diff --git a/crates/wasm-actor/tests/wasm_actor.rs b/crates/wasm-actor/tests/wasm_actor.rs
index d1ba56f..0bddffc 100644
--- a/crates/wasm-actor/tests/wasm_actor.rs
+++ b/crates/wasm-actor/tests/wasm_actor.rs
@@ -2604,3 +2604,135 @@ fn module_with_custom_section_builds_and_works() {
     let received = inbox.try_recv().expect("module with custom section should work");
     assert_eq!(received.0, vec![99]);
 }
+
+// ── Multiple engines: actors from different engines on same runtime ──────────
+
+#[test]
+fn actors_from_different_engines_coexist() {
+    // Two actors built from separate SharedEngine instances.
+    // Verifies that engine isolation doesn't cause issues when actors
+    // share the same runtime.
+    let engine_a = SharedEngine::new().unwrap();
+    let engine_b = SharedEngine::new().unwrap();
+    let wasm_bytes = guest_wasm("echo");
+
+    let actor_a = WasmActorBuilder::new(engine_a, wasm_bytes.clone()).build().unwrap();
+    let actor_b = WasmActorBuilder::new(engine_b, wasm_bytes).build().unwrap();
+
+    let rt = Runtime::new(RuntimeConfig::default());
+    let inbox_a = rt.new_inbox::<ByteMessage>().unwrap();
+    let inbox_b = rt.new_inbox::<ByteMessage>().unwrap();
+
+    let addr_a = rt.spawn(actor_a).unwrap();
+    let addr_b = rt.spawn(actor_b).unwrap();
+
+    rt.send_to(addr_a, framed_msg(inbox_a.addr(), b"engine-A")).unwrap();
+    rt.send_to(addr_b, framed_msg(inbox_b.addr(), b"engine-B")).unwrap();
+    rt.tick();
+
+    let recv_a = inbox_a.try_recv().expect("engine A actor should echo");
+    let recv_b = inbox_b.try_recv().expect("engine B actor should echo");
+    assert_eq!(recv_a.0, b"engine-A");
+    assert_eq!(recv_b.0, b"engine-B");
+}
+
+// ── Error formatting: WasmActorError Display ────────────────────────────────
+
+#[test]
+fn error_display_formats_correctly() {
+    let missing = WasmActorError::MissingExport("memory");
+    assert!(missing.to_string().contains("memory"));
+    assert!(missing.to_string().contains("missing"));
+
+    let garbage = vec![0u8, 1, 2, 3];
+    let engine = SharedEngine::new().unwrap();
+    let build_result = WasmActorBuilder::new(engine, garbage).build();
+    assert!(build_result.is_err());
+    let wasmtime_err = build_result.err().unwrap();
+    assert!(wasmtime_err.to_string().contains("wasmtime"));
+}
+
+// ── Rapid lifecycle: spawn, process, stop, repeat ───────────────────────────
+
+#[test]
+fn rapid_spawn_process_stop_cycle() {
+    // Rapidly spawn, send, tick, stop, tick, repeat for 20 iterations.
+    // Tests that the runtime cleanly handles rapid WASM actor lifecycle.
+    let engine = SharedEngine::new().unwrap();
+    let wasm_bytes = guest_wasm("echo");
+
+    let rt = Runtime::new(RuntimeConfig::default());
+    let inbox = rt.new_inbox::<ByteMessage>().unwrap();
+
+    for i in 0u8..20 {
+        let actor = WasmActorBuilder::new(engine.clone(), wasm_bytes.clone())
+            .build()
+            .unwrap();
+        let addr = rt.spawn(actor).unwrap();
+
+        rt.send_to(addr, framed_msg(inbox.addr(), &[i])).unwrap();
+        rt.tick();
+
+        let received = inbox.try_recv().expect("actor should echo before stop");
+        assert_eq!(received.0, vec![i]);
+
+        rt.stop_actor(addr).unwrap();
+        rt.tick();
+        rt.tick(); // cleanup
+    }
+}
+
+// ── Stop-send race: send after stop_actor but before tick ───────────────────
+
+#[test]
+fn send_after_stop_before_tick_is_silently_dropped() {
+    // Stop an actor, then immediately send a message before ticking.
+    // The message should be silently dropped (actor is stopping).
+    let engine = SharedEngine::new().unwrap();
+    let actor = WasmActorBuilder::new(engine, guest_wasm("echo"))
+        .build()
+        .unwrap();
+
+    let rt = Runtime::new(RuntimeConfig::default());
+    let inbox = rt.new_inbox::<ByteMessage>().unwrap();
+    let addr = rt.spawn(actor).unwrap();
+
+    // Verify it works first
+    rt.send_to(addr, framed_msg(inbox.addr(), b"alive")).unwrap();
+    rt.tick();
+    assert!(inbox.try_recv().is_some());
+
+    // Stop then immediately send before tick processes the stop
+    rt.stop_actor(addr).unwrap();
+    // This send may or may not succeed depending on mailbox state
+    let _ = rt.send_to(addr, framed_msg(inbox.addr(), b"after-stop"));
+    rt.tick(); // processes stop signal, clears mailbox
+    rt.tick(); // cleanup
+
+    // No response expected — either the send failed or the message was cleared
+    // The key assertion: no panic or corruption
+}
+
+// ── SharedEngine Debug impl ─────────────────────────────────────────────────
+
+#[test]
+fn shared_engine_debug_does_not_panic() {
+    let engine = SharedEngine::new().unwrap();
+    let debug_str = format!("{:?}", engine);
+    assert!(debug_str.contains("SharedEngine"));
+}
+
+// ── ByteMessage equality and clone ──────────────────────────────────────────
+
+#[test]
+fn byte_message_traits() {
+    let msg1 = ByteMessage(vec![1, 2, 3]);
+    let msg2 = msg1.clone();
+    assert_eq!(msg1, msg2);
+
+    let msg3 = ByteMessage(vec![4, 5, 6]);
+    assert_ne!(msg1, msg3);
+
+    let debug_str = format!("{:?}", msg1);
+    assert!(debug_str.contains("ByteMessage"));
+}