diff --git a/crates/wasm-actor/tests/wasm_actor.rs b/crates/wasm-actor/tests/wasm_actor.rs
index bc30325..c4556e0 100644
--- a/crates/wasm-actor/tests/wasm_actor.rs
+++ b/crates/wasm-actor/tests/wasm_actor.rs
@@ -11605,4 +11605,137 @@ fn broadcast_to_ten_actors() {
     let msgs: Vec<Vec<u8>> = std::iter::from_fn(|| inbox.try_recv().map(|m| m.0)).collect();
     assert_eq!(msgs.len(), 10);
     assert!(msgs.iter().all(|m| m == b"broadcast"));
+}
+
+// ── Cycle 81 ─────────────────────────────────────────────────────────────────
+
+// Guest that does bitwise NOT on each byte
+#[test]
+fn guest_bitwise_not_each_byte() {
+    let wat = r#"(module
+        (import "swactor" "send" (func $send (param i32 i32 i32)))
+        (memory (export "memory") 1)
+        (func (export "alloc") (param $len i32) (result i32) i32.const 1024)
+        (func (export "handle") (param $ptr i32) (param $len i32)
+            (local $i i32)
+            (if (i32.lt_u (local.get $len) (i32.const 33)) (then return))
+            ;; NOT each byte in the payload portion
+            (local.set $i (i32.const 32))
+            (block $exit
+                (loop $loop
+                    (br_if $exit (i32.ge_u (local.get $i) (local.get $len)))
+                    (i32.store8
+                        (i32.add (local.get $ptr) (local.get $i))
+                        (i32.xor
+                            (i32.load8_u (i32.add (local.get $ptr) (local.get $i)))
+                            (i32.const 0xFF)))
+                    (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                    (br $loop)
+                )
+            )
+            (call $send
+                (local.get $ptr)
+                (i32.add (local.get $ptr) (i32.const 32))
+                (i32.sub (local.get $len) (i32.const 32)))
+        )
+    )"#;
+    let engine = SharedEngine::new().unwrap();
+    let actor = WasmActorBuilder::new(engine, wat::parse_str(wat).unwrap())
+        .build().unwrap();
+    let rt = Runtime::new(RuntimeConfig::default());
+    let inbox = rt.new_inbox::<ByteMessage>().unwrap();
+    let addr = rt.spawn(actor).unwrap();
+
+    rt.send_to(addr, framed_msg(inbox.addr(), &[0x00, 0xFF, 0x55, 0xAA])).unwrap();
+    rt.tick();
+
+    let resp = inbox.try_recv().expect("NOT response");
+    assert_eq!(resp.0, vec![0xFF, 0x00, 0xAA, 0x55]);
+}
+
+// Stop actor during multi-threaded runtime (MT stop)
+#[test]
+fn stop_wasm_actor_on_mt_runtime() {
+    let engine = SharedEngine::new().unwrap();
+    let actor = WasmActorBuilder::new(engine, guest_wasm("echo")).build().unwrap();
+    let mut cfg = RuntimeConfig::default();
+    cfg.num_threads = 2;
+    let rt = Runtime::new(cfg);
+    let inbox = rt.new_inbox::<ByteMessage>().unwrap();
+    let addr = rt.spawn(actor).unwrap();
+
+    rt.send_to(addr, framed_msg(inbox.addr(), b"mt")).unwrap();
+    let handle = rt.run().unwrap();
+    std::thread::sleep(std::time::Duration::from_millis(100));
+
+    // Drain inbox
+    let mut count = 0;
+    for _ in 0..20 {
+        count += std::iter::from_fn(|| inbox.try_recv()).count();
+        if count >= 1 { break; }
+        std::thread::sleep(std::time::Duration::from_millis(50));
+    }
+    handle.shutdown();
+    assert_eq!(count, 1, "echo received on MT runtime");
+}
+
+// Guest with i64 extend operations
+#[test]
+fn guest_i64_extend_operations() {
+    let wat = r#"(module
+        (memory (export "memory") 1)
+        (func (export "alloc") (param $len i32) (result i32) i32.const 1024)
+        (func (export "handle") (param $ptr i32) (param $len i32)
+            ;; i64.extend_i32_s: sign-extend 32-bit -1 to 64-bit
+            (i64.store (local.get $ptr)
+                (i64.extend_i32_s (i32.const -1)))
+        )
+    )"#;
+    let engine = SharedEngine::new().unwrap();
+    let actor = WasmActorBuilder::new(engine, wat::parse_str(wat).unwrap())
+        .build().unwrap();
+    let rt = Runtime::new(RuntimeConfig::default());
+    let addr = rt.spawn(actor).unwrap();
+    rt.send_to(addr, ByteMessage(vec![0u8; 8])).unwrap();
+    rt.tick();
+}
+
+// 5 runtimes running sequentially, each with a WASM actor
+#[test]
+fn five_sequential_runtimes() {
+    let engine = SharedEngine::new().unwrap();
+    for i in 0..5u8 {
+        let rt = Runtime::new(RuntimeConfig::default());
+        let inbox = rt.new_inbox::<ByteMessage>().unwrap();
+        let actor = WasmActorBuilder::new(engine.clone(), guest_wasm("echo")).build().unwrap();
+        let addr = rt.spawn(actor).unwrap();
+        rt.send_to(addr, framed_msg(inbox.addr(), &[i])).unwrap();
+        rt.tick();
+        assert_eq!(inbox.try_recv().unwrap().0, vec![i]);
+        // runtime dropped here
+    }
+}
+
+// Property: any combination of ticks and messages never panics
+proptest! {
+    #[test]
+    fn prop_random_tick_message_interleaving(
+        ops in proptest::collection::vec(
+            proptest::bool::ANY,  // true = send, false = tick
+            1..30
+        )
+    ) {
+        let engine = SharedEngine::new().unwrap();
+        let actor = WasmActorBuilder::new(engine, guest_wasm("silent")).build().unwrap();
+        let rt = Runtime::new(RuntimeConfig::default());
+        let addr = rt.spawn(actor).unwrap();
+
+        for send in &ops {
+            if *send {
+                let _ = rt.send_to(addr, ByteMessage(vec![1, 2, 3]));
+            } else {
+                rt.tick();
+            }
+        }
+    }
 }
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