diff --git a/server/media.js b/server/media.js
new file mode 100644
index 0000000..4c40fcf
--- /dev/null
+++ b/server/media.js
@@ -0,0 +1,189 @@
+'use strict';
+//
+// media.js — web playback renditions for uploaded videos.
+//
+// WHY THIS EXISTS (measured, not guessed):
+//   Real uploads on this box were probed at 19.4 Mbps and 19.0 Mbps (1080p screen recordings) and
+//   3.6 Mbps (phone portrait). To play a 19 Mbps file the client must SUSTAIN a 19 Mbps download for
+//   the whole clip; no mobile link does, so the <video> buffer drains every few seconds and you get
+//   the classic "buffers, plays, buffers, plays". Server disk and CPU were idle throughout — the
+//   bottleneck is the media, not the delivery.
+//   Separately, phone MP4s often store `moov` AFTER `mdat` (not faststart), so the player has to
+//   fetch the tail before it can begin at all.
+//
+// WHAT WE DO:
+//   Leave the uploaded bytes untouched — that is what the download button serves, at full quality.
+//   Alongside it build <id>.web.mp4: longest side capped at 1280, ~2.5 Mbps ceiling, +faststart.
+//   /stream/<id> prefers that rendition and falls back to the original until it is ready, so a video
+//   is never unplayable while it transcodes.
+//
+const fs = require('fs');
+const path = require('path');
+const { execFile } = require('child_process');
+const { UPLOADS_DIR } = require('./config');
+
+const MAX_CONCURRENT = 2;        // transcoding is never urgent; leave the cores to the app
+const OK_BITRATE = 2500000;      // ≤2.5 Mbps streams fine over mobile
+const OK_DIMENSION = 1280;       // ...provided it isn't oversized as well
+const PROBE_TIMEOUT = 20000;
+const XCODE_TIMEOUT = 30 * 60 * 1000;
+
+let running = 0;
+const queue = [];
+const pending = new Set();       // ids queued or transcoding right now
+const failed = new Set();        // ffmpeg couldn't handle it — don't retry forever
+
+const webPath = (id) => path.join(UPLOADS_DIR, id + '.web.mp4');
+
+function hasWebRendition(id) {
+  try { return fs.statSync(webPath(id)).size > 0; } catch (e) { return false; }
+}
+
+// Walk the top-level MP4 box headers. If `mdat` comes before `moov` the index lives at the end of
+// the file and the player must seek there before it can start — that is what +faststart fixes.
+function isFastStart(fp) {
+  let fd;
+  try {
+    fd = fs.openSync(fp, 'r');
+    const buf = Buffer.alloc(4096);
+    const read = fs.readSync(fd, buf, 0, 4096, 0);
+    let off = 0;
+    while (off + 8 <= read) {
+      let size = buf.readUInt32BE(off);
+      const type = buf.toString('latin1', off + 4, off + 8);
+      if (type === 'moov') return true;
+      if (type === 'mdat') return false;
+      if (size === 1) {                                   // 64-bit largesize follows the header
+        if (off + 16 > read) return true;
+        size = Number(buf.readBigUInt64BE(off + 8));
+      } else if (size === 0) return true;                 // box runs to EOF
+      if (size < 8) return true;                          // malformed — leave it alone
+      off += size;
+    }
+    return true;                                          // couldn't tell; assume fine
+  } catch (e) {
+    return true;
+  } finally {
+    if (fd !== undefined) { try { fs.closeSync(fd); } catch (e) {} }
+  }
+}
+
+function probe(fp, cb) {
+  execFile('ffprobe', ['-v', 'error', '-select_streams', 'v:0',
+    '-show_entries', 'stream=width,height,codec_name',
+    '-show_entries', 'format=bit_rate,duration',
+    '-of', 'json', fp], { timeout: PROBE_TIMEOUT, maxBuffer: 1 << 20 }, (err, stdout) => {
+    if (err) return cb(err);
+    let info;
+    try { info = JSON.parse(stdout); } catch (e) { return cb(e); }
+    const s = (info.streams || [])[0], f = info.format || {};
+    if (!s || !s.width) return cb(new Error('no video stream'));
+    cb(null, {
+      width: +s.width, height: +s.height, codec: s.codec_name || '',
+      bitrate: +f.bit_rate || 0, duration: +f.duration || 0,
+    });
+  });
+}
+
+// Cap the LONGEST side at 1280 (so 1920x1080 → 1280x720, and portrait 1080x2340 → 591x1280) while
+// preserving aspect. force_divisible_by=2 keeps dimensions legal for H.264.
