diff --git a/CHANGELOG.md b/CHANGELOG.md
index f6e38b5e..db31b3d4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,7 +3,7 @@
 ## Unreleased
 
 ### Added
-- Stats Sync Without the Launcher: The stats sync engine now runs inside the app. The sync window no longer needs bun or the `subminer` command-line launcher installed, and a remote machine only needs SubMiner itself — the app answers `SubMiner --sync-cli sync ...` headless (works over SSH with no display), and remote lookup finds the app binary or the launcher automatically.
+- Stats Sync Without the Launcher: The stats sync engine now runs only inside the app. The sync window and the `subminer sync` command both delegate to `SubMiner --sync-cli` (headless, works over SSH with no display), so neither machine needs bun or the command-line launcher — a remote machine only needs SubMiner itself, found automatically as the app binary or via the launcher proxy.
 - Stats Sync With Windows Remotes: Sync now detects the remote shell (POSIX, cmd, or PowerShell) and manages remote temp files through SubMiner itself (`sync --make-temp`/`--remove-temp`) instead of `mktemp`/`rm`, so a Windows machine with the built-in OpenSSH Server works as a sync remote; SubMiner is found in its default Windows install location automatically.
 
 ## v0.18.0 (2026-07-10)
diff --git a/docs-site/changelog.md b/docs-site/changelog.md
index 49aa06af..41d288fd 100644
--- a/docs-site/changelog.md
+++ b/docs-site/changelog.md
@@ -3,7 +3,7 @@
 ## Unreleased
 
 **Added**
-- Stats Sync Without the Launcher: The stats sync engine now runs inside the app. The sync window no longer needs bun or the `subminer` command-line launcher installed, and a remote machine only needs SubMiner itself — the app answers `SubMiner --sync-cli sync ...` headless (works over SSH with no display), and remote lookup finds the app binary or the launcher automatically.
+- Stats Sync Without the Launcher: The stats sync engine now runs only inside the app. The sync window and the `subminer sync` command both delegate to `SubMiner --sync-cli` (headless, works over SSH with no display), so neither machine needs bun or the command-line launcher — a remote machine only needs SubMiner itself, found automatically as the app binary or via the launcher proxy.
 - Stats Sync With Windows Remotes: Sync now detects the remote shell (POSIX, cmd, or PowerShell) and manages remote temp files through SubMiner itself (`sync --make-temp`/`--remove-temp`) instead of `mktemp`/`rm`, so a Windows machine with the built-in OpenSSH Server works as a sync remote; SubMiner is found in its default Windows install location automatically.
 
 ## v0.18.0 (2026-07-10)
diff --git a/docs-site/launcher-script.md b/docs-site/launcher-script.md
index 35a1c635..d20d4ec2 100644
--- a/docs-site/launcher-script.md
+++ b/docs-site/launcher-script.md
@@ -81,7 +81,7 @@ Series whose directories are not currently accessible (e.g. an unmounted network
 
 ## Sync Between Machines
 
-`subminer sync <host>` merges immersion stats and watch history between two machines over SSH, so both end up with the union of sessions, lifetime totals, vocabulary counts, daily/monthly charts, and `--history` entries. `<host>` is anything `ssh` accepts (`user@hostname` or an ssh config alias); SubMiner must be installed on both machines at the same version. The command-line launcher is optional on both sides: the app itself answers sync commands (`SubMiner --sync-cli sync ...`), the sync window runs the engine in-app, and the remote side is found automatically whether it has the launcher or just the app.
+`subminer sync <host>` merges immersion stats and watch history between two machines over SSH, so both end up with the union of sessions, lifetime totals, vocabulary counts, daily/monthly charts, and `--history` entries. `<host>` is anything `ssh` accepts (`user@hostname` or an ssh config alias); SubMiner must be installed on both machines at the same version. The sync engine runs only inside the app (`SubMiner --sync-cli sync ...`): the sync window spawns it that way, `subminer sync` is a thin proxy that forwards to the installed app, and the remote side is found automatically whether it has the launcher or just the app — so the command-line launcher is optional everywhere.
 
 ```bash
 subminer sync macbook                  # two-way sync with the host "macbook"
diff --git a/launcher/commands/sync-command.test.ts b/launcher/commands/sync-command.test.ts
index df307877..f2a2f2f4 100644
--- a/launcher/commands/sync-command.test.ts
+++ b/launcher/commands/sync-command.test.ts
@@ -1,14 +1,13 @@
 import test from 'node:test';
 import assert from 'node:assert/strict';
-import fs from 'node:fs';
-import os from 'node:os';
-import path from 'node:path';
 import type { Args } from '../types.js';
-import { createEmptyMergeSummary } from '../sync/sync-shared.js';
 import type { LauncherCommandContext } from './context.js';
-import { ensureTrackerQuiescent, runSyncCommand, type SyncCommandDeps } from './sync-command.js';
+import { buildSyncCliArgv, runSyncCommand, type SyncCommandDeps } from './sync-command.js';
 
-function makeContext(overrides: Partial<Args>): LauncherCommandContext {
+function makeContext(
+  overrides: Partial<Args>,
+  appPath: string | null = '/opt/SubMiner/subminer-app',
+): LauncherCommandContext {
   return {
     args: {
       sync: true,
@@ -19,6 +18,8 @@ function makeContext(overrides: Partial<Args>): LauncherCommandContext {
       syncRemoteCmd: '',
       syncDbPath: '',
       syncForce: false,
+      syncJson: false,
+      syncCheck: false,
       syncMakeTemp: false,
       syncRemoveTempPath: '',
       logLevel: 'warn',
@@ -28,522 +29,120 @@ function makeContext(overrides: Partial<Args>): LauncherCommandContext {
     scriptName: 'subminer',
     mpvSocketPath: '',
     pluginRuntimeConfig: {},
-    appPath: null,
+    appPath,
     launcherJellyfinConfig: {},
-    processAdapter: process,
+    processAdapter: { platform: () => 'linux' },
   } as unknown as LauncherCommandContext;
 }
 
-function ok(stdout = ''): { status: number; stdout: string; stderr: string } {
-  return { status: 0, stdout, stderr: '' };
-}
-
-// Every host-sync test must stub recordHostSyncResult: the default writes the
-// real sync-hosts.json in the user's config directory.
