diff --git a/codex-rs/app-server/src/in_process.rs b/codex-rs/app-server/src/in_process.rs
index 0b1859f14a..a0d03a9de1 100644
--- a/codex-rs/app-server/src/in_process.rs
+++ b/codex-rs/app-server/src/in_process.rs
@@ -192,9 +192,9 @@ pub enum InProcessServerEvent {
 
 /// Internal message sent from [`InProcessClientHandle`] methods to the runtime task.
 ///
-/// Requests carry a oneshot sender for the response; notifications and server-request
-/// replies are fire-and-forget from the caller's perspective (transport errors are
-/// caught by `try_send` on the outer channel).
+/// Requests carry a oneshot sender for the response. Startup notifications can
+/// acknowledge processing; ordinary notifications and server-request replies are
+/// fire-and-forget (transport errors are caught by `try_send` on the outer channel).
 enum InProcessClientMessage {
     Request {
         request: Box<ClientRequest>,
@@ -203,6 +203,7 @@ enum InProcessClientMessage {
     },
     Notification {
         notification: ClientNotification,
+        processed_tx: Option<oneshot::Sender<()>>,
     },
     ServerRequestResponse {
         request_id: RequestId,
@@ -219,7 +220,10 @@ enum InProcessClientMessage {
 
 enum ProcessorCommand {
     Request(Box<ClientRequest>, tokio_util::sync::CancellationToken),
-    Notification(ClientNotification),
+    Notification {
+        notification: ClientNotification,
+        processed_tx: Option<oneshot::Sender<()>>,
+    },
 }
 
 #[derive(Clone)]
@@ -246,7 +250,10 @@ impl InProcessClientSender {
     }
 
     pub fn notify(&self, notification: ClientNotification) -> IoResult<()> {
-        self.try_send_client_message(InProcessClientMessage::Notification { notification })
+        self.try_send_client_message(InProcessClientMessage::Notification {
+            notification,
+            processed_tx: None,
+        })
     }
 
     pub fn respond_to_server_request(&self, request_id: RequestId, result: Result) -> IoResult<()> {
@@ -379,9 +386,10 @@ impl InProcessClientHandle {
 
 /// Starts an in-process app-server runtime and performs initialize handshake.
 ///
-/// This function sends `initialize` followed by `initialized` before returning
-/// the handle, so callers receive a ready-to-use runtime. If initialize fails,
-/// the runtime is shut down and an `InvalidData` error is returned.
+/// This function waits for `initialize` and `initialized` to be processed before
+/// returning the handle, so callers receive a ready-to-use runtime. Rejected
+/// initialization shuts down the runtime and returns an `InvalidData` error.
+/// Transport failures while processing `initialized` also shut down the runtime.
 pub async fn start(mut args: InProcessStartArgs) -> IoResult<InProcessClientHandle> {
     if let Ok(Some(err)) = check_execpolicy_for_warnings(&args.config.config_layer_stack).await {
         let (path, range) = crate::exec_policy_warning_location(&err);
@@ -409,7 +417,26 @@ pub async fn start(mut args: InProcessStartArgs) -> IoResult<InProcessClientHand
             format!("in-process initialize failed: {}", error.message),
         ));
     }
-    client.notify(ClientNotification::Initialized)?;
+    let (processed_tx, processed_rx) = oneshot::channel();
+    let initialized_result = async {
+        client
+            .client
+            .try_send_client_message(InProcessClientMessage::Notification {
+                notification: ClientNotification::Initialized,
+                processed_tx: Some(processed_tx),
+            })?;
+        processed_rx.await.map_err(|err| {
+            IoError::new(
+                ErrorKind::BrokenPipe,
+                format!("in-process initialized acknowledgment channel closed: {err}"),
+            )
+        })
+    }
+    .await;
+    if let Err(error) = initialized_result {
+        let _ = client.shutdown().await;
+        return Err(error);
+    }
 
     Ok(client)
 }
@@ -585,8 +612,11 @@ async fn start_uninitialized(mut args: InProcessStartArgs) -> IoResult<InProcess
                                     processor.send_initialize_notifications().await;
                                 }
                             }
-                            Some(ProcessorCommand::Notification(notification)) => {
+                            Some(ProcessorCommand::Notification { notification, processed_tx }) => {
                                 processor.process_client_notification(notification).await;
+                                if let Some(processed_tx) = processed_tx {
+                                    let _ = processed_tx.send(());
+                                }
                             }
                             None => {
                                 break;
@@ -678,8 +708,8 @@ async fn start_uninitialized(mut args: InProcessStartArgs) -> IoResult<InProcess
                                 }
                             }
                         }
-                        Some(InProcessClientMessage::Notification { notification }) => {
-                            match processor_tx.try_send(ProcessorCommand::Notification(notification)) {
+                        Some(InProcessClientMessage::Notification { notification, processed_tx }) => {
+                            match processor_tx.try_send(ProcessorCommand::Notification { notification, processed_tx }) {
                                 Ok(()) => {}
                                 Err(mpsc::error::TrySendError::Full(_)) => {
                                     warn!("dropping in-process client notification (queue full)");
@@ -1047,29 +1077,24 @@ mod tests {
         }
     }
 
+    #[test_case::test_case(0; "zero_capacity_is_clamped")]
+    #[test_case::test_case(1; "one_slot_capacity")]
     #[tokio::test]
-    async fn in_process_start_clamps_zero_channel_capacity() {
+    async fn in_process_start_accepts_immediate_request(channel_capacity: usize) {
         let client = start_test_client_with_options(
             SessionSource::Cli,
-            /*channel_capacity*/ 0,
+            channel_capacity,
             /*capabilities*/ None,
         )
         .await;
-        let response = loop {
-            match client
-                .request(ClientRequest::ConfigRequirementsRead {
-                    request_id: RequestId::Integer(4),
-                    params: None,
-                })
-                .await
-            {
-                Ok(response) => break response.expect("request should succeed"),
-                Err(err) if err.kind() == std::io::ErrorKind::WouldBlock => {
-                    tokio::task::yield_now().await;
-                }
-                Err(err) => panic!("request transport should work: {err}"),
-            }
-        };
+        let response = client
+            .request(ClientRequest::ConfigRequirementsRead {
+                request_id: RequestId::Integer(4),
+                params: None,
+            })
+            .await
+            .expect("the first request should not compete with the initialize handshake")
+            .expect("request should succeed");
         let _parsed: ConfigRequirementsReadResponse =
             serde_json::from_value(response).expect("response should match v2 schema");
         client