diff --git a/frontend/app-bootstrap.js b/frontend/app-bootstrap.js
index 3152544..398ca77 100644
--- a/frontend/app-bootstrap.js
+++ b/frontend/app-bootstrap.js
@@ -12,26 +12,46 @@
     // consumption can finish after that event; boot exactly once in that case.
     if(domReady)root.boot();
   }
+  async function external(url) {
+    await new Promise((resolve,reject)=>{
+      const node=doc.createElement('script');node.src=url;
+      node.onload=()=>{if(domReady&&!appSawDom)root.boot();resolve();};node.onerror=()=>reject(Error('Unable to load player'));
+      doc.head.append(node);
+    });
+  }
+  async function verified(response, expected) {
+    const bytes=await response.clone().arrayBuffer();
+    const digest=new Uint8Array(await root.crypto.subtle.digest('SHA-256',bytes));
+    const hex=Array.from(digest,b=>b.toString(16).padStart(2,'0')).join('');
+    if(hex.slice(0,10)!==expected)throw Error('Player body hash mismatch');
+    return {bytes,digest};
+  }
   async function start() {
-    if(!manifest.appCache || !root.crypto?.subtle) {
-      await new Promise((resolve,reject)=>{
-        const node=doc.createElement('script');node.src=key;
-        node.onload=()=>{if(domReady&&!appSawDom)root.boot();resolve();};node.onerror=()=>reject(Error('Unable to load player'));
-        doc.head.append(node);
-      });
-      return;
-    }
+    if(!manifest.appCache || !root.crypto?.subtle)return external(key);
     const expected=manifest.files['/app.js'].h;
     let cache;
     try { cache=await root.caches?.open('ytplayer-assets'); } catch {}
     let response=await cache?.match(key);
-    if(!response || response.headers.get('X-Asset-Hash')!==expected)response=await (root.Lazy.appResponse || root.fetch(key,{credentials:'same-origin'}));
-    if(!response.ok || response.headers.get('X-Asset-Hash')!==expected)throw Error('Player asset hash mismatch');
-    const bytes=await response.clone().arrayBuffer();
-    const digest=new Uint8Array(await root.crypto.subtle.digest('SHA-256',bytes));
-    const hex=Array.from(digest,b=>b.toString(16).padStart(2,'0')).join('');
+    if(!response || response.headers.get('X-Asset-Hash')!==expected) {
+      try { response=await (root.Lazy.appResponse || root.fetch(key,{credentials:'same-origin'})); } catch {}
+    }
+    if(!response?.ok || response.headers.get('X-Asset-Hash')!==expected) {
+      // Preserve staged N-1 boot after an individual cache eviction. Only the
+      // controller can serve this old URL safely; never fetch stale URLs bare.
+      const state=cache && await root.AssetSyncCore.state(cache);
+      const previousKey=state && root.AssetSyncCore.fallback(manifest,state.previous,'/app.js');
+      const previous=previousKey && await cache.match(previousKey);
+      const previousHash=state?.previous?.files['/app.js']?.h;
+      if(root.navigator?.serviceWorker?.controller && previous?.ok && previous.headers.get('X-Asset-Hash')===previousHash) {
+        await verified(previous,previousHash);
+        delete root.Lazy.appResponse;
+        return external(previousKey);
+      }
+      throw Error('Player asset hash mismatch');
+    }
+    const {bytes,digest}=await verified(response,expected);
     const permission="'sha256-"+root.btoa(String.fromCharCode(...digest))+"'";
-    if(hex.slice(0,10)!==expected || !doc.querySelector('meta[http-equiv="Content-Security-Policy"]').content.includes(permission))throw Error('Player body hash mismatch');
+    if(!doc.querySelector('meta[http-equiv="Content-Security-Policy"]').content.includes(permission))throw Error('Player body hash mismatch');
     // CacheStorage may share a full quota with saved music. Never remove data
     // to make room; boot from these verified bytes even if caching is refused.
