diff --git a/crates/wasm-actor/tests/wasm_actor.rs b/crates/wasm-actor/tests/wasm_actor.rs
index 7966ecc..582d4c5 100644
--- a/crates/wasm-actor/tests/wasm_actor.rs
+++ b/crates/wasm-actor/tests/wasm_actor.rs
@@ -2059,3 +2059,173 @@ fn alloc_that_grows_memory_works() {
     let received = inbox.try_recv().expect("alloc in grown region should work");
     assert_eq!(received.0, payload);
 }
+
+// ── Message budget fairness: WASM actor processes only its budget ────────────
+
+#[test]
+fn wasm_actor_respects_message_budget() {
+    // With actor_message_budget=2, sending 5 messages should process at most
+    // 2 per tick. This verifies the budget applies to WASM actors too.
+    let engine = SharedEngine::new().unwrap();
+    let actor = WasmActorBuilder::new(engine, guest_wasm("echo"))
+        .build()
+        .unwrap();
+
+    let config = RuntimeConfig {
+        actor_message_budget: 2,
+        ..RuntimeConfig::default()
+    };
+    let rt = Runtime::new(config);
+    let inbox = rt.new_inbox::<ByteMessage>().unwrap();
+    let addr = rt.spawn(actor).unwrap();
+
+    // Send 5 messages
+    for i in 0u8..5 {
+        rt.send_to(addr, framed_msg(inbox.addr(), &[i])).unwrap();
+    }
+
+    // First tick: should process at most 2
+    rt.tick();
+    let mut count_tick1 = 0;
+    while inbox.try_recv().is_some() {
+        count_tick1 += 1;
+    }
+    assert_eq!(count_tick1, 2, "first tick should process exactly budget=2 messages");
+
+    // Second tick: another 2
+    rt.tick();
+    let mut count_tick2 = 0;
+    while inbox.try_recv().is_some() {
+        count_tick2 += 1;
+    }
+    assert_eq!(count_tick2, 2, "second tick should process next 2 messages");
+
+    // Third tick: remaining 1
+    rt.tick();
+    let mut count_tick3 = 0;
+    while inbox.try_recv().is_some() {
+        count_tick3 += 1;
+    }
+    assert_eq!(count_tick3, 1, "third tick should process remaining 1 message");
+}
+
+// ── Data segment: guest module with pre-initialized memory ──────────────────
+
+#[test]
+fn guest_with_data_segment_handles_messages_correctly() {
+    // A guest module with a data segment that pre-fills bytes at offset 0.
+    // The host writes the incoming message starting at the alloc pointer (256),
+    // which shouldn't conflict with the data segment.
+    let wat = r#"
+        (module
+            (import "swactor" "send" (func $send (param i32 i32 i32)))
+            (memory (export "memory") 1)
+            ;; Pre-fill offset 200-203 with "DATA"
+            (data (i32.const 200) "DATA")
+
+            (func (export "alloc") (param i32) (result i32)
+                i32.const 256  ;; alloc above data segment
+            )
+            (func (export "handle") (param $ptr i32) (param $len i32)
+                ;; Send the pre-initialized data as payload
+                local.get $ptr    ;; dest_ptr
+                i32.const 200     ;; payload_ptr (data segment)
+                i32.const 4       ;; payload_len
+                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("should receive data segment content");
+    assert_eq!(received.0, b"DATA");
+}
+
+// ── Outbox isolation: two actors' outboxes don't interfere ──────────────────
+
+#[test]
+fn two_wasm_actors_outboxes_are_isolated() {
+    // Two WASM actors process messages in the same tick. Their outbox
+    // entries should not mix. Each Store has its own HostState.
+    let engine = SharedEngine::new().unwrap();
+    let wasm_bytes = guest_wasm("echo");
+
+    let actor_a = WasmActorBuilder::new(engine.clone(), wasm_bytes.clone()).build().unwrap();
+    let actor_b = WasmActorBuilder::new(engine, 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();
+
+    // Send to both in same tick
+    rt.send_to(addr_a, framed_msg(inbox_a.addr(), b"msg-A")).unwrap();
+    rt.send_to(addr_b, framed_msg(inbox_b.addr(), b"msg-B")).unwrap();
+    rt.tick();
+
+    let recv_a = inbox_a.try_recv().expect("actor A should echo");
+    let recv_b = inbox_b.try_recv().expect("actor B should echo");
+    assert_eq!(recv_a.0, b"msg-A");
+    assert_eq!(recv_b.0, b"msg-B");
+
+    // No cross-contamination
+    assert!(inbox_a.try_recv().is_none(), "inbox_a should have exactly 1 message");
+    assert!(inbox_b.try_recv().is_none(), "inbox_b should have exactly 1 message");
+}
+
+// ── Property: combined alloc + handle stress never crashes ──────────────────
+
+proptest! {
+    #[test]
+    fn prop_random_module_behavior_never_crashes(
+        alloc_val in -100i32..70000,
+        trap_handle in proptest::bool::ANY,
+        send_before_trap in proptest::bool::ANY,
+    ) {
+        // Fuzz the module behavior: random alloc return, optional trap in handle,
+        // optional send before the trap. The actor must never be poisoned.
+        let trap_code = if trap_handle { "unreachable" } else { "" };
+        let send_code = if send_before_trap {
+            "local.get $ptr i32.const 32 i32.const 1 call $send"
+        } else {
+            ""
+        };
+
+        let wat = format!(r#"
+            (module
+                (import "swactor" "send" (func $send (param i32 i32 i32)))
+                (memory (export "memory") 1)
+                (func (export "alloc") (param i32) (result i32)
+                    i32.const {alloc_val}
+                )
+                (func (export "handle") (param $ptr i32) (param $len i32)
+                    {send_code}
+                    {trap_code}
+                )
+            )
+        "#);
+        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![0u8; 64])).unwrap();
+        rt.tick();
+
+        let result = rt.send_to(addr, ByteMessage(vec![1]));
+        prop_assert!(result.is_ok(), "actor must survive: alloc={alloc_val} trap={trap_handle} send_before={send_before_trap}");
+    }
+}