+const SCALE = 'scale=w=' + OK_DIMENSION + ':h=' + OK_DIMENSION +
+  ':force_original_aspect_ratio=decrease:force_divisible_by=2';
+
+function buildArgs(src, out, meta) {
+  const maxDim = Math.max(meta.width, meta.height);
+  const lightEnough = meta.bitrate > 0 && meta.bitrate <= OK_BITRATE && maxDim <= OK_DIMENSION;
+  // Already small enough and only the atom order is wrong → remux, no re-encode. Seconds, not minutes.
+  if (lightEnough && /^(h264|avc1)$/i.test(meta.codec)) {
+    return ['-y', '-i', src, '-c', 'copy', '-movflags', '+faststart', '-f', 'mp4', out];
+  }
+  return ['-y', '-i', src,
+    '-map', '0:v:0', '-map', '0:a:0?',                    // audio optional — silent clips exist
+    '-vf', SCALE,
+    '-c:v', 'libx264', '-preset', 'veryfast', '-profile:v', 'high', '-level', '4.0',
+    '-crf', '24', '-maxrate', '2500k', '-bufsize', '5000k',
+    '-g', '48', '-pix_fmt', 'yuv420p',                    // 2s keyframes: smooth seeking
+    '-c:a', 'aac', '-b:a', '128k', '-ac', '2',
+    '-movflags', '+faststart', '-f', 'mp4', out];
+}
+
+function pump() {
+  while (running < MAX_CONCURRENT && queue.length) {
+    const id = queue.shift();
+    running++;
+    transcode(id, () => { running--; pending.delete(id); pump(); });
+  }
+}
+
+function transcode(id, done) {
+  const src = path.join(UPLOADS_DIR, id);
+  const out = webPath(id);
+  const tmp = out + '.part';
+  if (!src.startsWith(UPLOADS_DIR)) return done();
+  fs.stat(src, (e, srcStat) => {
+    if (e) return done();
+    probe(src, (perr, meta) => {
+      if (perr) { failed.add(id); return done(); }
+      const maxDim = Math.max(meta.width, meta.height);
+      // Nothing to gain: already light, already sized, already faststart.
+      if (meta.bitrate > 0 && meta.bitrate <= OK_BITRATE && maxDim <= OK_DIMENSION && isFastStart(src)) {
+        failed.add(id); // "no rendition needed" — same effect: stop reconsidering it
+        return done();
+      }
+      execFile('ffmpeg', buildArgs(src, tmp, meta), { timeout: XCODE_TIMEOUT, maxBuffer: 1 << 20 }, (xerr) => {
+        if (xerr) {
+          try { fs.unlinkSync(tmp); } catch (_) {}
+          failed.add(id);
+          return done();
+        }
+        let outStat;
+        try { outStat = fs.statSync(tmp); } catch (_) { failed.add(id); return done(); }
+        // A rendition bigger than the original helps nobody — throw it away and stream the original.
+        if (!outStat.size || outStat.size >= srcStat.size) {
+          try { fs.unlinkSync(tmp); } catch (_) {}
+          failed.add(id);
+          return done();
+        }
+        try { fs.renameSync(tmp, out); } catch (_) { try { fs.unlinkSync(tmp); } catch (__) {} }
+        done();
+      });
+    });
+  });
+}
+
+// Queue a freshly uploaded (or first-played) video. Cheap and idempotent: safe to call on every
+// /stream hit, which is also how pre-existing uploads get backfilled.
+function ensureWebRendition(id, mime) {
+  if (!/^video\//.test(mime || '')) return;
+  if (pending.has(id) || failed.has(id) || hasWebRendition(id)) return;
+  pending.add(id);
+  queue.push(id);
+  pump();
+}
+
+// Drop derived files when the attachment goes away.
