve(e.data.result);
+      this.pump();
+    };
+    w.onerror = (e) => {
+      e.preventDefault?.();
+      const c = this.current;
+      this.current = null;
+      if (this.w === w) { w.terminate(); this.w = null; }
+      c?.p.reject(new Error(`job worker crashed: ${e.message}`));
+      this.pump();
+    };
+    return (this.w = w);
+  }
+
+  private pump(): void {
+    while (!this.current && this.queue.length && !this.closed) {
+      const p = this.queue.shift()!;
+      if (p.signal?.aborted) { p.reject(new DOMException("aborted", "AbortError")); continue; }
+      const id = this.nextId++;
+      this.current = { id, p };
+      this.worker().postMessage({ id, job: p.job });
+    }
+  }
+}
diff --git a/src/align/jobs.ts b/src/align/jobs.ts
new file mode 100644
index 0000000..207d261
--- /dev/null
+++ b/src/align/jobs.ts
@@ -0,0 +1,36 @@
+// Worker thread for the alignment server's heavy, synchronous jobs (MIDI preview render, automatic alignment), so the
+// server's event loop keeps answering other requests (page, fit, audio) while they run. Started by ./jobs-pool.ts.
+/// <reference lib="webworker" />
+import { renderMidi } from "./render.ts";
+import { autoAlign } from "./auto.ts";
+import { makeWarp, type Anchor } from "./warp.ts";
+import type { Audio, Role } from "../core/types.ts";
+import type { RefTrack, Smf } from "./smf.ts";
+
+export type Job =
+  /** Mono, not normalised, audio time [startSec, startSec + durationSec) -> Float32Array. */
+  | { type: "render"; sf2Path: string; tracks: RefTrack[]; anchors: Anchor[]; sampleRate: number; startSec: number; durationSec: number }
+  | { type: "auto"; sf2Path: string; audio: Audio; smf: Smf; roles: Record<string, Role | null>; manual: Anchor[] };
+
+const sf2s = new Map<string, Promise<Uint8Array>>();
+const sf2 = (p: string) => { let x = sf2s.get(p); if (!x) sf2s.set(p, (x = Bun.file(p).arrayBuffer().then((b) => new Uint8Array(b)))); return x; };
+
+async function run(job: Job): Promise<unknown> {
+  if (job.type === "render") {
+    const a = renderMidi(job.tracks, makeWarp(job.anchors), await sf2(job.sf2Path), {
+      sampleRate: job.sampleRate, mono: true, startSec: job.startSec, durationSec: job.durationSec, normalize: false,
+    });
+    return a.channels[0]!; // transferred back
+  }
+  return autoAlign(job.audio, job.smf, job.roles, job.manual, await sf2(job.sf2Path));
+}
+
+self.onmessage = async (e: MessageEvent<{ id: number; job: Job }>) => {
+  const { id, job } = e.data;
+  try {
+    const result = await run(job);
+    self.postMessage({ id, result }, ArrayBuffer.isView(result) ? [result.buffer as ArrayBuffer] : []);
+  } catch (err) {
+    self.postMessage({ id, error: (err as Error).message ?? String(err) });
+  }
+};
diff --git a/src/align/preview.ts b/src/align/preview.ts
new file mode 100644
index 0000000..7f1cccd
--- /dev/null
+++ b/src/align/preview.ts
@@ -0,0 +1,52 @@
+// MIDI preview rendering split into time chunks that render in parallel (one per worker thread), then stitched.
+// Each chunk is rendered with a pre-roll (notes already sounding are re-attacked inside it and it is discarded) and a
+// short overlap that is cross-faded into the next chunk, so seams are inaudible for a preview. Pure: no I/O.