-const noRecord: Pick<SyncCommandDeps, 'recordHostSyncResult'> = {
-  recordHostSyncResult: () => {},
-};
-
-test('ensureTrackerQuiescent ignores stale sockets but rejects live sockets', async () => {
-  const context = makeContext({ syncDbPath: '/tmp/local.sqlite' });
-  context.mpvSocketPath = '/tmp/subminer-socket';
-  let socketConnectable = false;
-  const deps: Partial<SyncCommandDeps> = {
-    realpathSync: (() => '/tracker.sqlite') as unknown as typeof fs.realpathSync,
-    findLiveStatsDaemonPid: () => null,
-    canConnectUnixSocket: async () => socketConnectable,
-    fail: (message: string): never => {
-      throw new Error(message);
-    },
-  };
-
-  await ensureTrackerQuiescent(context, '/tmp/local.sqlite', deps);
-
-  socketConnectable = true;
-  await assert.rejects(
-    async () => ensureTrackerQuiescent(context, '/tmp/local.sqlite', deps),
-    /mpv\/SubMiner session appears to be running/,
-  );
-});
-
-test('runSyncCommand dispatches snapshot, merge, host, and missing-target modes', async () => {
-  const calls: string[] = [];
-  const deps: Partial<SyncCommandDeps> = {
-    ...noRecord,
-    createDbSnapshot: (dbPath: string, outPath: string) => {
-      calls.push(`snapshot:${dbPath}->${outPath}`);
-    },
-    mergeSnapshotIntoDb: (dbPath: string, snapshotPath: string) => {
-      calls.push(`merge:${dbPath}<-${snapshotPath}`);
-      return createEmptyMergeSummary();
-    },
-    formatMergeSummary: () => 'summary',
-    ensureTrackerQuiescent: async () => {
-      calls.push('quiescent');
-    },
-    assertSafeSshHost: (host: string) => {
-      calls.push(`host:${host}`);
-    },
-    resolveRemoteSubminerCommand: () => 'subminer',
-    runSsh: (_host: string, command: string) => {
-      calls.push(`ssh:${command}`);
-      return command.includes(' sync --make-temp') ? ok('/tmp/subminer-sync-remote\n') : ok();
-    },
-    runScp: (from: string, to: string) => {
-      calls.push(`scp:${from}->${to}`);
-    },
-    fail: (message: string): never => {
-      throw new Error(message);
-    },
-  };
-
-  assert.equal(
-    await runSyncCommand(
-      makeContext({ syncDbPath: '/tmp/local.sqlite', syncSnapshotPath: '/tmp/out.sqlite' }),
-      deps,
-    ),
-    true,
-  );
-  assert.ok(calls.includes('snapshot:/tmp/local.sqlite->/tmp/out.sqlite'));
-
-  await runSyncCommand(
-    makeContext({ syncDbPath: '/tmp/local.sqlite', syncMergePath: '/tmp/in.sqlite' }),
-    deps,
-  );
-  assert.ok(calls.includes('quiescent'));
-  assert.ok(calls.includes('merge:/tmp/local.sqlite<-/tmp/in.sqlite'));
-
-  await runSyncCommand(
-    makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box' }),
-    deps,
-  );
-  assert.ok(calls.includes('host:media-box'));
-
-  await assert.rejects(
-    () => runSyncCommand(makeContext({ syncDbPath: '/tmp/local.sqlite' }), deps),
-    /sync requires a host, --snapshot <file>, or --merge <file>/,
-  );
-});
-
-test('runHostSync keeps tracker quiescent through local and remote merge and cleans up after failure', async () => {
-  const calls: string[] = [];
-  let localTmpDir = '';
-  const deps: Partial<SyncCommandDeps> = {
-    ...noRecord,
-    createDbSnapshot: (_dbPath: string, outPath: string) => {
-      calls.push(`snapshot:${outPath}`);
-      fs.writeFileSync(outPath, 'snapshot');
-    },
-    mergeSnapshotIntoDb: () => {
-      calls.push('local-merge');
-      return createEmptyMergeSummary();
-    },
-    formatMergeSummary: () => 'summary',
-    ensureTrackerQuiescent: async () => {
-      calls.push('quiescent');
-    },
-    assertSafeSshHost: () => {},
-    resolveRemoteSubminerCommand: () => 'subminer',
-    mkdtempSync: ((prefix: string) => {
-      localTmpDir = fs.mkdtempSync(path.join(os.tmpdir(), path.basename(prefix)));
-      return localTmpDir;
-    }) as typeof fs.mkdtempSync,
-    runSsh: (_host: string, command: string) => {
-      calls.push(`ssh:${command}`);
-      if (command.includes(' sync --make-temp')) return ok('/tmp/subminer-sync-remote\n');
-      if (command.includes(' sync --snapshot ')) return ok();
-      if (command.includes(' sync --merge ')) {
-        return { status: 9, stdout: 'remote output', stderr: 'remote merge exploded' };
-      }
-      return ok();
-    },
-    runScp: (from: string, to: string) => {
-      calls.push(`scp:${from}->${to}`);
-      if (!to.includes(':')) fs.writeFileSync(to, 'pulled');
-    },
-  };
-
-  await assert.rejects(
-    () =>
-      runSyncCommand(makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box' }), deps),
-    /Remote merge failed on media-box[\s\S]*remote merge exploded/,
-  );
-  assert.equal(calls.filter((call) => call === 'quiescent').length, 3);
-  assert.ok(calls.indexOf('quiescent') < calls.findIndex((call) => call.startsWith('snapshot:')));
-  assert.ok(calls.includes('local-merge'));
-  assert.ok(calls.some((call) => call.includes(' sync --remove-temp ')));
-  assert.equal(fs.existsSync(localTmpDir), false);
-});
-
-test('runHostSync includes remote snapshot stderr in failures', async () => {
-  const deps: Partial<SyncCommandDeps> = {
-    ...noRecord,
-    createDbSnapshot: (_dbPath: string, outPath: string) => {
-      fs.writeFileSync(outPath, 'snapshot');
-    },
-    ensureTrackerQuiescent: async () => {},
-    assertSafeSshHost: () => {},
-    resolveRemoteSubminerCommand: () => 'subminer',
-    runSsh: (_host: string, command: string) => {
-      if (command.includes(' sync --make-temp')) return ok('/tmp/subminer-sync-remote\n');
-      if (command.includes(' sync --snapshot ')) {
-        return { status: 5, stdout: '', stderr: 'snapshot permission denied' };
-      }
-      return ok();
-    },
-    runScp: () => {},
-  };
-
-  await assert.rejects(
-    () =>
-      runSyncCommand(makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box' }), deps),
-    /Remote snapshot failed on media-box[\s\S]*snapshot permission denied/,
-  );
-});
-
-function makeDirectionDeps(calls: string[]): Partial<SyncCommandDeps> {
+function makeArgs(overrides: Partial<Parameters<typeof buildSyncCliArgv>[0]>) {
   return {
-    ...noRecord,
-    createDbSnapshot: (_dbPath: string, outPath: string) => {
-      calls.push(`snapshot:${outPath}`);
-      fs.writeFileSync(outPath, 'snapshot');
-    },
-    mergeSnapshotIntoDb: () => {
-      calls.push('local-merge');
-      return createEmptyMergeSummary();
-    },
-    formatMergeSummary: () => 'summary',
-    ensureTrackerQuiescent: async () => {
-      calls.push('quiescent');
-    },
-    assertSafeSshHost: () => {},
-    resolveRemoteSubminerCommand: () => 'subminer',
-    runSsh: (_host: string, command: string) => {
-      calls.push(`ssh:${command}`);
-      if (command.includes(' sync --make-temp')) return ok('/tmp/subminer-sync-remote\n');
-      return ok();
-    },
-    runScp: (from: string, to: string) => {
-      calls.push(`scp:${from}->${to}`);
-      if (!to.includes(':')) fs.writeFileSync(to, 'pulled');
-    },
+    syncHost: '',
+    syncSnapshotPath: '',
+    syncMergePath: '',
+    syncDirection: 'both' as const,
+    syncRemoteCmd: '',
+    syncDbPath: '',
+    syncForce: false,
+    syncJson: false,
+    syncCheck: false,
+    syncMakeTemp: false,
+    syncRemoveTempPath: '',
+    logLevel: 'warn' as const,
+    ...overrides,
   };
 }
 
-test('runHostSync push only snapshots locally and merges remotely', async () => {
-  const calls: string[] = [];
-
-  await runSyncCommand(
-    makeContext({
-      syncDbPath: '/tmp/local.sqlite',
-      syncHost: 'media-box',
-      syncDirection: 'push',
-    }),
-    makeDirectionDeps(calls),
-  );
-
-  assert.ok(calls.some((call) => call.startsWith('snapshot:')));
-  assert.ok(calls.some((call) => call.includes(' sync --merge ')));
-  assert.ok(calls.some((call) => call.startsWith('scp:') && call.includes('->media-box:')));
-  assert.ok(!calls.some((call) => call.includes(' sync --snapshot ')));
-  assert.ok(!calls.includes('local-merge'));
-});
-
-test('runHostSync pull only snapshots remotely and merges locally', async () => {
-  const calls: string[] = [];
-
-  await runSyncCommand(
-    makeContext({
-      syncDbPath: '/tmp/local.sqlite',
-      syncHost: 'media-box',
-      syncDirection: 'pull',
-    }),
-    makeDirectionDeps(calls),
-  );
-
-  assert.ok(calls.some((call) => call.includes(' sync --snapshot ')));
-  assert.ok(calls.some((call) => call.startsWith('scp:media-box:')));
-  assert.ok(calls.includes('local-merge'));
-  assert.ok(!calls.some((call) => call.startsWith('snapshot:')));
-  assert.ok(!calls.some((call) => call.includes(' sync --merge ')));
-});
-
-test('runSyncCommand --json emits NDJSON progress events for a two-way host sync', async () => {
-  const lines: string[] = [];
+test('runSyncCommand proxies sync argv to the app in --sync-cli mode', async () => {
+  const spawned: Array<{ appPath: string; appArgs: string[] }> = [];
   const deps: Partial<SyncCommandDeps> = {
-    ...makeDirectionDeps([]),
-    consoleLog: (line: string) => {
-      lines.push(line);
-    },
-    writeStdout: ((chunk: string) => {
-      lines.push(`stdout:${chunk}`);
-      return true;
-    }) as unknown as typeof process.stdout.write,
-    runSsh: (_host: string, command: string) => {
-      if (command.includes(' sync --make-temp')) return ok('/tmp/subminer-sync-remote\n');
-      if (command.includes(' sync --merge ')) return ok('remote summary text\n');
-      return ok();
-    },
-    recordHostSyncResult: () => {},
-  };
-
-  await runSyncCommand(
-    makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box', syncJson: true }),
-    deps,
-  );
-
-  const events = lines.map((line) => JSON.parse(line));
-  const types = events.map((event) => event.type);
-  assert.ok(types.includes('stage'));