+function dropDerived(id) {
+  for (const p of [webPath(id), webPath(id) + '.part', path.join(UPLOADS_DIR, id + '.thumb.jpg')]) {
+    try { fs.unlinkSync(p); } catch (e) {}
+  }
+  pending.delete(id); failed.delete(id);
+}
+
+// One-off catch-up for videos uploaded before this module existed (and after a restore). Renditions
+// persist on the data volume, so on a normal restart this finds nothing to do and costs one query.
+// Deliberately delayed and rate-limited by the same 2-at-a-time queue — boot must not stall on it.
+function backfill() {
+  setTimeout(() => {
+    let rows = [];
+    try { rows = require('./repos').attachments.allVideos(); } catch (e) { return; }
+    let queued = 0;
+    for (const r of rows) {
+      if (hasWebRendition(r.id)) continue;
+      try { if (!fs.statSync(path.join(UPLOADS_DIR, r.id)).size) continue; } catch (e) { continue; }
+      ensureWebRendition(r.id, r.mime); queued++;
+    }
+    if (queued) console.log('[media] backfilling streaming renditions for ' + queued + ' video(s)');
+  }, 15000).unref();
+}
+
+module.exports = { ensureWebRendition, hasWebRendition, webPath, dropDerived, backfill };
diff --git a/server/public/home.html b/server/public/home.html
index dfb4576..8c8703d 100644
--- a/server/public/home.html
+++ b/server/public/home.html
@@ -1165,7 +1165,7 @@
 </head>
 <body>
 <script src="/icons.js?v=6"></script>
-<script>window.__BUILD='2026-07-22-batch159';console.log('%cBiz Connect','color:#1F3B73;font-weight:bold','build '+window.__BUILD);
+<script>window.__BUILD='2026-07-23-batch160';console.log('%cBiz Connect','color:#1F3B73;font-weight:bold','build '+window.__BUILD);
 // Emoji are rendered with the OS's own (colour) emoji font — instant, zero network.
 //
 // We used to run Twemoji over every emoji, which swapped each one for an <img> pulled INDIVIDUALLY from
@@ -2013,7 +2013,7 @@ function bubbleHTML(m){
   const A = m.attachment;
   const att = A ? (
       A.isImage ? '<img class="att-img" src="/files/'+pEsc(A.id)+'" data-img="/files/'+pEsc(A.id)+'" alt="'+pEsc(A.name)+'" title="Click to view">'
-    : A.isVideo ? '<div class="att-vid"><video class="att-vid-v" src="/files/'+pEsc(A.id)+'#t=0.1" poster="/thumbs/'+pEsc(A.id)+'" controls playsinline preload="none" title="'+pEsc(A.name)+'"></video><span class="att-vid-spin" aria-hidden="true"></span></div>'
+    : A.isVideo ? '<div class="att-vid"><video class="att-vid-v" src="/stream/'+pEsc(A.id)+'#t=0.1" poster="/thumbs/'+pEsc(A.id)+'" controls playsinline preload="none" title="'+pEsc(A.name)+'"></video><span class="att-vid-spin" aria-hidden="true"></span></div>'
     : '<a class="att-file" href="/files/'+pEsc(A.id)+'" download="'+pEsc(A.name)+'">'+ic('file',15)+' <span>'+pEsc(A.name)+'</span> <span class="att-sz">'+fmtSize(A.size)+'</span></a>') : '';
   const mentionsMe=convoIsGroup && !mine && Array.isArray(m.mentions) && (m.mentions.includes(ME.id)||m.mentions.includes('everyone'));
   // DM ticks: sent (1 grey) → delivered (2 grey) → read (2 yellow).
diff --git a/server/repos.js b/server/repos.js
index 35f2606..f21e737 100644
--- a/server/repos.js
+++ b/server/repos.js
@@ -296,6 +296,8 @@ const attachments = {
     db.prepare('INSERT INTO attachments (id,team_id,uploader_id,name,mime,size,created_at) VALUES (?,?,?,?,?,?,?)')
