feat(rust): route cloud sessions through create_session

rust/README.md

@@ -84,6 +84,24 @@ With the default `CliProgram::Resolve`, `Client::start()` resolves the CLI in th
 
 Created via `Client::create_session` or `Client::resume_session`. Owns an internal event loop that dispatches CLI callbacks to the focused handler traits you install on `SessionConfig`, and broadcasts session events through `subscribe()`.
 
+#### Cloud sessions
+
+`Client::create_session` creates a Mission Control–backed cloud session when the config is built with `SessionConfig::with_cloud(...)`. The runtime owns the session ID: do **not** set `session_id` or `provider` on the config (the SDK rejects both with `Error::InvalidConfig`).
+
+```rust,ignore
+use github_copilot_sdk::types::{CloudSessionOptions, CloudSessionRepository, SessionConfig};
+
+let cloud = CloudSessionOptions::with_repository(
+    CloudSessionRepository::new("github", "copilot-sdk").with_branch("main"),
+);
+let session = client
+    .create_session(SessionConfig::default().with_cloud(cloud))
+    .await?;
+println!("cloud session id: {}", session.id());
+```
+
+The SDK buffers any `session.event` notifications or inbound JSON-RPC requests that arrive before the `session.create` response (bounded, drop-oldest) and replays them once the runtime-assigned session ID is registered.
+
 ```rust,ignore
 use github_copilot_sdk::MessageOptions;
 

rust/src/jsonrpc.rs

@@ -63,7 +63,6 @@ pub mod error_codes {
     /// Invalid method parameters (-32602).
     pub const INVALID_PARAMS: i32 = -32602;
     /// Internal server error (-32603).
-    #[allow(dead_code, reason = "standard JSON-RPC code, reserved for future use")]
     pub const INTERNAL_ERROR: i32 = -32603;
 }
 
@@ -490,6 +489,59 @@ impl JsonRpcClient {
             ))),
         }
     }
+
+    /// Clone a sync handle onto the outbound writer for fire-and-forget
+    /// frames. Use only for paths that cannot `.await` (currently the
+    /// session router, which holds a `parking_lot::Mutex` while deciding
+    /// to discard a buffered request).
+    pub(crate) fn writer_handle(&self) -> WriterHandle {
+        WriterHandle {
+            write_tx: self.write_tx.clone(),
+        }
+    }
+}
+
+/// Sync, fire-and-forget handle onto the JSON-RPC writer actor. Cloned
+/// from [`JsonRpcClient::writer_handle`]; serializes the message on the
+/// caller's thread and enqueues it without awaiting an ack. Loss of the
+/// ack means we'll never observe a write error here, which is acceptable
+/// for the one current caller (error responses to dropped pending
+/// requests): if the wire is broken, the runtime will time out the
+/// request on its own.
+pub(crate) struct WriterHandle {
+    write_tx: mpsc::UnboundedSender<WriteCommand>,
+}
+
+impl Clone for WriterHandle {
+    fn clone(&self) -> Self {
+        Self {
+            write_tx: self.write_tx.clone(),
+        }
+    }
+}
+
+impl WriterHandle {
+    /// Serialize and enqueue a JSON-RPC message without waiting for the
+    /// writer actor to flush it. Drops silently if serialization fails or
+    /// the writer actor has shut down — both indicate the transport is
+    /// already unusable.
+    pub(crate) fn send_fire_and_forget<T: serde::Serialize>(&self, message: &T) {
+        let body = match serde_json::to_vec(message) {
+            Ok(body) => body,
+            Err(e) => {
+                warn!(error = %e, "WriterHandle failed to serialize fire-and-forget message");
+                return;
+            }
+        };
+        let mut frame = Vec::with_capacity(CONTENT_LENGTH_HEADER.len() + 16 + body.len() + 4);
+        frame.extend_from_slice(CONTENT_LENGTH_HEADER.as_bytes());
+        frame.extend_from_slice(body.len().to_string().as_bytes());
+        frame.extend_from_slice(b"\r\n\r\n");
+        frame.extend_from_slice(&body);
+
+        let (ack_tx, _ack_rx) = oneshot::channel();
+        let _ = self.write_tx.send(WriteCommand { frame, ack: ack_tx });
+    }
 }
 
 /// RAII guard that removes a pending-request entry from the map if the

rust/src/lib.rs

@@ -1204,14 +1204,16 @@ impl Client {
         let pid = child.as_ref().and_then(|c| c.id());
         info!(pid = ?pid, "copilot CLI client ready");
 
+        let client_rpc_writer_handle = rpc.writer_handle();
+
         let client = Self {
             inner: Arc::new(ClientInner {
                 child: parking_lot::Mutex::new(child),
                 rpc,
                 cwd,
                 request_rx: parking_lot::Mutex::new(Some(request_rx)),
                 notification_tx: notification_broadcast_tx,
-                router: router::SessionRouter::new(),
+                router: router::SessionRouter::with_writer(client_rpc_writer_handle),
                 negotiated_protocol_version: OnceLock::new(),
                 state: parking_lot::Mutex::new(ConnectionState::Connected),
                 lifecycle_tx: broadcast::channel(256).0,
@@ -1224,6 +1226,10 @@ impl Client {
             }),
         };
         client.spawn_lifecycle_dispatcher();
+        client
+            .inner
+            .router
+            .start(&client.inner.notification_tx, &client.inner.request_rx);
         debug!(
             elapsed_ms = setup_start.elapsed().as_millis(),
             pid = ?pid,
@@ -1580,12 +1586,18 @@ impl Client {
         &self,
         session_id: &SessionId,
     ) -> crate::router::SessionChannels {
-        self.inner
-            .router
-            .ensure_started(&self.inner.notification_tx, &self.inner.request_rx);
         self.inner.router.register(session_id)
     }
 
+    /// Enter pending-routing mode on the router. While the returned guard is
+    /// alive, notifications and requests addressed to session ids that are
+    /// not yet registered are buffered instead of being dropped. Used when
+    /// creating cloud sessions so the SDK can receive events that the
+    /// runtime emits between `session.create` and the response.
+    pub(crate) fn begin_pending_session_routing(&self) -> crate::router::PendingSessionRouting {
+        self.inner.router.begin_pending_session_routing()
+    }
+
     /// Unregister a session, dropping its per-session channels.
     pub(crate) fn unregister_session(&self, session_id: &SessionId) {
         self.inner.router.unregister(session_id);