-  assert.ok(events.some((event) => event.type === 'stage' && event.stage === 'snapshot-local'));
-  assert.ok(events.some((event) => event.type === 'merge-summary' && event.target === 'local'));
-  assert.ok(
-    events.some(
-      (event) => event.type === 'remote-output' && event.text.includes('remote summary text'),
-    ),
-  );
-  const last = events[events.length - 1];
-  assert.deepEqual(last, { type: 'result', ok: true, error: null });
-  assert.ok(!lines.some((line) => line.startsWith('stdout:')));
-});
-
-test('runSyncCommand --json emits an error result when the sync fails', async () => {
-  const lines: string[] = [];
-  const deps: Partial<SyncCommandDeps> = {
-    ...makeDirectionDeps([]),
-    consoleLog: (line: string) => {
-      lines.push(line);
-    },
-    writeStdout: (() => true) as unknown as typeof process.stdout.write,
-    runSsh: (_host: string, command: string) => {
-      if (command.includes(' sync --make-temp')) return ok('/tmp/subminer-sync-remote\n');
-      if (command.includes(' sync --merge ')) {
-        return { status: 9, stdout: '', stderr: 'remote merge exploded' };
-      }
-      return ok();
-    },
-    recordHostSyncResult: () => {},
-  };
-
-  await assert.rejects(() =>
-    runSyncCommand(
-      makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box', syncJson: true }),
-      deps,
-    ),
-  );
-
-  const events = lines.map((line) => JSON.parse(line));
-  const last = events[events.length - 1];
-  assert.equal(last.type, 'result');
-  assert.equal(last.ok, false);
-  assert.match(last.error, /Remote merge failed/);
-});
-
-test('runSyncCommand records host sync results for saved-host bookkeeping', async () => {
-  const recorded: Array<{ host: string; status: string; detail: string | null }> = [];
-  const baseDeps = makeDirectionDeps([]);
-  const deps: Partial<SyncCommandDeps> = {
-    ...baseDeps,
-    recordHostSyncResult: (host: string, status: 'success' | 'error', detail: string | null) => {
-      recorded.push({ host, status, detail });
-    },
-  };
-
-  await runSyncCommand(
-    makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box' }),
-    deps,
-  );
-  assert.equal(recorded.length, 1);
-  assert.equal(recorded[0]!.host, 'media-box');
-  assert.equal(recorded[0]!.status, 'success');
-
-  const failingDeps: Partial<SyncCommandDeps> = {
-    ...deps,
-    runSsh: (_host: string, command: string) => {
-      if (command.includes(' sync --make-temp')) return ok('/tmp/subminer-sync-remote\n');
-      if (command.includes(' sync --merge ')) {
-        return { status: 9, stdout: '', stderr: 'boom' };
-      }
-      return ok();
-    },
-  };
-  await assert.rejects(() =>
-    runSyncCommand(
-      makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box' }),
-      failingDeps,
-    ),
-  );
-  assert.equal(recorded.length, 2);
-  assert.equal(recorded[1]!.status, 'error');
-});
-
-test('runSyncCommand --check --json reports ssh and remote launcher status', async () => {
-  const lines: string[] = [];
-  const deps: Partial<SyncCommandDeps> = {
-    ensureTrackerQuiescent: async () => {},
-    assertSafeSshHost: () => {},
-    resolveRemoteSubminerCommand: () => 'subminer',
-    consoleLog: (line: string) => {
-      lines.push(line);
-    },
-    runSsh: (_host: string, command: string) => {
-      if (command.includes('--version')) return ok('SubMiner 0.18.0\n');
-      return ok('subminer-check-ok\n');
-    },
-    fail: (message: string): never => {
-      throw new Error(message);
+    runAppCommand: (appPath, appArgs) => {
+      spawned.push({ appPath, appArgs });
     },
   };
 
   assert.equal(
-    await runSyncCommand(
-      makeContext({
-        syncDbPath: '/tmp/local.sqlite',
-        syncHost: 'media-box',
-        syncCheck: true,
-        syncJson: true,
-      }),
-      deps,
-    ),
+    await runSyncCommand(makeContext({ syncHost: 'media-box', syncJson: true }), deps),
     true,
   );
+  assert.deepEqual(spawned, [
+    {
+      appPath: '/opt/SubMiner/subminer-app',
+      appArgs: ['--sync-cli', 'sync', 'media-box', '--json', '--log-level', 'warn'],
+    },
+  ]);
 
-  const events = lines.map((line) => JSON.parse(line));
-  const check = events.find((event) => event.type === 'check-result');
-  assert.ok(check);
-  assert.equal(check.host, 'media-box');
-  assert.equal(check.sshOk, true);
-  assert.equal(check.remoteCommand, 'subminer');
-  assert.equal(check.remoteVersion, 'SubMiner 0.18.0');
-  assert.equal(check.ok, true);
-  assert.equal(check.error, null);
+  assert.equal(await runSyncCommand(makeContext({ sync: false }), deps), false);
+  assert.equal(spawned.length, 1);
 });
 
-test('runSyncCommand --check --json reports failures without throwing early', async () => {
-  const lines: string[] = [];
+test('buildSyncCliArgv forwards every sync option', () => {
+  assert.deepEqual(
+    buildSyncCliArgv(
+      makeArgs({
+        syncHost: 'media-box',
+        syncDirection: 'pull',
+        syncRemoteCmd: '/opt/SubMiner.AppImage',
+        syncDbPath: '/tmp/db.sqlite',
+        syncForce: true,
+        syncJson: true,
+        logLevel: 'debug',
+      }),
+    ),
+    [
+      '--sync-cli',
+      'sync',
+      'media-box',
+      '--pull',
+      '--remote-cmd',
+      '/opt/SubMiner.AppImage',
+      '--db',
+      '/tmp/db.sqlite',
+      '--force',
+      '--json',
+      '--log-level',
+      'debug',
+    ],
+  );
+
+  assert.deepEqual(
+    buildSyncCliArgv(makeArgs({ syncSnapshotPath: '/tmp/out.sqlite' })),
+    ['--sync-cli', 'sync', '--snapshot', '/tmp/out.sqlite', '--log-level', 'warn'],
+  );
+
+  assert.deepEqual(
+    buildSyncCliArgv(makeArgs({ syncHost: 'media-box', syncCheck: true })),
+    ['--sync-cli', 'sync', 'media-box', '--check', '--log-level', 'warn'],
+  );
+
+  assert.deepEqual(
+    buildSyncCliArgv(makeArgs({ syncMakeTemp: true })),
+    ['--sync-cli', 'sync', '--make-temp', '--log-level', 'warn'],
+  );
+
+  assert.deepEqual(
+    buildSyncCliArgv(makeArgs({ syncRemoveTempPath: '/tmp/subminer-sync-x' })),
+    ['--sync-cli', 'sync', '--remove-temp', '/tmp/subminer-sync-x', '--log-level', 'warn'],
+  );
+
+  assert.deepEqual(
+    buildSyncCliArgv(makeArgs({ syncMergePath: '/tmp/in.sqlite', syncForce: true })),
+    ['--sync-cli', 'sync', '--merge', '/tmp/in.sqlite', '--force', '--log-level', 'warn'],
+  );
+});
+
+test('runSyncCommand fails with a clear message when the app binary is missing', async () => {
   const deps: Partial<SyncCommandDeps> = {
-    ensureTrackerQuiescent: async () => {},
-    assertSafeSshHost: () => {},
-    resolveRemoteSubminerCommand: () => {
-      throw new Error('Could not find a runnable "subminer" on media-box.');
+    runAppCommand: () => {
+      throw new Error('should not spawn');
     },
-    consoleLog: (line: string) => {
-      lines.push(line);
-    },
-    runSsh: () => ok('subminer-check-ok\n'),
     fail: (message: string): never => {
       throw new Error(message);
     },
   };
 
-  await assert.rejects(() =>
-    runSyncCommand(
-      makeContext({
-        syncDbPath: '/tmp/local.sqlite',
-        syncHost: 'media-box',
-        syncCheck: true,
-        syncJson: true,
-      }),
-      deps,
-    ),
+  await assert.rejects(
+    () => runSyncCommand(makeContext({ syncHost: 'media-box' }, null), deps),
+    /SubMiner app binary not found \(sync runs inside the app\)/,
   );
-
-  const events = lines.map((line) => JSON.parse(line));
-  const check = events.find((event) => event.type === 'check-result');
-  assert.ok(check);
-  assert.equal(check.sshOk, true);
-  assert.equal(check.ok, false);
-  assert.match(check.error, /Could not find a runnable/);
-});
-
-test('runHostSync speaks Windows shells: app command, double quotes, temp protocol', async () => {
-  const sshCommands: string[] = [];
-  const scpCalls: string[] = [];
-  const winTemp = 'C:\\Users\\First Last\\AppData\\Local\\Temp\\subminer-sync-remote';
-  const deps: Partial<SyncCommandDeps> = {
-    ...noRecord,
-    createDbSnapshot: (_dbPath: string, outPath: string) => {
-      fs.writeFileSync(outPath, 'snapshot');
-    },
-    mergeSnapshotIntoDb: () => createEmptyMergeSummary(),
-    formatMergeSummary: () => 'summary',
-    ensureTrackerQuiescent: async () => {},
-    assertSafeSshHost: () => {},
-    detectRemoteShellFlavor: () => 'windows-cmd',
-    resolveRemoteSubminerCommand: () => '"%LOCALAPPDATA%\\Programs\\SubMiner\\SubMiner.exe" --sync-cli',
-    runSsh: (_host: string, command: string) => {
-      sshCommands.push(command);
-      if (command.includes(' sync --make-temp')) return ok(`${winTemp}\r\n`);
-      return ok();
-    },
-    runScp: (from: string, to: string) => {
-      scpCalls.push(`${from}->${to}`);
-      if (!to.includes(':')) fs.writeFileSync(to, 'pulled');
-    },
-  };
-
-  await runSyncCommand(
-    makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'win-box' }),
-    deps,
-  );
-
-  const expectedDir = 'C:/Users/First Last/AppData/Local/Temp/subminer-sync-remote';
-  assert.ok(
-    sshCommands.includes(
-      `"%LOCALAPPDATA%\\Programs\\SubMiner\\SubMiner.exe" --sync-cli sync --snapshot "${expectedDir}/snapshot.sqlite"`,
-    ),
-  );
-  assert.ok(
-    sshCommands.includes(
-      `"%LOCALAPPDATA%\\Programs\\SubMiner\\SubMiner.exe" --sync-cli sync --merge "${expectedDir}/incoming.sqlite"`,
-    ),
-  );
-  assert.ok(
-    sshCommands.includes(
-      `"%LOCALAPPDATA%\\Programs\\SubMiner\\SubMiner.exe" --sync-cli sync --remove-temp "${expectedDir}"`,
-    ),
-  );
-  assert.ok(scpCalls.some((call) => call.startsWith(`win-box:${expectedDir}/snapshot.sqlite->`)));
-  assert.ok(scpCalls.some((call) => call.endsWith(`->win-box:${expectedDir}/incoming.sqlite`)));
-  // No POSIX shell-isms reach a Windows remote.