+
+export interface Chunk {
+  /** Audio time of the chunk's first output sample (includes the pre-roll). */
+  renderStart: number;
+  /** Seconds to render from `renderStart`. */
+  renderDur: number;
+  /** The part of the timeline this chunk owns: [from, to). */
+  from: number; to: number;
+}
+
+export const PREROLL_SEC = 2;
+export const XFADE_SEC = 0.01;
+/** Chunks shorter than this are not worth a worker (synth start-up, pre-roll overhead). */
+export const MIN_CHUNK_SEC = 20;
+
+/** Splits [0, durSec) into at most `maxChunks` equal chunks of at least MIN_CHUNK_SEC (always at least one). */
+export function planChunks(durSec: number, maxChunks: number): Chunk[] {
+  const n = Math.max(1, Math.min(maxChunks, Math.floor(durSec / MIN_CHUNK_SEC)));
+  const out: Chunk[] = [];
+  for (let k = 0; k < n; k++) {
+    const from = (durSec * k) / n, to = k === n - 1 ? durSec : (durSec * (k + 1)) / n;
+    const renderStart = k === 0 ? 0 : from - PREROLL_SEC;
+    const end = k === n - 1 ? durSec : to + XFADE_SEC;
+    out.push({ renderStart, renderDur: end - renderStart, from, to });
+  }
+  return out;
+}
+
+/**
+ * Joins rendered chunks (mono, `sampleRate`, as planned by `planChunks`) into one signal of round(durSec·sr) samples:
+ * each chunk contributes [from, to), and its XFADE_SEC overlap past `to` fades out while the next chunk fades in.
+ */
+export function stitchChunks(chunks: Chunk[], pcm: Float32Array[], sampleRate: number, durSec: number): Float32Array {
+  const n = Math.round(durSec * sampleRate), out = new Float32Array(n);
+  const xf = Math.max(1, Math.round(XFADE_SEC * sampleRate));
+  chunks.forEach((c, k) => {
+    const x = pcm[k]!, s0 = Math.round(c.from * sampleRate), s1 = k === chunks.length - 1 ? n : Math.round(c.to * sampleRate);
+    const off = Math.round(c.renderStart * sampleRate); // output index of x[0]
+    for (let i = s0; i < s1; i++) {
+      const v = x[i - off] ?? 0;
+      // first xf samples of a later chunk fade in; the previous chunk's overlap was already added (fading out)
+      out[i] = k > 0 && i - s0 < xf ? out[i]! + v * ((i - s0) / xf) : v;
+    }
+    if (k < chunks.length - 1) {
+      for (let j = 0; j < xf && s1 + j < n; j++) out[s1 + j] = (x[s1 + j - off] ?? 0) * (1 - j / xf);
+    }
+  });
+  return out;
+}
diff --git a/src/align/render.ts b/src/align/render.ts
index 2feb12a..c54d4b8 100644
--- a/src/align/render.ts
+++ b/src/align/render.ts
@@ -14,12 +14,16 @@ const MELODIC_CHANNELS = [0, 1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15];
  * Render reference tracks to audio with `warp` applied (MIDI time -> audio time). Notes are placed on a fixed-tempo
  * tick grid so alphaSynth output time equals audio time. Peak-normalised to -1 dBFS; `durationSec` pads/trims.
  * Pure: no I/O. Notes landing before audio t=0 are clamped to 0 (dropped if they end before it).
+ * `startSec` renders a window: output sample 0 is audio time `startSec`; notes sounding at `startSec` are re-attacked
+ * there (callers render a pre-roll and discard it) and notes starting after the window are skipped.
+ * `normalize: false` skips the peak normalisation (e.g. for chunks normalised together later).
