--- a/server/media-cache.js
+++ b/server/media-cache.js
@@ -24,6 +24,7 @@
   statSync, statfsSync, unlinkSync,
 } from 'node:fs';
 import { join } from 'node:path';
+import { sha256File } from './hash.js';
 
 export const HIGH = 0; // explicit save / Broken re-download
 export const LOW  = 1; // auto-cache after a play
@@ -167,6 +168,9 @@
   freeBytes = () => { const s = statfsSync(dir); return s.bavail * s.bsize; },
   now = () => Date.now(),
   log = console,
+  hashFile = sha256File,     // server-computed SHA-256 = the file's P2P content id
+  onReady = null,            // ({ id, gen, path, sha256, size, … }) after a validated copy lands
+  backfillDelayMs = 30_000,  // hash pre-existing copies this long after init()
 } = {}) {
   const tmpDir = join(dir, '.tmp');
   const fileFor = (id, gen, kind = 'mp4') => join(dir, `${id}.${gen}.${kind}`);
@@ -236,6 +240,20 @@
     }
   }
 
+  // Tell the P2P layer a validated file with a server-computed hash exists.
+  // Never throws and never delays the caller.
+  function notifyReady(id, gen, sha256, probe, metaJson) {
+    if (!onReady || !sha256) return;
+    let meta = {};
+    try { meta = JSON.parse(metaJson || '{}'); } catch { /* keep {} */ }
+    Promise.resolve()
+      .then(() => onReady({
+        id, gen, path: fileFor(id, gen), sha256, size: probe.size, height: probe.height,
+        vcodec: probe.vcodec, acodec: probe.acodec, duration: probe.duration, meta,
+      }))
+      .catch((e) => log.warn?.(`[media] onReady ${id} failed: ${e.message}`));
+  }
+
   async function setStatus(job, status) {
     job.status = status;
     await db.upsertMedia(job.id, { status, updated_at: now() });
@@ -308,6 +326,7 @@
         '-map', '0:a:0', '-c', 'copy', '-movflags', '+faststart', '-f', 'mp4', m4a]);
       if (ex.code !== 0 || sizeOf(m4a) < 1024) throw new Error('audio sidecar failed: ' + tail(ex.stderr));
 
+      const sha256 = await hashFile(norm);
       const prev = await db.getMedia(id);
       const gen = ((prev && prev.gen) || 0) + 1;
       renameSync(norm, fileFor(id, gen));
@@ -318,13 +337,14 @@
         size: probe.size + sizeOf(fileFor(id, gen, 'm4a')),
         height: probe.height, vcodec: probe.vcodec, acodec: probe.acodec, duration: probe.duration,
         optimized: 0, meta: JSON.stringify(metaFromInfo(info, probe.duration)),
-        attempts: 0, error: null, retry_at: 0, updated_at: t, last_access: t,
+        attempts: 0, error: null, retry_at: 0, updated_at: t, last_access: t, sha256,
       };
       await db.upsertMedia(id, fields);
       removeFiles(id, gen);
       job.status = 'ready';
       log.info?.(`[media] cached ${id} ${probe.height}p ${fmtMB(fields.size)}`);
       enqueueOptimize(id);
+      notifyReady(id, gen, sha256, probe, fields.meta);
       return { ...(prev || {}), video_id: id, ...fields };
     } catch (err) {
       job.status = 'failed';
@@ -427,6 +447,7 @@
         log.info?.(`[media] kept original ${id}: ${probe.vcodec} ${fmtMB(probe.size)} vs ${fmtMB(oldSize)}`);
         return;
       }
+      const sha256 = await hashFile(out);
       const gen = row.gen + 1;
       renameSync(out, fileFor(id, gen));
       const oldAudio = fileFor(id, row.gen, 'm4a');
@@ -434,8 +455,9 @@
       try { linkSync(oldAudio, newAudio); } catch { copyFileSync(oldAudio, newAudio); }
       await db.upsertMedia(id, {
         gen, size: probe.size + sizeOf(newAudio), height: probe.height, vcodec: probe.vcodec,
-        duration: probe.duration, optimized: OPT_REV, updated_at: now(),
+        duration: probe.duration, optimized: OPT_REV, updated_at: now(), sha256,
       });
+      notifyReady(id, gen, sha256, probe, cur.meta);
       removeFiles(id, gen, OLD_GEN_GRACE_MS);
       log.info?.(`[media] optimized ${id}: ${fmtMB(oldSize)} → ${fmtMB(probe.size)} (${probe.vcodec})`);
     } catch (err) {
@@ -622,7 +644,34 @@
     }
     const s = await db.mediaStats();
     log.info?.(`[media] ${s.count} cached (${fmtMB(s.bytes)}), ${resumed} job(s) resumed, dir ${dir}`);
+    if (backfillDelayMs >= 0) {
+      const t = setTimeout(() => backfillHashes().catch(() => {}), backfillDelayMs);
+      t.unref?.();
+    }
+  }
+
+  // Copies cached before hashing existed get their sha256 in the background,
+  // one at a time, so their content ids can be admitted too.
+  async function backfillHashes() {
+    let n = 0;
+    for (const r of await db.listMedia()) {
+      if (r.status !== 'ready' || r.sha256) continue;
+      const p = fileFor(r.video_id, r.gen);
+      if (!existsSync(p)) continue;
+      try {
+        const sha256 = await hashFile(p);
+        const cur = await db.getMedia(r.video_id);
+        if (!cur || cur.status !== 'ready' || cur.gen !== r.gen) continue;
+        await db.upsertMedia(r.video_id, { sha256 });
+        notifyReady(r.video_id, r.gen, sha256,
+          { size: sizeOf(p), height: r.height, vcodec: r.vcodec, acodec: r.acodec, duration: r.duration }, r.meta);
+        n++;
+      } catch (e) {
+        log.warn?.(`[media] hash backfill ${r.video_id}: ${e.message}`);
+      }
+    }
+    return n;
   }
 
-  return { init, ensureCached, redownload, verify, getReady, filePath, touch, status, stats, isMediaId };
+  return { init, ensureCached, redownload, verify, getReady, filePath, touch, status, stats, isMediaId, backfillHashes };
 }