Match Qwen compiled verification gate math

src/engine/metal/qwen_mtplx/session_checkpoint.rs

@@ -30,6 +30,8 @@ struct Manifest {
     identity: Identity,
     tag: [u8; 32],
     #[serde(default)]
+    verify_gate: bool,
+    #[serde(default)]
     images: Vec<crate::engine::qwen_vision::Identity>,
     tokens: Vec<i32>,
     lazy_kv: bool,
@@ -116,6 +118,7 @@ impl LiveSession {
             check_abort(&abort)?;
             let offset = file.stream_position().map_err(|e| e.to_string())?;
             let manifest = serde_json::to_vec(&Manifest {
+                verify_gate: true,
                 identity,
                 tag,
                 images: self.current_images.clone(),
@@ -185,6 +188,11 @@ impl LiveSession {
         if !crate::engine::qwen_vision::compatible(&manifest.images) {
             return Err("Qwen vision cache policy differs; rebuild the cache".into());
         }
+        if identity.mtp && !manifest.verify_gate {
+            return Err(
+                "Qwen checkpoint uses obsolete MTP attention-gate math; rebuild the cache".into(),
+            );
+        }
         if !manifest.payload["mtp_history_snapshot"].is_null() != identity.mtp {
             return Err("Qwen checkpoint MTP payload differs".into());
         }
@@ -433,7 +441,46 @@ fn qwen_checkpoint_roundtrip_and_failed_restore_preserve_live_state() {
     }
     assert_eq!(live.tag, [9; 32]);
     assert_eq!(fingerprint(&live), original);
+    // Files predating the verify-phase law lack this field. Only MTP state
+    // changed numerically; old AR files must remain exactly restorable.
+    let without_gate_policy = |bytes: &[u8]| {
+        let offset = u64::from_le_bytes(bytes[8..16].try_into().unwrap()) as usize;
+        let mut manifest: serde_json::Value = serde_json::from_slice(&bytes[offset..]).unwrap();
+        manifest.as_object_mut().unwrap().remove("verify_gate");
+        let mut result = bytes[..offset].to_vec();
+        result.extend(serde_json::to_vec(&manifest).unwrap());
+        result
+    };
+    fs::write(&broken, without_gate_policy(&valid)).unwrap();
+    assert!(
+        live.load_checkpoint(&broken, identity, &streams, stream, |_| {}, || false)
+            .unwrap_err()
+            .contains("obsolete MTP attention-gate")
+    );
+    assert_eq!(fingerprint(&live), original);
+    live.current.as_mut().unwrap().mtp = None;
+    let ar_identity = Identity {
+        mtp: false,
+        ..identity
+    };
+    let ar_original = fingerprint(&live);
+    live.save_checkpoint(
+        &path,
+        ar_identity,
+        [10; 32],
+        &streams,
+        stream,
+        |_| {},
+        || false,
+    )
+    .unwrap();
+    fs::write(&broken, without_gate_policy(&fs::read(&path).unwrap())).unwrap();
+    assert!(
+        live.load_checkpoint(&broken, ar_identity, &streams, stream, |_| {}, || false)
+            .unwrap()
+    );
+    assert_eq!(fingerprint(&live), ar_original);
     println!(
-        "Qwen checkpoint: exact payload roundtrip, interrupted save, incompatible/corrupt restore passed"
+        "Qwen checkpoint: exact roundtrip, cancellation/corruption controls, old MTP rejected and old AR restored"
     );
 }