     try { await cache?.put(key,response); } catch(error) { root.console.warn('[app-cache]',error.message); }
diff --git a/frontend/app-bootstrap.test.js b/frontend/app-bootstrap.test.js
index 912b9e0..ea11df6 100644
--- a/frontend/app-bootstrap.test.js
+++ b/frontend/app-bootstrap.test.js
@@ -3,16 +3,18 @@ const assert=require('node:assert/strict');
 const {webcrypto,createHash}=require('node:crypto');
 const vm=require('node:vm');
 const {readFileSync}=require('node:fs');
-function fixture({cached=false,tampered=false,enabled=true,loading=false,quota=false,insecure=false}={}){
+function fixture({cached=false,tampered=false,enabled=true,loading=false,quota=false,insecure=false,previous=false,contract=1}={}){
  const source='window.appRuns=(window.appRuns||0)+1; window.boot=()=>window.bootRuns=(window.bootRuns||0)+1; document.addEventListener("DOMContentLoaded",boot);',hash=createHash('sha256').update(source).digest('hex'), key='/app.js?v='+hash.slice(0,10);
+ const oldSource='window.oldApp=true;'+source, oldHash=createHash('sha256').update(oldSource).digest('hex').slice(0,10), oldKey='/app.js?v='+oldHash;
  const held=new Map(), appended=[],listeners={};let calls=0;
  const response=()=>new Response(tampered?'bad':source,{headers:{'X-Asset-Hash':hash.slice(0,10)}});
  if(cached)held.set(key,response());
- const root={crypto:insecure?undefined:webcrypto,TextDecoder,Uint8Array,btoa,console,Lazy:{manifest:{appCache:enabled,files:{'/app.js':{h:hash.slice(0,10)}}},url:()=>key},fetch:async()=>{calls++;return response();},caches:{open:async()=>({match:async k=>held.get(k)?.clone(),put:async(k,r)=>{if(quota)throw Error('quota');held.set(k,r.clone());}})}};
- const doc=root.document={readyState:loading?'loading':'complete',querySelector:()=>({content:"script-src 'self' 'sha256-"+Buffer.from(hash,'hex').toString('base64')+"'"}),addEventListener:(name,fn)=>(listeners[name] ||= []).push(fn),createElement:()=>({remove(){this.removed=true;}}),head:{append(node){appended.push(node);if(node.textContent)vm.runInContext(node.textContent,context);else {vm.runInContext(source,context);queueMicrotask(()=>node.onload());}}}};
+ if(previous){held.set(oldKey,new Response(oldSource,{headers:{'X-Asset-Hash':oldHash}}));held.set('/__ytp_asset_state',Response.json({previous:{files:{'/app.js':{h:oldHash}},groups:{core:{contract,files:['/app.js']}}}}));}
+ const root={crypto:insecure?undefined:webcrypto,TextDecoder,Uint8Array,btoa,console,AssetSyncCore:require('./asset-sync-core'),navigator:{serviceWorker:{controller:{}}},Lazy:{manifest:{groups:{core:{contract:1,files:['/app.js']}},appCache:enabled,files:{'/app.js':{h:hash.slice(0,10)}}},url:()=>key},fetch:async()=>{calls++;return previous?new Response(oldSource,{headers:{'X-Asset-Hash':oldHash}}):response();},caches:{open:async()=>({match:async k=>held.get(k)?.clone(),put:async(k,r)=>{if(quota)throw Error('quota');held.set(k,r.clone());}})}};
+ const doc=root.document={readyState:loading?'loading':'complete',querySelector:()=>({content:"script-src 'self' 'sha256-"+Buffer.from(hash,'hex').toString('base64')+"'"}),addEventListener:(name,fn)=>(listeners[name] ||= []).push(fn),createElement:()=>({remove(){this.removed=true;}}),head:{append(node){appended.push(node);if(node.textContent)vm.runInContext(node.textContent,context);else {vm.runInContext(node.src===oldKey?oldSource:source,context);queueMicrotask(()=>node.onload());}}}};
  root.window=root;const context=vm.createContext(root);
  vm.runInContext(readFileSync(require.resolve('./app-bootstrap.js'),'utf8'),context);
- return {root,held,appended,listeners,key,calls:()=>calls,doc,response};
+ return {root,held,appended,listeners,key,calls:()=>calls,doc,response,oldKey};
 }
 test('cold boot caches verified app before execution so worker skips its download',async()=>{
  const f=fixture();await f.root.AppBootstrap.ready;assert.equal(f.calls(),1);assert.ok(f.held.has(f.key));assert.equal(f.root.appRuns,1);assert.equal(f.root.bootRuns,1);assert.equal(f.appended[0].src,undefined);assert.equal(f.appended[0].removed,true);
@@ -43,3 +45,12 @@ test('the early head response is consumed without a second page fetch',async()=>
 test('insecure local HTTP boot retains external execution without WebCrypto',async()=>{
  const f=fixture({insecure:true});await f.root.AppBootstrap.ready;assert.equal(f.calls(),0);assert.equal(f.appended[0].src,f.key);assert.equal(f.root.bootRuns,1);
 });
+
+
+test('missing current app uses verified same-contract N-1 offline through the controlling worker',async()=>{
+ const f=fixture({previous:true});await f.root.AppBootstrap.ready;
+ assert.equal(f.appended[0].src,f.oldKey);assert.equal(f.root.oldApp,true);assert.equal(f.root.bootRuns,1);assert.equal(f.held.has(f.key),false);
+});
+test('an incompatible previous core never executes against the new shell',async()=>{
+ const f=fixture({previous:true,contract:2});await assert.rejects(f.root.AppBootstrap.ready,/hash/);assert.equal(f.root.appRuns,undefined);
+});