diff --git a/src/servers/sidecar/music/index.ts b/src/servers/sidecar/music/index.ts
index b93d1f8e..f29bbb4d 100644
--- a/src/servers/sidecar/music/index.ts
+++ b/src/servers/sidecar/music/index.ts
@@ -1,6 +1,14 @@
 import type { SidecarCommand, SidecarEvent } from '../protocol';
 import { createSidecarConnector } from '../connect';
 import { streamAudioFile } from './stream-audio';
+import {
+  buildMusicIndex,
+  getIndexStatus,
+  getManifest,
+  albumVersion,
+  metaFilePath,
+  coverFilePath,
+} from './indexer';
 
 // The music sidecar (officer-music). Same philosophy as the other officer-* sidecars: a singleton
 // process that registers with the API server. It OWNS an audio-streaming HTTP server (all path
@@ -28,12 +36,47 @@ const server = Bun.serve({
   hostname: '127.0.0.1',
   async fetch(req) {
     const url = new URL(req.url);
+    const json = (data: unknown, init?: ResponseInit) =>
+      new Response(JSON.stringify(data), { ...init, headers: { 'Content-Type': 'application/json', ...init?.headers } });
+
     if (url.pathname === '/health') return new Response('ok');
+
+    // ── Streaming ──
     if (url.pathname === '/stream') {
       const path = url.searchParams.get('path');
       if (!path) return new Response('path is required', { status: 400 });
       return streamAudioFile(path, req.headers.get('range'));
     }
+
+    // ── Index build ──
+    if (url.pathname === '/reindex') {
+      if (req.method !== 'POST') return new Response('Method not allowed', { status: 405 });
+      void buildMusicIndex(); // fire-and-forget; sets running=true synchronously before the first await
+      return json(getIndexStatus());
+    }
+    if (url.pathname === '/reindex/status') return json(getIndexStatus());
+
+    // ── Sync surface ──
+    if (url.pathname === '/manifest') return json(await getManifest());
+
+    if (url.pathname === '/meta' || url.pathname === '/cover') {
+      const rel = url.searchParams.get('path');
+      if (rel === null) return new Response('path is required', { status: 400 });
+      const isMeta = url.pathname === '/meta';
+      const file = isMeta ? metaFilePath(rel) : coverFilePath(rel);
+      if (!file) return new Response('Invalid path', { status: 400 });
+      if (!(await Bun.file(file).exists())) return new Response('Not found', { status: 404 });
+
+      const v = await albumVersion(rel);
+      if (v && req.headers.get('if-none-match') === v) return new Response(null, { status: 304 });
+      return new Response(Bun.file(file), {
+        headers: {
+          'Content-Type': isMeta ? 'application/json' : 'image/jpeg',
+          ...(v ? { ETag: v } : {}),
+        },
+      });
+    }
+
     return new Response('Not found', { status: 404 });
   },
 });
diff --git a/src/servers/sidecar/music/indexer.ts b/src/servers/sidecar/music/indexer.ts
new file mode 100644
index 00000000..9362b519
--- /dev/null
+++ b/src/servers/sidecar/music/indexer.ts
@@ -0,0 +1,307 @@
+import { readdir, stat, mkdir, writeFile, readFile, rm } from 'node:fs/promises';
+import { existsSync } from 'node:fs';
+import { join, relative } from 'node:path';
+import { homedir } from 'node:os';
+
+// Server-side music library indexer — the platform counterpart of the app's music-index.ts. Walks the
+// library and builds a cache tree that MIRRORS the library layout (so it syncs cleanly), using ffprobe
+// for tags + duration and ffmpeg to compress covers for the phone. Output, per album folder at <rel>
+// (relative to the Music root):
+//
+//   DATA_PATH/music/cache/<rel>/meta.json   { path, cover?, tracks: [{ file, title, artist, albumArtist,
+//                                             album, track, year, durationSec }] }   (phone IndexMeta schema)
+//   DATA_PATH/music/cache/<rel>/cover.jpg    compressed (<=600px, jpeg q5)
+//
+// A manifest (cache root) carries a per-album version `v` = hash of the album's source signature (track
+// name+size+mtime, cover size+mtime). It drives BOTH incremental build (skip unchanged albums) and the
+// phone's resync diff (fetch only changed `v`s).