  */
 export function renderMidi(
   tracks: RefTrack[], warp: Warp, sf2: Uint8Array,
-  opts: { sampleRate?: number; mono?: boolean; durationSec?: number } = {},
+  opts: { sampleRate?: number; mono?: boolean; durationSec?: number; startSec?: number; normalize?: boolean } = {},
 ): Audio {
   const sampleRate = opts.sampleRate ?? RENDER_SR;
+  const shift = opts.startSec ?? 0, endSec = opts.durationSec ?? Infinity;
   const midi = new at.midi.MidiFile();
   midi.division = 960;
   midi.addEvent(new at.midi.TempoChangeEvent(0, 500000));
@@ -30,9 +34,9 @@ export function renderMidi(
     const ch = t.isDrums ? 9 : MELODIC_CHANNELS[next++ % MELODIC_CHANNELS.length]!;
     if (!seen.has(ch)) { seen.add(ch); midi.addEvent(new at.midi.ProgramChangeEvent(0, 0, ch, t.isDrums ? 0 : t.program)); }
     for (const n of t.notes) {
-      const offSec = warp.toAudio(n.offset);
-      if (offSec <= 0) continue;
-      const on = Math.round(Math.max(0, warp.toAudio(n.onset)) * TPS);
+      const offSec = warp.toAudio(n.offset) - shift, onSec = warp.toAudio(n.onset) - shift;
+      if (offSec <= 0 || onSec >= endSec) continue;
+      const on = Math.round(Math.max(0, onSec) * TPS);
       const off = Math.max(on + 1, Math.round(offSec * TPS));
       midi.addEvent(new at.midi.NoteOnEvent(0, on, ch, n.pitch, n.velocity));
       midi.addEvent(new at.midi.NoteOffEvent(0, off, ch, n.pitch, 0));
@@ -49,8 +53,13 @@ export function renderMidi(
     chans = [L.map((v, i) => (v + R[i]!) / 2)];
   }
   for (const c of chans) for (let i = 0; i < c.length; i++) if (!Number.isFinite(c[i]!)) throw new Error("renderMidi: non-finite samples from synth");
+  if (opts.normalize !== false) normalizePeak(chans);
+  return { sampleRate, channels: chans };
+}
+
+/** Scales in place so the peak is -1 dBFS (silence stays silent). */
+export function normalizePeak(chans: Float32Array[]): void {
   let peak = 0;
   for (const c of chans) for (let i = 0; i < c.length; i++) { const a = Math.abs(c[i]!); if (a > peak) peak = a; }
   if (peak > 0) { const g = PEAK / peak; for (const c of chans) for (let i = 0; i < c.length; i++) c[i]! *= g; }
-  return { sampleRate, channels: chans };
 }
diff --git a/src/align/server.ts b/src/align/server.ts
index 458a3f0..7211c34 100644
--- a/src/align/server.ts
+++ b/src/align/server.ts
@@ -8,8 +8,10 @@ import type { Role } from "../core/types.ts";
 import { audioOgg, loadSong, readAlignment, readManifest, writeAlignment, type AlignmentFile, type LoadedSong } from "../ref/manifest.ts";
 import { trackKeys } from "./roles.ts";
 import { makeWarp, stretches, type Anchor } from "./warp.ts";
-import { autoAlign, fitFromCues, recordingCues, type AutoResult, type RecordingCues } from "./auto.ts";
-import { renderMidi } from "./render.ts";
+import { fitFromCues, recordingCues, type AutoResult, type RecordingCues } from "./auto.ts";
+import { normalizePeak } from "./render.ts";
+import { JobWorker } from "./jobs-pool.ts";
+import { planChunks, stitchChunks } from "./preview.ts";
 import { peaks } from "./peaks.ts";
 import { stretchInfo } from "./tempo.ts";
 import type { FitReport } from "./fit.ts";
@@ -24,7 +26,11 @@ const MAX_BODY_BYTES = 1 << 20;
 const PEAKS_PER_SEC = 2000 / 60;
 const PEAKS_VERSION = 1;
 const RENDER_SR = 22050;
-const RENDER_CACHE = 6;
+const RENDER_CACHE = 24; // Opus previews are ~1.5 MB per 5 min
+/** Parallel render workers (each renders one time chunk of a preview). */
+const RENDER_WORKERS = Math.max(1, Math.min(4, (navigator.hardwareConcurrency ?? 2) - 1));
+// Preview transport: mono Opus. 48 kb/s at compression level 0 encodes 5 min in ~0.6 s to ~1.5 MB (vs 12 MB WAV).
