diff --git a/server/test/playlist-build-cost.test.js b/server/test/playlist-build-cost.test.js
new file mode 100644
index 0000000..269f800
--- /dev/null
+++ b/server/test/playlist-build-cost.test.js
@@ -0,0 +1,58 @@
+'use strict';
+
+// #146 P2.5 — classify buildPlaylistPayload under a legitimate fleet-wide simultaneous
+// reconnect (e.g. 230 disty devices after one network blip). Each first connect passes
+// the flap/reconnect gates and runs buildPlaylistPayload SYNCHRONOUSLY. Measure the
+// per-call cost with a large published_snapshot and confirm it stays well under the
+// ~50ms invariant (the loop yields between socket.io handler invocations, so the risk is
+// per-call, not one aggregate block).
+
+const os = require('node:os');
+const path = require('node:path');
+const crypto = require('node:crypto');
+process.env.DATA_DIR = path.join(os.tmpdir(), 'st-plcost-' + crypto.randomBytes(4).toString('hex'));
+process.env.SELF_HOSTED = 'true';
+process.env.NODE_ENV = 'test';
+
+const { test, before, after } = require('node:test');
+const assert = require('node:assert/strict');
+const http = require('node:http');
+const { Server } = require('socket.io');
+const { db } = require('../db/database');
+const setupDeviceSocket = require('../ws/deviceSocket');
+
+let httpServer, io, buildPlaylistPayload;
+before(async () => {
+  httpServer = http.createServer(); io = new Server(httpServer); setupDeviceSocket(io);
+  buildPlaylistPayload = setupDeviceSocket.buildPlaylistPayload;
+  await new Promise((r) => httpServer.listen(0, r));
+
+  // a realistically LARGE published snapshot: 200 items.
+  const snap = JSON.stringify(Array.from({ length: 200 }, (_, i) => ({
+    content_id: 'c' + i, type: 'image', duration_sec: 10, url: '/uploads/content/c' + i + '.jpg', name: 'item ' + i,
+  })));
+  db.pragma('foreign_keys = OFF');
+  db.prepare("INSERT INTO playlists (id, user_id, name, status, published_snapshot) VALUES ('pl-big', 'u', 'Big', 'published', ?)").run(snap);
+  db.prepare("INSERT INTO devices (id, status, playlist_id) VALUES ('perf-dev', 'online', 'pl-big')").run();
+  db.pragma('foreign_keys = ON');
+});
+after(() => { try { io.close(); } catch { /* */ } try { httpServer.close(); } catch { /* */ } });
+
+test('buildPlaylistPayload is synchronous and cheap enough for a fleet-wide reconnect', () => {
+  assert.equal(typeof buildPlaylistPayload, 'function');
+  const payload = buildPlaylistPayload('perf-dev');
+  assert.equal(payload.assignments.length, 200, 'parsed the 200-item snapshot');
+
+  // Warm, then measure per-call over a disty-scale fleet count.
+  for (let i = 0; i < 50; i++) buildPlaylistPayload('perf-dev');
+  const N = 230, times = [];
+  for (let i = 0; i < N; i++) { const t = process.hrtime.bigint(); buildPlaylistPayload('perf-dev'); times.push(Number(process.hrtime.bigint() - t) / 1e6); }
+  const max = Math.max(...times);
+  const avg = times.reduce((a, b) => a + b, 0) / N;
+  const aggregate = avg * N;
+  console.log(`[P2.5] buildPlaylistPayload x${N}: avg=${avg.toFixed(3)}ms max=${max.toFixed(3)}ms aggregate=${aggregate.toFixed(1)}ms (spread across ${N} handler invocations, not one block)`);
+
+  // Per-call is the real unit (socket.io yields between handlers) — must be far under 50ms.
+  assert.ok(max < 50, `single build max ${max.toFixed(2)}ms < 50ms invariant`);
+  assert.ok(avg < 5, `single build avg ${avg.toFixed(3)}ms is cheap`);
+});