       .run(id, teamId, uploaderId, name, mime || null, size || 0, now()),
   byId: (id) => db.prepare('SELECT * FROM attachments WHERE id=?').get(id),
+  // Newest first: media.js backfills streaming renditions for uploads that predate it.
+  allVideos: () => db.prepare("SELECT id, mime FROM attachments WHERE mime LIKE 'video/%' ORDER BY created_at DESC").all(),
 };
 
 const scheduledMeetings = {
diff --git a/server/routes.js b/server/routes.js
index eed8015..75de229 100644
--- a/server/routes.js
+++ b/server/routes.js
@@ -1716,6 +1716,9 @@ route('POST', '/api/messages/upload', async (req, res) => {
       try { fs.renameSync(tmp, path.join(UPLOADS_DIR, id)); }
       catch (e) { try { fs.unlinkSync(tmp); } catch (_) {} return finish(500, { error: 'could not store file' }); }
       R.attachments.create({ id, teamId: u.team_id, uploaderId: u.id, name, mime, size: total });
+      // Videos: build the capped/faststart streaming rendition in the background so it is ready before
+      // anyone taps play. Never blocks the upload response, and playback falls back to the original.
+      try { require('./media').ensureWebRendition(id, mime); } catch (e) {}
       finish(200, { id, name, mime, size: total });
     });
   });
diff --git a/server/server.js b/server/server.js
index c5474a1..a77fd55 100644
--- a/server/server.js
+++ b/server/server.js
@@ -34,6 +34,7 @@ wss.on('connection', onConnection);
 
 server.listen(PORT, () => {
   console.log(`HTTP  on http://localhost:${PORT}`);
+  try { require('./media').backfill(); } catch (e) {} // streaming renditions for older video uploads
 });
 
 // HTTPS — required so other devices can share their screen (browsers block
diff --git a/server/static.js b/server/static.js
index 6e41d6c..2a156c5 100644
--- a/server/static.js
+++ b/server/static.js
@@ -6,6 +6,7 @@ const R = require('./repos');
 const { json } = require('./lib');
 const { currentUser } = require('./session');
 const { PUBLIC_DIR, REC_DIR, TRANS_DIR, UPLOADS_DIR, DOWNLOADS_DIR } = require('./config');
+const media = require('./media');
 
 const MIME = { '.html': 'text/html', '.js': 'text/javascript', '.css': 'text/css', '.png': 'image/png', '.jpg': 'image/jpeg', '.jpeg': 'image/jpeg', '.svg': 'image/svg+xml', '.ico': 'image/x-icon', '.json': 'application/json', '.webmanifest': 'application/manifest+json' };
 
@@ -37,6 +38,32 @@ function authAttachment(id, u) {
   return a;
 }
 
+// Serve a file with HTTP Range support (206 Partial Content) — required for reliable iOS video
+// streaming and for seeking anywhere other than the start.
+function sendRanged(req, res, fp, headers) {
+  return fs.stat(fp, (err, st) => {
+    if (err) return json(res, 404, { error: 'not found' });
+    const total = st.size;
+    const base = Object.assign({ 'Accept-Ranges': 'bytes' }, headers);
+    const mm = req.headers.range && /^bytes=(\d*)-(\d*)$/.exec(req.headers.range);
+    if (mm) {
+      let start = mm[1] ? parseInt(mm[1], 10) : 0;
+      let end = mm[2] ? parseInt(mm[2], 10) : total - 1;
+      if (isNaN(start)) start = 0;
+      if (isNaN(end) || end >= total) end = total - 1;
+      if (start > end || start >= total) { res.writeHead(416, { 'Content-Range': 'bytes */' + total }); return res.end(); }
+      res.writeHead(206, Object.assign({}, base, { 'Content-Range': 'bytes ' + start + '-' + end + '/' + total, 'Content-Length': (end - start + 1) }));
+      const rs = fs.createReadStream(fp, { start, end });
+      rs.on('error', () => { try { res.destroy(); } catch (e) {} });
+      return rs.pipe(res);
+    }
+    res.writeHead(200, Object.assign({}, base, { 'Content-Length': total }));
+    const rs = fs.createReadStream(fp);
+    rs.on('error', () => { try { res.destroy(); } catch (e) {} });
+    rs.pipe(res);
+  });
+}
+
 function serveStatic(req, res) {
   let p = req.url.split('?')[0];
   if (p === '/') p = '/index.html';
@@ -201,6 +228,26 @@ function handleGet(req, res) {
       send();
     });
   }
+  // PLAYBACK source for in-chat videos. Prefers the capped/faststart rendition built by media.js; falls
+  // back to the original bytes while that is still transcoding, so a video is never unplayable. The
+  // download button keeps pointing at /files, which always serves the untouched original.