+const OPUS_ARGS = ["-c:a", "libopus", "-b:a", "48k", "-compression_level", "0", "-f", "ogg", "pipe:1"];
 const SONG_CACHE = 2;
 
 export interface ServeOptions { root?: string; host?: string; port?: number; urlHost?: string; quiet?: boolean; sf2Path?: string }
@@ -34,11 +40,26 @@ interface SongState extends LoadedSong {
   roles: Record<string, Role | null>;
   cues: RecordingCues;
   peaks: { bucketSec: number; min: number[]; max: number[] };
-  auto?: AutoResult; // first automatic pass, reused while no alignment.json exists
+  auto?: Promise<AutoResult>; // first automatic pass, reused while no alignment.json exists
 }
 
 class HttpError extends Error { constructor(readonly status: number, msg: string) { super(msg); } }
 
+/** Mono float PCM -> Ogg Opus via ffmpeg (a separate process: the event loop stays free). */
+async function encodeOpus(pcm: Float32Array, sampleRate: number): Promise<Uint8Array<ArrayBuffer>> {
+  const p = Bun.spawn(["ffmpeg", "-nostdin", "-loglevel", "error", "-f", "f32le", "-ar", String(sampleRate), "-ac", "1", "-i", "pipe:0", ...OPUS_ARGS], {
+    stdin: new Uint8Array(pcm.buffer, pcm.byteOffset, pcm.byteLength), stdout: "pipe", stderr: "pipe",
+  });
+  const [out, err, code] = await Promise.all([new Response(p.stdout).arrayBuffer(), new Response(p.stderr).text(), p.exited]);
+  if (code !== 0) throw new Error(`ffmpeg (opus) failed (${code}): ${err.trim()}`);
+  return new Uint8Array(out);
+}
+
+/** Copies audio into SharedArrayBuffers so worker jobs can read it without a copy per job. */
+function shareAudio(a: LoadedSong["audio"]): LoadedSong["audio"] {
+  return { sampleRate: a.sampleRate, channels: a.channels.map((c) => { const x = new Float32Array(new SharedArrayBuffer(c.byteLength)); x.set(c); return x; }) };
+}
+
 /** Small insertion-ordered LRU. */
 class Lru<V> {
   private m = new Map<string, V>();
@@ -84,10 +105,15 @@ export async function startServer(opts: ServeOptions = {}): Promise<AlignServer>
   const host = opts.host ?? "127.0.0.1";
   const log = (msg: string) => { if (!opts.quiet) console.log(msg); };
   const token = Buffer.from(crypto.getRandomValues(new Uint8Array(18))).toString("base64url");
-  const sf2 = new Uint8Array(await Bun.file(opts.sf2Path ?? SF2_PATH).arrayBuffer());
+  const sf2Path = opts.sf2Path ?? SF2_PATH;
+  if (!(await Bun.file(sf2Path).exists())) throw new Error(`soundfont not found: ${sf2Path}`);
+  // Heavy work runs on worker threads so this event loop keeps serving while a render or auto-align is in progress.