+
+const HOME = process.env.HOME_DIR ?? homedir();
+const MUSIC_ROOT = join(HOME, 'Music');
+const CACHE_ROOT = join(process.env.DATA_PATH ?? join(process.cwd(), 'data'), 'music', 'cache');
+const MANIFEST_PATH = join(CACHE_ROOT, 'manifest.json');
+
+const AUDIO_EXT = new Set(['mp3', 'flac', 'm4a', 'aac', 'ogg', 'opus', 'wav', 'wma', 'mp4']);
+const COVER_FILES = ['folder.jpg', 'cover.jpg', 'folder.png', 'cover.png'];
+const TRACK_CONCURRENCY = 6;
+const COVER_MAX_PX = 600;
+
+// ── Types (meta.json matches the app's IndexMeta/IndexTrack) ──
+
+export type IndexTrack = {
+  file: string;
+  title?: string;
+  artist?: string;
+  albumArtist?: string;
+  album?: string;
+  track?: string;
+  year?: string;
+  durationSec?: number;
+};
+export type IndexMeta = { path: string; cover?: string; tracks: IndexTrack[] };
+
+type ManifestEntry = { v: string; cover: boolean; tracks: number };
+type Manifest = { version: number; generatedAt: number; albums: Record<string, ManifestEntry> };
+
+export type IndexStatus = {
+  running: boolean;
+  startedAt: number | null;
+  finishedAt: number | null;
+  foldersScanned: number;
+  albumsBuilt: number; // rebuilt this run (changed)
+  albumsSkipped: number; // unchanged (v matched)
+  tracksIndexed: number;
+  coversSaved: number;
+  currentPath: string;
+  error: string | null;
+};
+
+// ── In-memory manifest (loaded from disk on first access, updated on build) ──
+
+let manifest: Manifest = { version: 1, generatedAt: 0, albums: {} };
+let manifestLoaded = false;
+
+async function loadManifest(): Promise<Manifest> {
+  if (manifestLoaded) return manifest;
+  try {
+    manifest = JSON.parse(await readFile(MANIFEST_PATH, 'utf8')) as Manifest;
+  } catch {
+    manifest = { version: 1, generatedAt: 0, albums: {} };
+  }
+  manifestLoaded = true;
+  return manifest;
+}
+
+export async function getManifest(): Promise<Manifest> {
+  return loadManifest();
+}
+
+/** The version stamp for an album, for ETag/diff use (null if unknown). */
+export async function albumVersion(rel: string): Promise<string | null> {
+  const m = await loadManifest();
+  return m.albums[rel]?.v ?? null;
+}
+
+// ── Status ──
+
+const status: IndexStatus = {
+  running: false,
+  startedAt: null,
+  finishedAt: null,
+  foldersScanned: 0,
+  albumsBuilt: 0,
+  albumsSkipped: 0,
+  tracksIndexed: 0,
+  coversSaved: 0,
+  currentPath: '',
+  error: null,
+};
+
+export function getIndexStatus(): IndexStatus {
+  return { ...status };
+}
+
+// ── Helpers ──
+
+function isAudio(name: string): boolean {
+  const dot = name.lastIndexOf('.');
+  return dot >= 0 && AUDIO_EXT.has(name.slice(dot + 1).toLowerCase());
+}
+
+/** Bounded-concurrency map preserving order. */
+async function mapPool<T, R>(items: T[], limit: number, fn: (item: T) => Promise<R>): Promise<R[]> {
+  const out: R[] = new Array(items.length);
+  let next = 0;
+  const worker = async () => {
+    while (true) {
+      const i = next++;
+      if (i >= items.length) break;
+      out[i] = await fn(items[i]!);
+    }
+  };
+  await Promise.all(Array.from({ length: Math.min(limit, items.length) || 0 }, worker));
+  return out;
+}
+
+async function ffprobeTrack(absPath: string, file: string): Promise<IndexTrack> {
+  try {
+    const proc = Bun.spawn(
+      [
+        'ffprobe',
+        '-v',
+        'error',
+        '-print_format',
+        'json',
+        '-show_entries',
+        'format=duration:format_tags=title,artist,album,album_artist,track,date',
+        absPath,
+      ],
+      { stdout: 'pipe', stderr: 'ignore' },
+    );
+    const out = await new Response(proc.stdout).text();
+    await proc.exited;
+    const data = JSON.parse(out) as { format?: { duration?: string; tags?: Record<string, string> } };
+    const fmt = data.format ?? {};
+    // Tag keys vary in case across containers — normalize to lowercase.
+    const tags: Record<string, string> = {};
+    for (const [k, val] of Object.entries(fmt.tags ?? {})) tags[k.toLowerCase()] = val;
+    const durNum = fmt.duration ? Math.round(parseFloat(fmt.duration)) : undefined;
+    const yearMatch = tags.date?.match(/\d{4}/);
+    return {
+      file,
+      title: tags.title || undefined,
+      artist: tags.artist || undefined,
+      albumArtist: tags.album_artist || undefined,
+      album: tags.album || undefined,
+      track: tags.track || undefined,
+      year: yearMatch ? yearMatch[0] : undefined,
+      durationSec: Number.isFinite(durNum) ? durNum : undefined,
+    };
+  } catch {
+    return { file };
+  }
+}
+
+/** Compress a cover to <=COVER_MAX_PX (no upscaling), jpeg q5. Returns true on success. */
+async function compressCover(srcAbs: string, destAbs: string): Promise<boolean> {
+  try {
+    const proc = Bun.spawn(
+      [
+        'ffmpeg',
+        '-y',
+        '-i',
+        srcAbs,
+        '-vf',
+        `scale='min(iw,${COVER_MAX_PX})':'min(ih,${COVER_MAX_PX})':force_original_aspect_ratio=decrease`,
+        '-q:v',
+        '5',
+        destAbs,
+      ],
+      { stdout: 'ignore', stderr: 'ignore' },
+    );
+    await proc.exited;
+    return proc.exitCode === 0 && existsSync(destAbs);
+  } catch {
+    return false;
+  }
+}
+
+// ── Build ──
+
+export async function buildMusicIndex(): Promise<IndexStatus> {
+  if (status.running) return getIndexStatus();
+  Object.assign(status, {
+    running: true,
+    startedAt: Date.now(),
+    finishedAt: null,
+    foldersScanned: 0,
+    albumsBuilt: 0,
+    albumsSkipped: 0,
+    tracksIndexed: 0,
+    coversSaved: 0,
+    currentPath: '',
+    error: null,
+  });
+
+  const prev = await loadManifest();
+  const next: Manifest = { version: 1, generatedAt: status.startedAt!, albums: {} };
+
+  try {
+    await mkdir(CACHE_ROOT, { recursive: true });
+    await walk(MUSIC_ROOT, prev, next);
+
+    // Prune cache dirs for albums that vanished from the library.