-  assert.ok(!sshCommands.some((command) => command.startsWith('mktemp') || command.startsWith('rm ')));
-  assert.ok(!sshCommands.some((command) => command.includes('PATH="$HOME')));
-});
-
-test('runSyncCommand --make-temp prints a temp dir and --remove-temp only removes sync temp dirs', async () => {
-  const printed: string[] = [];
-  const removed: string[] = [];
-  const madeDir = fs.mkdtempSync(path.join(os.tmpdir(), 'subminer-sync-'));
-  try {
-    await runSyncCommand(makeContext({ syncMakeTemp: true }), {
-      ...noRecord,
-      mkdtempSync: (() => madeDir) as unknown as typeof fs.mkdtempSync,
-      consoleLog: (line: string) => {
-        printed.push(line);
-      },
-    });
-    assert.deepEqual(printed, [madeDir]);
-
-    await runSyncCommand(makeContext({ syncRemoveTempPath: madeDir }), {
-      ...noRecord,
-      rmSync: ((target: string) => {
-        removed.push(target);
-      }) as typeof fs.rmSync,
-    });
-    assert.deepEqual(removed, [madeDir]);
-
-    await assert.rejects(
-      () =>
-        runSyncCommand(makeContext({ syncRemoveTempPath: '/etc' }), {
-          ...noRecord,
-          rmSync: ((target: string) => {
-            removed.push(target);
-          }) as typeof fs.rmSync,
-        }),
-      /Refusing to remove a directory outside the sync temp area/,
-    );
-    assert.equal(removed.length, 1);
-  } finally {
-    fs.rmSync(madeDir, { recursive: true, force: true });
-  }
 });
diff --git a/launcher/commands/sync-command.ts b/launcher/commands/sync-command.ts
index 8d89910a..9f80ff4b 100644
--- a/launcher/commands/sync-command.ts
+++ b/launcher/commands/sync-command.ts
@@ -1,111 +1,73 @@
-import fs from 'node:fs';
-import { fail, log } from '../log.js';
-import type { LogLevel } from '../types.js';
-import { resolveImmersionDbPath } from '../history-db.js';
-import {
-  createDbSnapshot,
-  findLiveStatsDaemonPid,
-  formatMergeSummary,
-  mergeSnapshotIntoDb,
-} from '../sync/sync-db.js';
-import {
-  assertSafeSshHost,
-  detectRemoteShellFlavor,
-  resolveRemoteSubminerCommand,
-  runScp,
-  runSsh,
-} from '../sync/ssh.js';
-import { resolvePathMaybe } from '../util.js';
-import { canConnectUnixSocket } from '../mpv.js';
-import { recordHostSyncResultToDisk } from '../sync/sync-hosts.js';
-import {
-  ensureTrackerQuiescentFlow,
-  runSyncFlow,
-  runCheckMode as runCheckModeFlow,
-  runHostSync as runHostSyncFlow,
-  runMergeMode as runMergeModeFlow,
-  runSnapshotMode as runSnapshotModeFlow,
-  type SyncFlowContext,
-  type SyncFlowDeps,
-} from '../../src/core/services/stats-sync/sync-flow.js';
+import { fail } from '../log.js';
+import { runAppCommandInteractive } from '../mpv.js';
+import type { Args } from '../types.js';
 import type { LauncherCommandContext } from './context.js';
 
-// The sync flow itself lives in src/core/services/stats-sync (shared with the
-// app's --sync-cli mode); this module binds it to the launcher's bun:sqlite
-// engine, logger, and config paths.
-export type SyncCommandDeps = SyncFlowDeps;
+export interface SyncCommandDeps {
+  runAppCommand: (appPath: string, appArgs: string[]) => void;
+  fail: (message: string) => never;
+}
 
 const defaultSyncCommandDeps: SyncCommandDeps = {
-  createDbSnapshot,
-  mergeSnapshotIntoDb,
-  formatMergeSummary,
-  findLiveStatsDaemonPid,
-  assertSafeSshHost,
-  detectRemoteShellFlavor,
-  resolveRemoteSubminerCommand,
-  runScp,
-  runSsh,
+  runAppCommand: runAppCommandInteractive,
   fail,
-  log: (level, configured, message) => log(level as LogLevel, configured as LogLevel, message),
-  canConnectUnixSocket,
-  realpathSync: fs.realpathSync,
-  mkdtempSync: fs.mkdtempSync,
-  rmSync: fs.rmSync,
-  consoleLog: console.log,
-  writeStdout: process.stdout.write.bind(process.stdout),
-  ensureTrackerQuiescent: async (context, dbPath) => ensureTrackerQuiescent(context, dbPath),
-  emitEvent: () => {},
-  recordHostSyncResult: recordHostSyncResultToDisk,
-  resolveDefaultDbPath: resolveImmersionDbPath,
-  resolvePath: resolvePathMaybe,
 };
 
-function resolveSyncCommandDeps(inputDeps: Partial<SyncCommandDeps> = {}): SyncCommandDeps {
-  return { ...defaultSyncCommandDeps, ...inputDeps };
-}
-
-export async function ensureTrackerQuiescent(
-  context: SyncFlowContext,
-  dbPath: string,
-  inputDeps: Partial<SyncCommandDeps> = {},
-): Promise<void> {
-  await ensureTrackerQuiescentFlow(context, dbPath, resolveSyncCommandDeps(inputDeps));
-}
-
-export function runSnapshotMode(
-  context: LauncherCommandContext,
-  dbPath: string,
-  inputDeps: Partial<SyncCommandDeps> = {},
-): void {
-  runSnapshotModeFlow(context, dbPath, resolveSyncCommandDeps(inputDeps));
-}
-
-export async function runMergeMode(
-  context: LauncherCommandContext,
-  dbPath: string,
-  inputDeps: Partial<SyncCommandDeps> = {},
-): Promise<void> {
-  await runMergeModeFlow(context, dbPath, resolveSyncCommandDeps(inputDeps));
-}
-
-export async function runCheckMode(
-  context: LauncherCommandContext,
-  inputDeps: Partial<SyncCommandDeps> = {},
-): Promise<void> {
-  await runCheckModeFlow(context, resolveSyncCommandDeps(inputDeps));
-}
-
-export async function runHostSync(
-  context: LauncherCommandContext,
-  dbPath: string,
-  inputDeps: Partial<SyncCommandDeps> = {},
-): Promise<void> {
-  await runHostSyncFlow(context, dbPath, resolveSyncCommandDeps(inputDeps));
+type SyncArgs = Pick<
+  Args,
+  | 'syncHost'
+  | 'syncSnapshotPath'
+  | 'syncMergePath'
+  | 'syncDirection'
+  | 'syncRemoteCmd'
+  | 'syncDbPath'
+  | 'syncForce'
+  | 'syncJson'
+  | 'syncCheck'
+  | 'syncMakeTemp'
+  | 'syncRemoveTempPath'
+  | 'logLevel'
+>;
+
+/** Rebuild the app's --sync-cli argv from the launcher's parsed sync args. */
+export function buildSyncCliArgv(args: SyncArgs): string[] {
+  const argv = ['--sync-cli', 'sync'];
+  if (args.syncHost) argv.push(args.syncHost);
+  if (args.syncSnapshotPath) argv.push('--snapshot', args.syncSnapshotPath);
+  if (args.syncMergePath) argv.push('--merge', args.syncMergePath);
+  if (args.syncMakeTemp) argv.push('--make-temp');
+  if (args.syncRemoveTempPath) argv.push('--remove-temp', args.syncRemoveTempPath);
+  if (args.syncDirection === 'push') argv.push('--push');
+  if (args.syncDirection === 'pull') argv.push('--pull');
+  if (args.syncCheck) argv.push('--check');
+  if (args.syncRemoteCmd) argv.push('--remote-cmd', args.syncRemoteCmd);
+  if (args.syncDbPath) argv.push('--db', args.syncDbPath);
+  if (args.syncForce) argv.push('--force');
+  if (args.syncJson) argv.push('--json');
+  argv.push('--log-level', args.logLevel);
+  return argv;
 }
 
+/**
+ * `subminer sync` is a thin proxy: the sync engine only executes inside the
+ * SubMiner app (--sync-cli mode, libsql), so the launcher and the app cannot
+ * drift apart. The launcher contributes its parser/help and app discovery;
+ * the child owns the terminal and its exit code becomes the launcher's.