+  const renderWorkers = Array.from({ length: RENDER_WORKERS }, () => new JobWorker());
+  const autoWorker = new JobWorker();
 
   const songs = new Lru<Promise<SongState>>(SONG_CACHE);
   const renders = new Lru<Uint8Array<ArrayBuffer>>(RENDER_CACHE);
+  const inflight = new Map<string, Promise<Uint8Array<ArrayBuffer>>>();
 
   async function entryExists(id: string): Promise<boolean> {
     return (await readManifest(root)).some((s) => s.id === id);
@@ -95,7 +121,8 @@ export async function startServer(opts: ServeOptions = {}): Promise<AlignServer>
 
   async function buildState(id: string): Promise<SongState> {
     const t0 = performance.now();
-    const s = await loadSong(id, root);
+    const loaded = await loadSong(id, root);
+    const s = { ...loaded, audio: shareAudio(loaded.audio) };
     const keys = trackKeys(s.smf.tracks);
     const roles = Object.fromEntries(keys.map((k) => [k, s.entry.roles[k] ?? null]));
     const cues = recordingCues(s.audio);
@@ -131,9 +158,9 @@ export async function startServer(opts: ServeOptions = {}): Promise<AlignServer>
   const stretchesOf = (s: SongState, anchors: Anchor[]) => stretchInfo(stretches(anchors, s.smf.durationSec), s.smf.tempoMap);
   const state = (s: SongState, anchors: Anchor[], fit = fitOf(s, anchors)) => ({ anchors, stretches: stretchesOf(s, anchors), fit });
 
-  function runAuto(s: SongState, manual: Anchor[]) {
+  async function runAuto(s: SongState, manual: Anchor[]): Promise<AutoResult> {
     const t0 = performance.now();
-    const r = autoAlign(s.audio, s.smf, s.roles, manual, sf2);
+    const r = await autoWorker.run<AutoResult>({ type: "auto", sf2Path, audio: s.audio, smf: s.smf, roles: s.roles, manual });
     log(`auto-aligned ${s.entry.id}: ${r.anchors.length} anchors (${manual.length} manual, ${r.chosen}) in ${((performance.now() - t0) / 1000).toFixed(1)} s`);
     return r;
   }
@@ -151,6 +178,25 @@ export async function startServer(opts: ServeOptions = {}): Promise<AlignServer>
     }
   }
 
+  /** Warped MIDI rendered in parallel time chunks on the render workers, stitched, normalised and encoded. */
+  async function renderPreview(s: SongState, anchors: Anchor[], format: "wav" | "opus", signal: AbortSignal): Promise<Uint8Array<ArrayBuffer>> {
+    const t0 = performance.now();
+    const warp = makeWarp(anchors);
+    const dur = Math.max(s.audioDur, Math.min(warp.toAudio(s.smf.durationSec) + 1, s.audioDur + 60));
+    const chunks = planChunks(dur, renderWorkers.length);
+    const pcm = await Promise.all(chunks.map((c) => {
+      const w = renderWorkers.reduce((a, b) => (b.load < a.load ? b : a));
+      return w.run<Float32Array>({ type: "render", sf2Path, tracks: s.smf.tracks, anchors, sampleRate: RENDER_SR, startSec: c.renderStart, durationSec: c.renderDur }, signal);
+    }));
+    const mono = stitchChunks(chunks, pcm, RENDER_SR, dur);
+    for (let i = 0; i < mono.length; i++) if (!Number.isFinite(mono[i]!)) throw new Error("render: non-finite samples from synth");
+    normalizePeak([mono]);
+    const t1 = performance.now();
+    const out = format === "wav" ? encodeWav({ sampleRate: RENDER_SR, channels: [mono] }) : await encodeOpus(mono, RENDER_SR);
+    log(`rendered ${s.entry.id} (${anchors.length} anchors, ${chunks.length} chunks) in ${((t1 - t0) / 1000).toFixed(2)} s + ${format} ${((performance.now() - t1) / 1000).toFixed(2)} s, ${(out.length / 1e6).toFixed(2)} MB`);
