diff --git a/crates/wasm-actor/tests/wasm_actor.rs b/crates/wasm-actor/tests/wasm_actor.rs
index 582d4c5..b522cc8 100644
--- a/crates/wasm-actor/tests/wasm_actor.rs
+++ b/crates/wasm-actor/tests/wasm_actor.rs
@@ -1300,9 +1300,18 @@ fn wasm_actor_works_on_multi_worker_runtime() {
 
     // Use run() to drive the runtime on background threads
     let handle = rt.run().unwrap();
-    std::thread::sleep(std::time::Duration::from_millis(50));
 
-    let received = inbox.try_recv().expect("wasm actor should echo on MT runtime");
+    // Poll with retries — MT runtime timing is non-deterministic
+    let mut received = None;
+    for _ in 0..20 {
+        std::thread::sleep(std::time::Duration::from_millis(25));
+        if let Some(msg) = inbox.try_recv() {
+            received = Some(msg);
+            break;
+        }
+    }
+
+    let received = received.expect("wasm actor should echo on MT runtime");
     assert_eq!(received.0, payload);
 
     handle.shutdown();
@@ -1841,9 +1850,18 @@ fn wasm_actors_communicate_across_threads() {
     rt.send_to(addr_a, msg_for_a).unwrap();
 
     let handle = rt.run().unwrap();
-    std::thread::sleep(std::time::Duration::from_millis(100));
 
-    let received = inbox.try_recv().expect("cross-thread relay should deliver");
+    // Poll with retries — MT runtime timing is non-deterministic
+    let mut received = None;
+    for _ in 0..20 {
+        std::thread::sleep(std::time::Duration::from_millis(25));
+        if let Some(msg) = inbox.try_recv() {
+            received = Some(msg);
+            break;
+        }
+    }
+
+    let received = received.expect("cross-thread relay should deliver");
     assert_eq!(received.0, final_payload);
 
     handle.shutdown();
@@ -2229,3 +2247,157 @@ proptest! {
         prop_assert!(result.is_ok(), "actor must survive: alloc={alloc_val} trap={trap_handle} send_before={send_before_trap}");
     }
 }
+
+// ── Stack overflow: deep recursion in handle ────────────────────────────────
+
+#[test]
+fn guest_stack_overflow_traps_actor_survives() {
+    // Guest handle calls itself recursively until stack overflow.
+    // Wasmtime should trap with a stack overflow error; actor must survive.
+    let wat = r#"
+        (module
+            (import "swactor" "send" (func $send (param i32 i32 i32)))
+            (memory (export "memory") 1)
+            (func (export "alloc") (param i32) (result i32) i32.const 256)
+            (func $recurse (param $ptr i32) (param $len i32)
+                local.get $ptr
+                local.get $len
+                call $recurse
+            )
+            (func (export "handle") (param $ptr i32) (param $len i32)
+                local.get $ptr
+                local.get $len
+                call $recurse
+            )
+        )
+    "#;
+    let wasm = wat::parse_str(wat).unwrap();
+    let engine = SharedEngine::new().unwrap();
+    let actor = WasmActorBuilder::new(engine, wasm).build().unwrap();
+
+    let rt = Runtime::new(RuntimeConfig::default());
+    let addr = rt.spawn(actor).unwrap();
+
+    rt.send_to(addr, ByteMessage(vec![1, 2, 3])).unwrap();
+    rt.tick(); // stack overflow trap
+
+    // Actor survives
+    rt.send_to(addr, ByteMessage(vec![4, 5, 6])).unwrap();
+    rt.tick();
+}
+
+// ── Bulk memory: memory.fill and memory.copy ────────────────────────────────
+
+#[test]
+fn guest_using_bulk_memory_ops_works() {
+    // The engine enables bulk_memory. Guest uses memory.fill to write a
+    // pattern, then sends it. Verifies bulk memory operations work.
+    let wat = r#"
+        (module
+            (import "swactor" "send" (func $send (param i32 i32 i32)))
+            (memory (export "memory") 1)
+
+            (func (export "alloc") (param i32) (result i32) i32.const 256)
+            (func (export "handle") (param $ptr i32) (param $len i32)
+                ;; Fill bytes [500..510] with value 42 using memory.fill
+                (memory.fill (i32.const 500) (i32.const 42) (i32.const 10))
+
+                ;; Send 10 bytes from [500..510]
+                local.get $ptr
+                i32.const 500
+                i32.const 10
+                call $send
+            )
+        )
+    "#;
+    let wasm = wat::parse_str(wat).unwrap();
+    let engine = SharedEngine::new().unwrap();
+    let actor = WasmActorBuilder::new(engine, wasm).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(), b"trigger")).unwrap();
+    rt.tick();
+
+    let received = inbox.try_recv().expect("bulk memory fill should work");
+    assert_eq!(received.0, vec![42u8; 10]);
+}
+
+// ── Self-amplification: bounded by message budget ───────────────────────────
+
+#[test]
+fn self_amplification_bounded_by_budget_no_crash() {
+    // Guest sends 3 copies of the message back to itself. With budget=4
+    // each tick processes at most 4 messages. Run for a few ticks — should
+    // not crash or OOM.
+    let wat = r#"
+        (module
+            (import "swactor" "send" (func $send (param i32 i32 i32)))
+            (memory (export "memory") 1)
+            (func (export "alloc") (param i32) (result i32) i32.const 4096)
+            (func (export "handle") (param $ptr i32) (param $len i32)
+                (local $i i32)
+                (local.set $i (i32.const 0))
+                (block $break
+                    (loop $loop
+                        (br_if $break (i32.ge_u (local.get $i) (i32.const 3)))
+                        local.get $ptr
+                        local.get $ptr
+                        i32.const 33
+                        call $send
+                        (local.set $i (i32.add (local.get $i) (i32.const 1)))
+                        (br $loop)
+                    )
+                )
+            )
+        )
+    "#;
+    let wasm = wat::parse_str(wat).unwrap();
+    let engine = SharedEngine::new().unwrap();
+    let actor = WasmActorBuilder::new(engine, wasm).build().unwrap();
+
+    let config = RuntimeConfig {
+        actor_message_budget: 4,
+        ..RuntimeConfig::default()
+    };
+    let rt = Runtime::new(config);
+    let addr = rt.spawn(actor).unwrap();
+
+    // Initial seed: [self_addr | marker]
+    let mut seed = Vec::new();
+    seed.extend_from_slice(&addr.0);
+    seed.push(0xFF);
+    rt.send_to(addr, ByteMessage(seed)).unwrap();
+
+    // Run for 5 ticks — should not crash
+    for _ in 0..5 {
+        rt.tick();
+    }
+
+    // Actor alive
+    rt.send_to(addr, ByteMessage(vec![0])).unwrap();
+    rt.tick();
+}
+
+// ── Double stop: stopping an already-stopped actor ──────────────────────────
+
+#[test]
+fn double_stop_is_idempotent() {
+    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();
+
+    rt.stop_actor(addr).unwrap();
+    rt.tick();
+    rt.tick();
+
+    // Second stop should fail gracefully (not panic)
+    let result = rt.stop_actor(addr);
+    assert!(result.is_err(), "stopping already-stopped actor should error");
+}