+ */
 export async function runSyncCommand(
   context: LauncherCommandContext,
   inputDeps: Partial<SyncCommandDeps> = {},
 ): Promise<boolean> {
-  return runSyncFlow(context, resolveSyncCommandDeps(inputDeps));
+  const deps = { ...defaultSyncCommandDeps, ...inputDeps };
+  if (!context.args.sync) return false;
+  if (!context.appPath) {
+    deps.fail(
+      context.processAdapter.platform() === 'darwin'
+        ? 'SubMiner app binary not found (sync runs inside the app). Install SubMiner.app to /Applications or ~/Applications, or set SUBMINER_APPIMAGE_PATH.'
+        : 'SubMiner app binary not found (sync runs inside the app). Install the SubMiner app, or set SUBMINER_APPIMAGE_PATH.',
+    );
+    return true; // fail() never returns; this only satisfies control-flow analysis
+  }
+  deps.runAppCommand(context.appPath, buildSyncCliArgv(context.args));
+  return true;
 }
diff --git a/launcher/mpv.ts b/launcher/mpv.ts
index d249b9ff..208887e4 100644
--- a/launcher/mpv.ts
+++ b/launcher/mpv.ts
@@ -1462,6 +1462,30 @@ export function runAppCommandWithInherit(appPath: string, appArgs: string[]): vo
   });
 }
 
+/**
+ * Like runAppCommandWithInherit, but with the terminal fully attached: the
+ * child owns stdin (ssh password/host-key prompts must reach the user) and
+ * writes stdout/stderr directly (NDJSON --json output must stay unwrapped).
+ * Used for `subminer sync`, which proxies to the app's --sync-cli mode.
+ */
+export function runAppCommandInteractive(appPath: string, appArgs: string[]): void {
+  if (maybeCaptureAppArgs(appArgs)) {
+    process.exit(0);
+  }
+
+  const target = resolveAppSpawnTarget(appPath, appArgs);
+  const proc = spawn(target.command, target.args, {
+    stdio: ['inherit', 'inherit', 'inherit'],
+    env: buildAppEnv(process.env, target.env),
+  });
+  proc.once('error', (error) => {
+    fail(`Failed to run app command: ${error.message}`);
+  });
+  proc.once('close', (code) => {
+    process.exit(code ?? 0);
+  });
+}
+
 export function runAppCommandSilently(appPath: string, appArgs: string[]): void {
   if (maybeCaptureAppArgs(appArgs)) {
     process.exit(0);
diff --git a/launcher/sync/bun-driver.ts b/launcher/sync/bun-driver.ts
deleted file mode 100644
index 86ab0bb8..00000000
--- a/launcher/sync/bun-driver.ts
+++ /dev/null
@@ -1,23 +0,0 @@
-import { Database } from 'bun:sqlite';
-import type {
-  OpenSyncDb,
-  SyncDb,
-  SyncDbStatement,
-} from '../../src/core/services/stats-sync/driver.js';
-
-// bun:sqlite's Database.query() already caches prepared statements per SQL
-// string, which is exactly what the SyncDb contract asks for.
-export const openBunSyncDb: OpenSyncDb = (dbPath, options): SyncDb => {
-  const db = new Database(dbPath, options);
-  return {
-    query(sql: string): SyncDbStatement {
-      return db.query(sql);
-    },
-    exec(sql: string): void {
-      db.run(sql);
-    },
-    close(): void {
-      db.close();
-    },
-  };
-};
diff --git a/launcher/sync/engine-drivers.test.ts b/launcher/sync/engine-drivers.test.ts
deleted file mode 100644
index 1903306f..00000000
--- a/launcher/sync/engine-drivers.test.ts
+++ /dev/null
@@ -1,76 +0,0 @@
-import test from 'node:test';
-import assert from 'node:assert/strict';
-import fs from 'node:fs';
-import os from 'node:os';
-import path from 'node:path';
-import {
-  createImmersionDbFixture,
-  insertFixtureSession,
-} from '../test-support/immersion-db-fixture.js';
-import { openLibsqlSyncDb } from '../../src/core/services/stats-sync/libsql-driver.js';
-import { createDbSnapshot } from '../../src/core/services/stats-sync/shared.js';
-import { mergeSnapshotIntoDb } from '../../src/core/services/stats-sync/merge.js';
-
-const BASE_MS = Date.UTC(2026, 5, 1, 12, 0, 0);
-
-function makeTmpDir(): string {
-  return fs.mkdtempSync(path.join(os.tmpdir(), 'subminer-libsql-sync-test-'));
-}
-
-function countRows(dbPath: string, sql: string): number {
-  const db = openLibsqlSyncDb(dbPath, { readonly: true });
-  try {
-    const row = db.query(sql).get() as { n: number } | undefined;
-    return Number(row?.n ?? 0);
-  } finally {
-    db.close();
-  }
-}
-
-// End-to-end pass of the shared engine over the libsql driver (the binding the
-// Electron app's --sync-cli mode uses): snapshot a fixture DB, merge it into
-// another, and re-merge to confirm idempotence.
-test('libsql driver runs snapshot and merge through the shared engine', () => {
-  const dir = makeTmpDir();
-  try {
-    const localPath = path.join(dir, 'local.sqlite');
-    const remotePath = path.join(dir, 'remote.sqlite');
-    createImmersionDbFixture(localPath);
-    createImmersionDbFixture(remotePath);
-    insertFixtureSession(localPath, {
-      uuid: 'local-1',
-      videoKey: 'showa-e1',
-      animeTitleKey: 'showa',
-      startedAtMs: BASE_MS,
-      applyLifetime: true,
-      words: [{ headword: '見る', word: '見た', reading: 'みた', count: 2 }],
-    });
-    insertFixtureSession(remotePath, {
-      uuid: 'remote-1',
-      videoKey: 'showb-e1',
-      animeTitleKey: 'showb',
-      startedAtMs: BASE_MS + 86_400_000,
-      activeWatchedMs: 900_000,
-      cardsMined: 3,
-      applyLifetime: true,
-      words: [{ headword: '食べる', word: '食べた', reading: 'たべた', count: 1 }],
-    });
-
-    const snapshotPath = path.join(dir, 'remote-snapshot.sqlite');
-    createDbSnapshot(openLibsqlSyncDb, remotePath, snapshotPath);
-    assert.ok(fs.existsSync(snapshotPath));
-    assert.equal(countRows(snapshotPath, 'SELECT COUNT(*) AS n FROM imm_sessions'), 1);
-
-    const summary = mergeSnapshotIntoDb(openLibsqlSyncDb, localPath, snapshotPath);
-    assert.equal(summary.sessionsMerged, 1);
-    assert.equal(summary.videosAdded, 1);
-    assert.equal(countRows(localPath, 'SELECT COUNT(*) AS n FROM imm_sessions'), 2);
-
-    const again = mergeSnapshotIntoDb(openLibsqlSyncDb, localPath, snapshotPath);
-    assert.equal(again.sessionsMerged, 0);
-    assert.equal(again.sessionsAlreadyPresent, 1);
-    assert.equal(countRows(localPath, 'SELECT COUNT(*) AS n FROM imm_sessions'), 2);
-  } finally {
-    fs.rmSync(dir, { recursive: true, force: true });
-  }
-});
diff --git a/launcher/sync/sync-db.test.ts b/launcher/sync/engine-merge.test.ts
similarity index 96%
rename from launcher/sync/sync-db.test.ts
rename to launcher/sync/engine-merge.test.ts
index 6f72b190..d76b0123 100644
--- a/launcher/sync/sync-db.test.ts
+++ b/launcher/sync/engine-merge.test.ts
@@ -4,7 +4,17 @@ import fs from 'node:fs';
 import os from 'node:os';
 import path from 'node:path';
 import { Database } from 'bun:sqlite';
-import { createDbSnapshot, mergeSnapshotIntoDb } from './sync-db.js';
+import { openLibsqlSyncDb } from '../../src/core/services/stats-sync/libsql-driver.js';
+import { createDbSnapshot as createDbSnapshotWith } from '../../src/core/services/stats-sync/shared.js';
+import { mergeSnapshotIntoDb as mergeSnapshotIntoDbWith } from '../../src/core/services/stats-sync/merge.js';
+
+// The engine only executes inside the app (libsql driver) in production, so
+// these merge tests run through that same binding; bun:sqlite is used only to
+// build fixtures and inspect results.