+  if (pathOnly.startsWith('/stream/')) {
+    const u = currentUser(req);
+    if (!u) return json(res, 401, { error: 'unauthorized' });
+    const id = path.basename(decodeURIComponent(pathOnly));
+    const a = authAttachment(id, u);
+    if (!a) return json(res, 404, { error: 'not found' });
+    media.ensureWebRendition(id, a.mime);        // idempotent — also backfills pre-existing uploads
+    const ready = media.hasWebRendition(id);
+    const fp = ready ? media.webPath(id) : path.join(UPLOADS_DIR, id);
+    if (!fp.startsWith(UPLOADS_DIR)) return json(res, 403, { error: 'forbidden' });
+    return sendRanged(req, res, fp, {
+      'Content-Type': ready ? 'video/mp4' : (a.mime || 'application/octet-stream'),
+      'Content-Disposition': 'inline',
+      // Don't let a browser cache the heavy original past the point the rendition lands.
+      'Cache-Control': ready ? 'private, max-age=86400' : 'private, max-age=60',
+    });
+  }
   if (pathOnly.startsWith('/files/')) {
     const u = currentUser(req);
     if (!u) return json(res, 401, { error: 'unauthorized' });
@@ -211,28 +258,10 @@ function handleGet(req, res) {
     if (!fp.startsWith(UPLOADS_DIR)) return json(res, 403, { error: 'forbidden' });
     const inline = /^(image|video|audio)\//.test(a.mime || ''); // media opens/plays inline; other files download
     const safeName = String(a.name || 'file').replace(/[\r\n"]/g, '');
-    return fs.stat(fp, (err, st) => {
-      if (err) return json(res, 404, { error: 'not found' });
-      const total = st.size;
-      const base = { 'Content-Type': a.mime || 'application/octet-stream', 'Content-Disposition': (inline ? 'inline' : 'attachment') + '; filename="' + safeName + '"', 'Cache-Control': 'private, max-age=86400', 'Accept-Ranges': 'bytes' };
-      // Range support — required for reliable iOS video streaming/seeking (206 Partial Content).
-      const range = req.headers.range;
-      const mm = range && /^bytes=(\d*)-(\d*)$/.exec(range);
-      if (mm) {
-        let start = mm[1] ? parseInt(mm[1], 10) : 0;
-        let end = mm[2] ? parseInt(mm[2], 10) : total - 1;
-        if (isNaN(start)) start = 0;
-        if (isNaN(end) || end >= total) end = total - 1;
-        if (start > end || start >= total) { res.writeHead(416, { 'Content-Range': 'bytes */' + total }); return res.end(); }
-        res.writeHead(206, Object.assign({}, base, { 'Content-Range': 'bytes ' + start + '-' + end + '/' + total, 'Content-Length': (end - start + 1) }));
-        const rs = fs.createReadStream(fp, { start, end });
-        rs.on('error', () => { try { res.destroy(); } catch (e) {} });
-        return rs.pipe(res);
-      }
-      res.writeHead(200, Object.assign({}, base, { 'Content-Length': total }));
-      const rs = fs.createReadStream(fp);
-      rs.on('error', () => { try { res.destroy(); } catch (e) {} });
-      rs.pipe(res);
+    return sendRanged(req, res, fp, {
+      'Content-Type': a.mime || 'application/octet-stream',
+      'Content-Disposition': (inline ? 'inline' : 'attachment') + '; filename="' + safeName + '"',
+      'Cache-Control': 'private, max-age=86400',
     });
   }
   return serveStatic(req, res);