diff --git a/crates/weft-runtime/src/main.rs b/crates/weft-runtime/src/main.rs
index 111f668..8fb03bd 100644
--- a/crates/weft-runtime/src/main.rs
+++ b/crates/weft-runtime/src/main.rs
@@ -127,6 +127,14 @@ fn run_module(
 
     let mut linker: Linker<State> = Linker::new(&engine);
     add_to_linker_sync(&mut linker).context("add WASI to linker")?;
+    linker
+        .instance("weft:app/notify@0.1.0")
+        .context("define weft:app/notify instance")?
+        .func_wrap("ready", |_: wasmtime::StoreContextMut<'_, State>, ()| {
+            println!("READY");
+            Ok::<(), wasmtime::Error>(())
+        })
+        .context("define weft:app/notify#ready")?;
 
     let mut ctx_builder = WasiCtxBuilder::new();
     ctx_builder.inherit_stdout().inherit_stderr();
@@ -150,8 +158,6 @@ fn run_module(
         },
     );
 
-    println!("READY");
-
     let command =
         Command::instantiate(&mut store, &component, &linker).context("instantiate component")?;
 
diff --git a/crates/weft-runtime/wit/weft-app.wit b/crates/weft-runtime/wit/weft-app.wit
new file mode 100644
index 0000000..6b7c526
--- /dev/null
+++ b/crates/weft-runtime/wit/weft-app.wit
@@ -0,0 +1,10 @@
+package weft:app@0.1.0;
+
+/// Host interface that a WEFT app component imports to signal lifecycle events.
+interface notify {
+    /// Signal to the runtime that the application has finished initialising
+    /// and is ready to serve requests.  The runtime forwards this signal to
+    /// the session supervisor (appd), which transitions the session to the
+    /// Running state.
+    ready: func();
+}