+const createDbSnapshot = (dbPath: string, outPath: string) =>
+  createDbSnapshotWith(openLibsqlSyncDb, dbPath, outPath);
+const mergeSnapshotIntoDb = (localDbPath: string, snapshotPath: string) =>
+  mergeSnapshotIntoDbWith(openLibsqlSyncDb, localDbPath, snapshotPath);
 import {
   createImmersionDbFixture,
   insertFixtureSession,
diff --git a/launcher/sync/ssh.ts b/launcher/sync/ssh.ts
deleted file mode 100644
index 0bdef08a..00000000
--- a/launcher/sync/ssh.ts
+++ /dev/null
@@ -1,13 +0,0 @@
-// Re-exported from the shared stats-sync engine so the launcher and the
-// app's --sync-cli mode resolve remotes and shell out identically.
-export {
-  assertSafeSshHost,
-  detectRemoteShellFlavor,
-  quoteForRemoteShell,
-  resolveRemoteSubminerCommand,
-  runScp,
-  runSsh,
-  shellQuote,
-  type RemoteRunResult,
-  type RemoteShellFlavor,
-} from '../../src/core/services/stats-sync/ssh.js';
diff --git a/launcher/sync/sync-db.ts b/launcher/sync/sync-db.ts
deleted file mode 100644
index 0d45f611..00000000
--- a/launcher/sync/sync-db.ts
+++ /dev/null
@@ -1,12 +0,0 @@
-import { mergeSnapshotIntoDb as mergeSnapshotIntoDbWith } from '../../src/core/services/stats-sync/merge.js';
-import { openBunSyncDb } from './bun-driver.js';
-import type { SyncMergeSummary } from './sync-shared.js';
-
-export type { SyncMergeSummary } from './sync-shared.js';
-export { createDbSnapshot, findLiveStatsDaemonPid } from './sync-shared.js';
-export { formatMergeSummary } from '../../src/core/services/stats-sync/merge.js';
-
-/** bun:sqlite binding of the shared snapshot-merge engine. */
-export function mergeSnapshotIntoDb(localDbPath: string, snapshotPath: string): SyncMergeSummary {
-  return mergeSnapshotIntoDbWith(openBunSyncDb, localDbPath, snapshotPath);
-}
diff --git a/launcher/sync/sync-hosts.ts b/launcher/sync/sync-hosts.ts
deleted file mode 100644
index ab6d41a8..00000000
--- a/launcher/sync/sync-hosts.ts
+++ /dev/null
@@ -1,41 +0,0 @@
-import fs from 'node:fs';
-import os from 'node:os';
-import { resolveConfigDir } from '../../src/config/path-resolution.js';
-import {
-  getSyncHostsPath,
-  readSyncHostsState,
-  recordSyncResult,
-  writeSyncHostsState,
-  type SyncResultStatus,
-} from '../../src/shared/sync/sync-hosts-store.js';
-
-export function resolveSyncHostsFilePath(): string {
-  const configDir = resolveConfigDir({
-    platform: process.platform,
-    appDataDir: process.env.APPDATA,
-    xdgConfigHome: process.env.XDG_CONFIG_HOME,
-    homeDir: os.homedir(),
-    existsSync: fs.existsSync,
-  });
-  return getSyncHostsPath(configDir);
-}
-
-// Best-effort bookkeeping so hosts synced from the CLI show up in the sync UI;
-// a failure to persist must never fail the sync itself.
-export function recordHostSyncResultToDisk(
-  host: string,
-  status: SyncResultStatus,
-  detail: string | null,
-): void {
-  try {
-    const filePath = resolveSyncHostsFilePath();
-    const state = recordSyncResult(readSyncHostsState(filePath), host, {
-      atMs: Date.now(),
-      status,
-      detail,
-    });
-    writeSyncHostsState(filePath, state);
-  } catch {
-    // best effort
-  }
-}
diff --git a/launcher/sync/sync-shared.ts b/launcher/sync/sync-shared.ts
deleted file mode 100644
index eeb1c49e..00000000
--- a/launcher/sync/sync-shared.ts
+++ /dev/null
@@ -1,31 +0,0 @@
-// bun:sqlite bindings for the shared stats-sync engine. The engine itself
-// lives in src/core/services/stats-sync so the Electron app (libsql) can run
-// the exact same snapshot/merge code via its own driver.
-import {
-  createDbSnapshot as createDbSnapshotWith,
-  createEmptyMergeSummary,
-  findLiveStatsDaemonPid,
-  nowDbTimestamp,
-  readSchemaVersion,
-  assertMergeableSchema,
-  insertRow,
-  tableExists,
-  SCHEMA_VERSION,
-} from '../../src/core/services/stats-sync/shared.js';
-import { openBunSyncDb } from './bun-driver.js';
-
-export {
-  createEmptyMergeSummary,
-  findLiveStatsDaemonPid,
-  nowDbTimestamp,
-  readSchemaVersion,
-  assertMergeableSchema,
-  insertRow,
-  tableExists,
-  SCHEMA_VERSION,
-};
-export type { SyncMergeSummary } from '../../src/core/services/stats-sync/shared.js';
-
-export function createDbSnapshot(dbPath: string, outPath: string): void {
-  createDbSnapshotWith(openBunSyncDb, dbPath, outPath);
-}
diff --git a/launcher/sync/ssh.test.ts b/src/core/services/stats-sync/ssh.test.ts
similarity index 99%
rename from launcher/sync/ssh.test.ts
rename to src/core/services/stats-sync/ssh.test.ts
index 4693a58e..e78e59e4 100644
--- a/launcher/sync/ssh.test.ts
+++ b/src/core/services/stats-sync/ssh.test.ts
@@ -8,7 +8,7 @@ import {
   runScp,
   shellQuote,
   type RemoteRunResult,
-} from './ssh.js';
+} from './ssh';
 
 function remoteResult(status: number, stdout = ''): RemoteRunResult {
   return { status, stdout, stderr: '' };
diff --git a/src/core/services/stats-sync/sync-flow.test.ts b/src/core/services/stats-sync/sync-flow.test.ts
new file mode 100644
index 00000000..e6e8f3fd
--- /dev/null
+++ b/src/core/services/stats-sync/sync-flow.test.ts
@@ -0,0 +1,463 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import { createEmptyMergeSummary } from './shared';
+import {
+  ensureTrackerQuiescentFlow,
+  runSyncFlow,
+  type SyncFlowContext,
+  type SyncFlowDeps,
+} from './sync-flow';
+
+function makeContext(overrides: Partial<SyncFlowContext['args']> = {}): SyncFlowContext {
+  return {
+    args: {
+      sync: true,
+      syncHost: '',
+      syncSnapshotPath: '',
+      syncMergePath: '',
+      syncDirection: 'both',
+      syncRemoteCmd: '',
+      syncDbPath: '',
+      syncForce: false,
+      syncJson: false,
+      syncCheck: false,
+      syncMakeTemp: false,
+      syncRemoveTempPath: '',
+      logLevel: 'warn',
+      ...overrides,
+    },
+    mpvSocketPath: '',
+  };
+}
+
+function ok(stdout = ''): { status: number; stdout: string; stderr: string } {
+  return { status: 0, stdout, stderr: '' };
+}
+
+// recordHostSyncResult defaults to a no-op here: the real disk-writing
+// implementations are bound by the entry points, never by the flow itself.