+    for (const rel of Object.keys(prev.albums)) {
+      if (!next.albums[rel]) {
+        await rm(join(CACHE_ROOT, rel), { recursive: true, force: true }).catch(() => {});
+      }
+    }
+
+    await writeFile(MANIFEST_PATH, JSON.stringify(next));
+    manifest = next;
+    manifestLoaded = true;
+  } catch (err) {
+    status.error = err instanceof Error ? err.message : String(err);
+  } finally {
+    status.running = false;
+    status.finishedAt = Date.now();
+    status.currentPath = '';
+  }
+  return getIndexStatus();
+}
+
+async function walk(dirAbs: string, prev: Manifest, next: Manifest): Promise<void> {
+  status.currentPath = relative(MUSIC_ROOT, dirAbs) || '.';
+  status.foldersScanned += 1;
+
+  let entries: import('node:fs').Dirent[];
+  try {
+    entries = await readdir(dirAbs, { withFileTypes: true });
+  } catch {
+    return;
+  }
+
+  const files = entries.filter((e) => e.isFile());
+  const subdirs = entries.filter((e) => e.isDirectory());
+  const audio = files.filter((e) => isAudio(e.name)).map((e) => e.name);
+  const coverName = COVER_FILES.find((c) => files.some((e) => e.name === c));
+  const rel = relative(MUSIC_ROOT, dirAbs); // '' at root
+
+  if (audio.length || coverName) {
+    // Source signature → version.
+    const sigParts: string[] = [];
+    for (const name of [...audio].sort()) {
+      const st = await stat(join(dirAbs, name)).catch(() => null);
+      if (st) sigParts.push(`${name}:${st.size}:${Math.round(st.mtimeMs)}`);
+    }
+    if (coverName) {
+      const st = await stat(join(dirAbs, coverName)).catch(() => null);
+      if (st) sigParts.push(`cover:${coverName}:${st.size}:${Math.round(st.mtimeMs)}`);
+    }
+    const v = Bun.hash(sigParts.join('|')).toString(16);
+    const cacheDir = join(CACHE_ROOT, rel);
+    const metaExists = existsSync(join(cacheDir, 'meta.json'));
+
+    if (prev.albums[rel]?.v === v && metaExists) {
+      status.albumsSkipped += 1;
+    } else {
+      await mkdir(cacheDir, { recursive: true });
+
+      if (coverName) {
+        const ok = await compressCover(join(dirAbs, coverName), join(cacheDir, 'cover.jpg'));
+        if (ok) status.coversSaved += 1;
+      }
+
+      if (audio.length) {
+        const tracks = await mapPool(audio, TRACK_CONCURRENCY, async (name) => {
+          const t = await ffprobeTrack(join(dirAbs, name), name);
+          status.tracksIndexed += 1;
+          return t;
+        });
+        const meta: IndexMeta = { path: rel, cover: coverName ? 'cover.jpg' : undefined, tracks };
+        await writeFile(join(cacheDir, 'meta.json'), JSON.stringify(meta));
+      }
+      status.albumsBuilt += 1;
+    }
+
+    next.albums[rel] = { v, cover: !!coverName, tracks: audio.length };
+  }
+
+  for (const d of subdirs) await walk(join(dirAbs, d.name), prev, next);
+}
+
+// ── Serving helpers (path-safe within CACHE_ROOT) ──
+
+function resolveCachePath(rel: string, file: string): string | null {
+  const clean = rel.replace(/^\/+/, '');
+  const abs = join(CACHE_ROOT, clean, file);
+  if (abs !== CACHE_ROOT && !abs.startsWith(CACHE_ROOT + '/')) return null;
+  return abs;
+}
+
+export function metaFilePath(rel: string): string | null {
+  return resolveCachePath(rel, 'meta.json');
+}
+export function coverFilePath(rel: string): string | null {
+  return resolveCachePath(rel, 'cover.jpg');
+}