diff --git a/crates/wasm-actor/tests/wasm_actor.rs b/crates/wasm-actor/tests/wasm_actor.rs
index 8b49808..cba9a32 100644
--- a/crates/wasm-actor/tests/wasm_actor.rs
+++ b/crates/wasm-actor/tests/wasm_actor.rs
@@ -10875,4 +10875,131 @@ fn build_error_display_contains_export_name() {
         .build().err().expect("should fail");
     let msg = format!("{err}");
     assert!(msg.contains("alloc"), "error should mention missing 'alloc' export: {msg}");
+}
+
+// ── Cycle 75 ─────────────────────────────────────────────────────────────────
+
+// Guest with max function params — handle still only takes (i32, i32) though
+// Extra internal functions can have many params
+#[test]
+fn guest_internal_function_with_many_params() {
+    let wat = r#"(module
+        (memory (export "memory") 1)
+        (func $helper (param i32 i32 i32 i32 i32 i32 i32 i32) (result i32)
+            ;; Sum all 8 params
+            (i32.add (local.get 0) (i32.add (local.get 1) (i32.add (local.get 2)
+                (i32.add (local.get 3) (i32.add (local.get 4) (i32.add (local.get 5)
+                    (i32.add (local.get 6) (local.get 7))))))))
+        )
+        (func (export "alloc") (param $len i32) (result i32) i32.const 1024)
+        (func (export "handle") (param $ptr i32) (param $len i32)
+            (i32.store (local.get $ptr)
+                (call $helper
+                    (i32.const 1) (i32.const 2) (i32.const 3) (i32.const 4)
+                    (i32.const 5) (i32.const 6) (i32.const 7) (i32.const 8)))
+        )
+    )"#;
+    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; 4])).unwrap();
+    rt.tick(); // stores 36 at ptr
+}
+
+// Echo actor relays message through native forwarder back to inbox
+#[test]
+fn wasm_to_native_to_inbox_relay() {
+    let engine = SharedEngine::new().unwrap();
+    let echo = WasmActorBuilder::new(engine, guest_wasm("echo")).build().unwrap();
+    let rt = Runtime::new(RuntimeConfig::default());
+    let inbox = rt.new_inbox::<ByteMessage>().unwrap();
+
+    // Native forwarder: receives ByteMessage, sends to inbox
+    struct NativeForward(ActorAddress);
+    impl ActorInterface for NativeForward {
+        type Incoming = ByteMessage;
+        type Response = ();
+        fn handle(&mut self, ctx: &Ctx, msg: ByteMessage) {
+            let _ = ctx.send(self.0, msg);
+        }
+    }
+
+    let fwd_addr = rt.spawn(NativeForward(*inbox.addr())).unwrap();
+    let echo_addr = rt.spawn(echo).unwrap();
+
+    // Send to echo, echo sends to forwarder, forwarder sends to inbox
+    rt.send_to(echo_addr, framed_msg(&fwd_addr, &inbox.addr().0.to_vec())).unwrap();
+    rt.tick(); // echo → forwarder
+    rt.tick(); // forwarder → inbox
+
+    let resp = inbox.try_recv().expect("relayed through native forwarder");
+    assert_eq!(resp.0, inbox.addr().0.to_vec());
+}
+
+// Property: any message size from 0 to 10000 round-trips through echo
+proptest! {
+    #[test]
+    fn prop_any_message_size_round_trips(size in 0usize..10000) {
+        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();
+
+        let payload = vec![0x42u8; size];
+        rt.send_to(addr, framed_msg(inbox.addr(), &payload)).unwrap();
+        rt.tick();
+
+        let resp = inbox.try_recv().expect("echo should respond");
+        assert_eq!(resp.0.len(), size);
+    }
+}
+
+// Guest with i32 conversion: extend 16-bit signed
+#[test]
+fn guest_sign_extend_16() {
+    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)
+            ;; Sign-extend 16-bit value: 0xFFFF -> -1
+            (i32.store (local.get $ptr)
+                (i32.extend16_s (i32.const 0xFFFF))
+            )
+        )
+    )"#;
+    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; 4])).unwrap();
+    rt.tick();
+}
+
+// Echo actor processes message, then gets more after 100 idle ticks
+#[test]
+fn echo_after_hundred_idle_ticks() {
+    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();
+
+    // First message
+    rt.send_to(addr, framed_msg(inbox.addr(), b"before")).unwrap();
+    rt.tick();
+    assert_eq!(inbox.try_recv().unwrap().0, b"before");
+
+    // 100 idle ticks
+    for _ in 0..100 {
+        rt.tick();
+    }
+
+    // Second message — still works
+    rt.send_to(addr, framed_msg(inbox.addr(), b"after")).unwrap();
+    rt.tick();
+    assert_eq!(inbox.try_recv().unwrap().0, b"after");
 }
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