+function makeDeps(overrides: Partial<SyncFlowDeps> = {}): SyncFlowDeps {
+  return {
+    createDbSnapshot: () => {},
+    mergeSnapshotIntoDb: () => createEmptyMergeSummary(),
+    formatMergeSummary: () => 'summary',
+    findLiveStatsDaemonPid: () => null,
+    assertSafeSshHost: () => {},
+    detectRemoteShellFlavor: () => 'posix',
+    resolveRemoteSubminerCommand: () => 'subminer',
+    runScp: () => {},
+    runSsh: () => ok(),
+    fail: (message: string): never => {
+      throw new Error(message);
+    },
+    log: () => {},
+    canConnectUnixSocket: async () => false,
+    realpathSync: (candidate) => candidate,
+    mkdtempSync: (prefix) => fs.mkdtempSync(prefix),
+    rmSync: (target, options) => fs.rmSync(target, options),
+    consoleLog: () => {},
+    writeStdout: () => true,
+    ensureTrackerQuiescent: async () => {},
+    emitEvent: () => {},
+    recordHostSyncResult: () => {},
+    resolveDefaultDbPath: () => '/tracker.sqlite',
+    resolvePath: (value) => value,
+    ...overrides,
+  };
+}
+
+test('ensureTrackerQuiescentFlow ignores stale sockets but rejects live sockets', async () => {
+  const context = makeContext({ syncDbPath: '/tmp/local.sqlite' });
+  context.mpvSocketPath = '/tmp/subminer-socket';
+  let socketConnectable = false;
+  const deps = makeDeps({
+    realpathSync: () => '/tracker.sqlite',
+    canConnectUnixSocket: async () => socketConnectable,
+  });
+
+  await ensureTrackerQuiescentFlow(context, '/tmp/local.sqlite', deps);
+
+  socketConnectable = true;
+  await assert.rejects(
+    async () => ensureTrackerQuiescentFlow(context, '/tmp/local.sqlite', deps),
+    /mpv\/SubMiner session appears to be running/,
+  );
+});
+
+test('ensureTrackerQuiescentFlow rejects a live stats daemon and honors --force', async () => {
+  const context = makeContext({ syncDbPath: '/tmp/local.sqlite' });
+  const deps = makeDeps({
+    realpathSync: () => '/tracker.sqlite',
+    findLiveStatsDaemonPid: () => 4242,
+  });
+
+  await assert.rejects(
+    async () => ensureTrackerQuiescentFlow(context, '/tmp/local.sqlite', deps),
+    /stats server is running \(pid 4242\)/,
+  );
+
+  const forced = makeContext({ syncDbPath: '/tmp/local.sqlite', syncForce: true });
+  await ensureTrackerQuiescentFlow(forced, '/tmp/local.sqlite', deps);
+});
+
+test('runSyncFlow dispatches snapshot, merge, host, and missing-target modes', async () => {
+  const calls: string[] = [];
+  const deps = makeDeps({
+    createDbSnapshot: (dbPath, outPath) => {
+      calls.push(`snapshot:${dbPath}->${outPath}`);
+    },
+    mergeSnapshotIntoDb: (dbPath, snapshotPath) => {
+      calls.push(`merge:${dbPath}<-${snapshotPath}`);
+      return createEmptyMergeSummary();
+    },
+    ensureTrackerQuiescent: async () => {
+      calls.push('quiescent');
+    },
+    assertSafeSshHost: (host) => {
+      calls.push(`host:${host}`);
+    },
+    runSsh: (_host, command) => {
+      calls.push(`ssh:${command}`);
+      return command.includes(' sync --make-temp') ? ok('/tmp/subminer-sync-remote\n') : ok();
+    },
+    runScp: (from, to) => {
+      calls.push(`scp:${from}->${to}`);
+    },
+  });
+
+  assert.equal(
+    await runSyncFlow(
+      makeContext({ syncDbPath: '/tmp/local.sqlite', syncSnapshotPath: '/tmp/out.sqlite' }),
+      deps,
+    ),
+    true,
+  );
+  assert.ok(calls.includes('snapshot:/tmp/local.sqlite->/tmp/out.sqlite'));
+
+  await runSyncFlow(
+    makeContext({ syncDbPath: '/tmp/local.sqlite', syncMergePath: '/tmp/in.sqlite' }),
+    deps,
+  );
+  assert.ok(calls.includes('quiescent'));
+  assert.ok(calls.includes('merge:/tmp/local.sqlite<-/tmp/in.sqlite'));
+
+  await runSyncFlow(makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box' }), deps);
+  assert.ok(calls.includes('host:media-box'));
+
+  await assert.rejects(
+    () => runSyncFlow(makeContext({ syncDbPath: '/tmp/local.sqlite' }), deps),
+    /sync requires a host, --snapshot <file>, or --merge <file>/,
+  );
+
+  assert.equal(await runSyncFlow(makeContext({ sync: false }), deps), false);
+});
+
+function makeHostDeps(calls: string[], overrides: Partial<SyncFlowDeps> = {}): SyncFlowDeps {
+  return makeDeps({
+    createDbSnapshot: (_dbPath, outPath) => {
+      calls.push(`snapshot:${outPath}`);
+      fs.writeFileSync(outPath, 'snapshot');
+    },
+    mergeSnapshotIntoDb: () => {
+      calls.push('local-merge');
+      return createEmptyMergeSummary();
+    },
+    ensureTrackerQuiescent: async () => {
+      calls.push('quiescent');
+    },
+    runSsh: (_host, command) => {
+      calls.push(`ssh:${command}`);
+      if (command.includes(' sync --make-temp')) return ok('/tmp/subminer-sync-remote\n');
+      return ok();
+    },
+    runScp: (from, to) => {
+      calls.push(`scp:${from}->${to}`);
+      if (!to.includes(':')) fs.writeFileSync(to, 'pulled');
+    },
+    ...overrides,
+  });
+}
+
+test('runHostSync keeps tracker quiescent through both merges and cleans up after failure', async () => {
+  const calls: string[] = [];
+  let localTmpDir = '';
+  const deps = makeHostDeps(calls, {
+    mkdtempSync: (prefix) => {
+      localTmpDir = fs.mkdtempSync(prefix);
+      return localTmpDir;
+    },
+    runSsh: (_host, command) => {
+      calls.push(`ssh:${command}`);
+      if (command.includes(' sync --make-temp')) return ok('/tmp/subminer-sync-remote\n');
+      if (command.includes(' sync --merge ')) {
+        return { status: 9, stdout: 'remote output', stderr: 'remote merge exploded' };
+      }
+      return ok();
+    },
+  });
+
+  await assert.rejects(
+    () => runSyncFlow(makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box' }), deps),
+    /Remote merge failed on media-box[\s\S]*remote merge exploded/,
+  );
+  assert.equal(calls.filter((call) => call === 'quiescent').length, 3);
+  assert.ok(calls.indexOf('quiescent') < calls.findIndex((call) => call.startsWith('snapshot:')));
+  assert.ok(calls.includes('local-merge'));
+  assert.ok(calls.some((call) => call.includes(' sync --remove-temp ')));
+  assert.equal(fs.existsSync(localTmpDir), false);
+});
+
+test('runHostSync includes remote snapshot stderr in failures', async () => {
+  const deps = makeHostDeps([], {
+    runSsh: (_host, command) => {
+      if (command.includes(' sync --make-temp')) return ok('/tmp/subminer-sync-remote\n');
+      if (command.includes(' sync --snapshot ')) {
+        return { status: 5, stdout: '', stderr: 'snapshot permission denied' };
+      }
+      return ok();
+    },
+  });
+
+  await assert.rejects(
+    () => runSyncFlow(makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box' }), deps),
+    /Remote snapshot failed on media-box[\s\S]*snapshot permission denied/,
+  );
+});
+
+test('runHostSync push only snapshots locally and merges remotely', async () => {
+  const calls: string[] = [];
+  await runSyncFlow(
+    makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box', syncDirection: 'push' }),
+    makeHostDeps(calls),
+  );
+
+  assert.ok(calls.some((call) => call.startsWith('snapshot:')));
+  assert.ok(calls.some((call) => call.includes(' sync --merge ')));
+  assert.ok(calls.some((call) => call.startsWith('scp:') && call.includes('->media-box:')));
+  assert.ok(!calls.some((call) => call.includes(' sync --snapshot ')));
+  assert.ok(!calls.includes('local-merge'));
+});
+
+test('runHostSync pull only snapshots remotely and merges locally', async () => {
+  const calls: string[] = [];
+  await runSyncFlow(
+    makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box', syncDirection: 'pull' }),
+    makeHostDeps(calls),
+  );
+
+  assert.ok(calls.some((call) => call.includes(' sync --snapshot ')));
+  assert.ok(calls.some((call) => call.startsWith('scp:media-box:')));
+  assert.ok(calls.includes('local-merge'));
+  assert.ok(!calls.some((call) => call.startsWith('snapshot:')));