+    return out;
+  }
+
   async function api(req: Request, path: string): Promise<Response> {
     const method = req.method;
     if (path === "/api/songs" && method === "GET") {
@@ -172,12 +218,17 @@ export async function startServer(opts: ServeOptions = {}): Promise<AlignServer>
       const { file: saved, error } = await savedAlignment(id);
       let anchors: Anchor[], fit: FitReport | undefined;
       if (saved) anchors = saved.anchors;
-      else { s.auto ??= runAuto(s, []); anchors = s.auto.anchors; fit = s.auto.fit; }
+      else {
+        s.auto ??= runAuto(s, []);
+        s.auto.catch(() => { s.auto = undefined; });
+        const r = await s.auto;
+        anchors = r.anchors; fit = r.fit;
+      }
       return Response.json({
         id, title: s.entry.title, audioDur: s.audioDur, midiDur: s.smf.durationSec,
         tracks: trackKeys(s.smf.tracks).map((name) => ({ name, role: s.roles[name] ?? null })),
         recordingPeaks: s.peaks, ...state(s, anchors, fit), midiTempo: s.smf.tempoMap,
-        source: saved ? "saved" : "auto", savedAt: saved?.savedAt ?? null, ...(saved ? {} : { auto: autoInfo(s.auto!) }),
+        source: saved ? "saved" : "auto", savedAt: saved?.savedAt ?? null, ...(saved ? {} : { auto: autoInfo(await s.auto!) }),
         ...(error ? { warning: `${error}. Showing the automatic alignment instead; saving will overwrite the file.` } : {}),
       });
     }
@@ -186,21 +237,28 @@ export async function startServer(opts: ServeOptions = {}): Promise<AlignServer>
       return new Response(Bun.file(await audioOgg(e, root)), { headers: { "content-type": "audio/ogg", "cache-control": "private, max-age=3600" } });
     }
     if (route === "POST render") {
+      // ?format=wav: 16-bit PCM (tests, debugging). Default: Ogg Opus, ~8x smaller, for the page over the LAN.
+      const format = new URL(req.url).searchParams.get("format") === "wav" ? "wav" : "opus";
       const anchors = parseAnchors(await readJson(req));
       const s = await song(id);
       const hash = Bun.hash(JSON.stringify(anchors)).toString(16);
-      const key = `${id}:${hash}`;
-      let wav = renders.get(key);
-      const hit = wav !== undefined;
-      if (!wav) {
-        const t0 = performance.now();
-        const warp = makeWarp(anchors);
-        const dur = Math.max(s.audioDur, Math.min(warp.toAudio(s.smf.durationSec) + 1, s.audioDur + 60));
-        wav = encodeWav(renderMidi(s.smf.tracks, warp, sf2, { sampleRate: RENDER_SR, mono: true, durationSec: dur }));
-        renders.set(key, wav);
-        log(`rendered ${id} (${anchors.length} anchors) in ${((performance.now() - t0) / 1000).toFixed(1)} s`);
+      const key = `${id}:${format}:${hash}`;
+      let body = renders.get(key);
+      const hit = body !== undefined;
+      if (!body) {
+        const start = () => {
+          const p = renderPreview(s, anchors, format, req.signal).finally(() => { if (inflight.get(key) === p) inflight.delete(key); });
+          inflight.set(key, p);
+          return p;
+        };
+        try { body = await (inflight.get(key) ?? start()); } catch (e) {
+          if ((e as Error).name !== "AbortError") throw e;
+          if (req.signal.aborted) return new Response(null, { status: 499 }); // client gave up (superseded edit)
+          body = await start(); // a request we shared the render with was cancelled: render for this one
+        }