+  assert.ok(!calls.some((call) => call.includes(' sync --merge ')));
+});
+
+test('runSyncFlow --json emits NDJSON progress events and a final result', async () => {
+  const lines: string[] = [];
+  const deps = makeHostDeps([], {
+    consoleLog: (line) => {
+      lines.push(line);
+    },
+    runSsh: (_host, command) => {
+      if (command.includes(' sync --make-temp')) return ok('/tmp/subminer-sync-remote\n');
+      if (command.includes(' sync --merge ')) return ok('remote summary text\n');
+      return ok();
+    },
+  });
+
+  await runSyncFlow(
+    makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box', syncJson: true }),
+    deps,
+  );
+
+  const events = lines.map((line) => JSON.parse(line));
+  assert.ok(events.some((event) => event.type === 'stage' && event.stage === 'snapshot-local'));
+  assert.ok(events.some((event) => event.type === 'merge-summary' && event.target === 'local'));
+  assert.ok(
+    events.some(
+      (event) => event.type === 'remote-output' && event.text.includes('remote summary text'),
+    ),
+  );
+  assert.deepEqual(events[events.length - 1], { type: 'result', ok: true, error: null });
+});
+
+test('runSyncFlow --json emits an error result when the sync fails', async () => {
+  const lines: string[] = [];
+  const deps = makeHostDeps([], {
+    consoleLog: (line) => {
+      lines.push(line);
+    },
+    runSsh: (_host, command) => {
+      if (command.includes(' sync --make-temp')) return ok('/tmp/subminer-sync-remote\n');
+      if (command.includes(' sync --merge ')) return { status: 9, stdout: '', stderr: 'boom' };
+      return ok();
+    },
+  });
+
+  await assert.rejects(() =>
+    runSyncFlow(
+      makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box', syncJson: true }),
+      deps,
+    ),
+  );
+
+  const events = lines.map((line) => JSON.parse(line));
+  const last = events[events.length - 1];
+  assert.equal(last.type, 'result');
+  assert.equal(last.ok, false);
+  assert.match(last.error, /Remote merge failed/);
+});
+
+test('runHostSync records host sync results for saved-host bookkeeping', async () => {
+  const recorded: Array<{ host: string; status: string; detail: string | null }> = [];
+  const record: SyncFlowDeps['recordHostSyncResult'] = (host, status, detail) => {
+    recorded.push({ host, status, detail });
+  };
+
+  await runSyncFlow(
+    makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box' }),
+    makeHostDeps([], { recordHostSyncResult: record }),
+  );
+  assert.deepEqual(recorded[0], {
+    host: 'media-box',
+    status: 'success',
+    detail: '0 sessions merged; pushed local stats',
+  });
+
+  await assert.rejects(() =>
+    runSyncFlow(
+      makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box' }),
+      makeHostDeps([], {
+        recordHostSyncResult: record,
+        runSsh: (_host, command) => {
+          if (command.includes(' sync --make-temp')) return ok('/tmp/subminer-sync-remote\n');
+          if (command.includes(' sync --merge ')) return { status: 9, stdout: '', stderr: 'boom' };
+          return ok();
+        },
+      }),
+    ),
+  );
+  assert.equal(recorded.length, 2);
+  assert.equal(recorded[1]!.status, 'error');
+});
+
+test('runCheckMode --json reports ssh and remote SubMiner status', async () => {
+  const lines: string[] = [];
+  const deps = makeDeps({
+    consoleLog: (line) => {
+      lines.push(line);
+    },
+    runSsh: (_host, command) => {
+      if (command.includes('--version')) return ok('SubMiner 0.18.0\n');
+      return ok('subminer-check-ok\n');
+    },
+  });
+
+  await runSyncFlow(
+    makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box', syncCheck: true, syncJson: true }),
+    deps,
+  );
+
+  const events = lines.map((line) => JSON.parse(line));
+  const check = events.find((event) => event.type === 'check-result');
+  assert.ok(check);
+  assert.equal(check.host, 'media-box');
+  assert.equal(check.sshOk, true);
+  assert.equal(check.remoteCommand, 'subminer');
+  assert.equal(check.remoteVersion, 'SubMiner 0.18.0');
+  assert.equal(check.ok, true);
+  assert.equal(check.error, null);
+
+  const failLines: string[] = [];
+  await assert.rejects(() =>
+    runSyncFlow(
+      makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'media-box', syncCheck: true, syncJson: true }),
+      makeDeps({
+        consoleLog: (line) => {
+          failLines.push(line);
+        },
+        resolveRemoteSubminerCommand: () => {
+          throw new Error('Could not find a runnable "subminer" on media-box.');
+        },
+        runSsh: () => ok('subminer-check-ok\n'),
+      }),
+    ),
+  );
+  const failEvents = failLines.map((line) => JSON.parse(line));
+  const failedCheck = failEvents.find((event) => event.type === 'check-result');
+  assert.ok(failedCheck);
+  assert.equal(failedCheck.ok, false);
+  assert.match(failedCheck.error, /Could not find a runnable/);
+});
+
+test('runHostSync speaks Windows shells: app command, double quotes, temp protocol', async () => {
+  const sshCommands: string[] = [];
+  const scpCalls: string[] = [];
+  const winTemp = 'C:\\Users\\First Last\\AppData\\Local\\Temp\\subminer-sync-remote';
+  const appCmd = '"%LOCALAPPDATA%\\Programs\\SubMiner\\SubMiner.exe" --sync-cli';
+  const deps = makeHostDeps([], {
+    detectRemoteShellFlavor: () => 'windows-cmd',
+    resolveRemoteSubminerCommand: () => appCmd,
+    createDbSnapshot: (_dbPath, outPath) => {
+      fs.writeFileSync(outPath, 'snapshot');
+    },
+    runSsh: (_host, command) => {
+      sshCommands.push(command);
+      if (command.includes(' sync --make-temp')) return ok(`${winTemp}\r\n`);
+      return ok();
+    },
+    runScp: (from, to) => {
+      scpCalls.push(`${from}->${to}`);
+      if (!to.includes(':')) fs.writeFileSync(to, 'pulled');
+    },
+  });
+
+  await runSyncFlow(makeContext({ syncDbPath: '/tmp/local.sqlite', syncHost: 'win-box' }), deps);
+
+  const expectedDir = 'C:/Users/First Last/AppData/Local/Temp/subminer-sync-remote';
+  assert.ok(sshCommands.includes(`${appCmd} sync --snapshot "${expectedDir}/snapshot.sqlite"`));
+  assert.ok(sshCommands.includes(`${appCmd} sync --merge "${expectedDir}/incoming.sqlite"`));
+  assert.ok(sshCommands.includes(`${appCmd} sync --remove-temp "${expectedDir}"`));
+  assert.ok(scpCalls.some((call) => call.startsWith(`win-box:${expectedDir}/snapshot.sqlite->`)));
+  assert.ok(scpCalls.some((call) => call.endsWith(`->win-box:${expectedDir}/incoming.sqlite`)));
+  // No POSIX shell-isms reach a Windows remote.
+  assert.ok(!sshCommands.some((command) => command.startsWith('mktemp') || command.startsWith('rm ')));
+  assert.ok(!sshCommands.some((command) => command.includes('PATH="$HOME')));
+});
+
+test('runSyncFlow --make-temp prints a temp dir and --remove-temp only removes sync temp dirs', async () => {
+  const printed: string[] = [];
+  const removed: string[] = [];
+  const madeDir = fs.mkdtempSync(path.join(os.tmpdir(), 'subminer-sync-'));
+  try {
+    await runSyncFlow(
+      makeContext({ syncMakeTemp: true }),
+      makeDeps({
+        mkdtempSync: () => madeDir,
+        consoleLog: (line) => {
+          printed.push(line);
+        },
+      }),
+    );
+    assert.deepEqual(printed, [madeDir]);
+
+    const removeDeps = makeDeps({
+      rmSync: (target) => {
+        removed.push(target);
+      },
+    });
+    await runSyncFlow(makeContext({ syncRemoveTempPath: madeDir }), removeDeps);
+    assert.deepEqual(removed, [madeDir]);
+
+    await assert.rejects(
+      () => runSyncFlow(makeContext({ syncRemoveTempPath: '/etc' }), removeDeps),
+      /Refusing to remove a directory outside the sync temp area/,
+    );
+    assert.equal(removed.length, 1);
+  } finally {
+    fs.rmSync(madeDir, { recursive: true, force: true });
+  }
+});