+        renders.set(key, body);
       }
-      return new Response(wav, { headers: { "content-type": "audio/wav", "x-anchors-hash": hash, "x-cache": hit ? "hit" : "miss" } });
+      return new Response(body, { headers: { "content-type": format === "wav" ? "audio/wav" : "audio/ogg", "x-anchors-hash": hash, "x-cache": hit ? "hit" : "miss" } });
     }
     if (route === "POST fit") {
       const anchors = parseAnchors(await readJson(req));
@@ -209,7 +267,7 @@ export async function startServer(opts: ServeOptions = {}): Promise<AlignServer>
     if (route === "POST auto") {
       const manual = parseAnchors(await readJson(req)).filter((a) => a.kind === "manual");
       const s = await song(id);
-      const r = runAuto(s, manual);
+      const r = await runAuto(s, manual);
       return Response.json({ ...state(s, r.anchors, r.fit), auto: autoInfo(r) });
     }
     if (route === "POST reset") return Response.json(state(await song(id), []));
@@ -276,6 +334,9 @@ export async function startServer(opts: ServeOptions = {}): Promise<AlignServer>
   const origin = `http://${opts.urlHost ?? (host === "0.0.0.0" || host === "::" ? "localhost" : host)}:${outer.port}`;
   return {
     url: `${origin}/?key=${token}`, origin, token, port: outer.port!,
-    async stop() { await outer.stop(true); await inner.stop(true); },
+    async stop() {
+      await outer.stop(true); await inner.stop(true);
+      for (const w of [...renderWorkers, autoWorker]) w.terminate();
+    },
   };
 }
diff --git a/tests/align/preview.test.ts b/tests/align/preview.test.ts
new file mode 100644
index 0000000..255a63a
--- /dev/null
+++ b/tests/align/preview.test.ts
@@ -0,0 +1,54 @@
+import { test, expect } from "bun:test";
+import { planChunks, stitchChunks, MIN_CHUNK_SEC, PREROLL_SEC } from "../../src/align/preview.ts";
+import { renderMidi, normalizePeak } from "../../src/align/render.ts";
+import { makeWarp } from "../../src/align/warp.ts";
+import type { RefNote, RefTrack } from "../../src/align/smf.ts";
+
+test("planChunks covers [0, dur) contiguously, with pre-roll after the first chunk", () => {
+  const c = planChunks(280, 4);
+  expect(c).toHaveLength(4);
+  expect(c[0]!.from).toBe(0);
+  expect(c[3]!.to).toBe(280);
+  for (let k = 1; k < 4; k++) {
+    expect(c[k]!.from).toBe(c[k - 1]!.to);
+    expect(c[k]!.renderStart).toBeCloseTo(c[k]!.from - PREROLL_SEC, 9);
+  }
+  expect(planChunks(MIN_CHUNK_SEC * 1.5, 4)).toHaveLength(1);
+  expect(planChunks(5, 4)).toEqual([{ renderStart: 0, renderDur: 5, from: 0, to: 5 }]);
+});
+
+test("stitchChunks reproduces a signal from overlapping slices of it", () => {
+  const sr = 1000, dur = 90, n = dur * sr;
+  const x = Float32Array.from({ length: n }, (_, i) => Math.sin(i / 37) * 0.5);
+  const chunks = planChunks(dur, 3);
+  const pcm = chunks.map((c) => x.slice(Math.round(c.renderStart * sr), Math.round((c.renderStart + c.renderDur) * sr)));
+  const y = stitchChunks(chunks, pcm, sr, dur);
+  expect(y.length).toBe(n);
+  let maxErr = 0;
+  for (let i = 0; i < n; i++) maxErr = Math.max(maxErr, Math.abs(y[i]! - x[i]!));
+  expect(maxErr).toBeLessThan(1e-6);
+});
+
+test("a chunked render matches the whole render (onsets in place, small seam error)", async () => {
+  const SF2 = new Uint8Array(await Bun.file("node_modules/@coderline/alphatab/dist/soundfont/sonivox.sf2").arrayBuffer());
+  const notes: RefNote[] = [];
+  for (let t = 0.25; t < 58; t += 0.5) notes.push({ onset: t, offset: t + 0.4, pitch: 52 + ((t * 2) % 12), velocity: 100, channel: 0 });
+  notes.push({ onset: 18, offset: 23, pitch: 40, velocity: 100, channel: 0 }); // sustains across the first seam (20 s)
+  const tracks: RefTrack[] = [{ index: 0, name: "g", program: 25, isDrums: false, notes }];
+  const warp = makeWarp([{ midiSec: 0, audioSec: 0.3, kind: "manual" }, { midiSec: 50, audioSec: 51, kind: "manual" }]);
+  const sr = 22050, dur = 60;
+  const whole = renderMidi(tracks, warp, SF2, { sampleRate: sr, mono: true, durationSec: dur }).channels[0]!;
+  const chunks = planChunks(dur, 3);
+  const pcm = chunks.map((c) => renderMidi(tracks, warp, SF2, { sampleRate: sr, mono: true, startSec: c.renderStart, durationSec: c.renderDur, normalize: false }).channels[0]!);
+  const y = stitchChunks(chunks, pcm, sr, dur);
+  normalizePeak([y]);
+  // energy per 50 ms frame agrees closely everywhere (identical away from seams)
+  const frame = Math.floor(sr / 20);
+  let worst = 0;
+  for (let f = 0; f + frame <= y.length; f += frame) {
+    let a = 0, b = 0;
+    for (let i = f; i < f + frame; i++) { a += whole[i]! ** 2; b += y[i]! ** 2; }
+    worst = Math.max(worst, Math.abs(Math.sqrt(a / frame) - Math.sqrt(b / frame)));
+  }
+  expect(worst).toBeLessThan(0.05);
+}, 60_000);
diff --git a/tests/align/server.test.ts b/tests/align/server.test.ts
index 3f7f93a..43a1b9b 100644
--- a/tests/align/server.test.ts
+++ b/tests/align/server.test.ts
@@ -151,10 +151,23 @@ describe("API", () => {
     expect(String.fromCharCode(...b.subarray(0, 4))).toBe("OggS");
     expect(await Bun.file(join(root, "fix", "audio.ogg")).exists()).toBe(true);
   }, BIG);
-  test("render: RIFF WAV, mono 22.05 kHz, cached by anchors", async () => {
+  test("render: Ogg Opus by default (small), cached by anchors", async () => {
     const body = json({ anchors: first.anchors });
     const r = await api("/api/song/fix/render", body);
     expect(r.status).toBe(200);
+    expect(r.headers.get("content-type")).toBe("audio/ogg");
+    const b = new Uint8Array(await r.arrayBuffer());
+    expect(String.fromCharCode(...b.subarray(0, 4))).toBe("OggS");
+    expect(new TextDecoder().decode(b.subarray(28, 36))).toBe("OpusHead");
+    expect(b.length).toBeLessThan(first.audioDur * 22050 * 2 / 4); // far below 16-bit PCM
+    const again = await api("/api/song/fix/render", body);
+    expect(again.headers.get("x-cache")).toBe("hit");
+    expect(again.headers.get("x-anchors-hash")).toBe(r.headers.get("x-anchors-hash"));
+  }, BIG);
+  test("render ?format=wav: RIFF WAV, mono 22.05 kHz, cached by anchors", async () => {
+    const body = json({ anchors: first.anchors });
+    const r = await api("/api/song/fix/render?format=wav", body);
+    expect(r.status).toBe(200);
     expect(r.headers.get("content-type")).toBe("audio/wav");
     const b = new Uint8Array(await r.arrayBuffer());
     const dv = new DataView(b.buffer);
@@ -162,7 +175,7 @@ describe("API", () => {
     expect(dv.getUint16(22, true)).toBe(1);
     expect(dv.getUint32(24, true)).toBe(22050);
     expect(b.length).toBeGreaterThan(first.audioDur * 22050 * 2);
-    const again = await api("/api/song/fix/render", body);
+    const again = await api("/api/song/fix/render?format=wav", body);
     expect(again.headers.get("x-cache")).toBe("hit");
     expect(again.headers.get("x-anchors-hash")).toBe(r.headers.get("x-anchors-hash"));
   }, BIG);