diff --git a/crates/harness/src/monolith.rs b/crates/harness/src/monolith.rs
index adf8225d0..e85babbb1 100644
--- a/crates/harness/src/monolith.rs
+++ b/crates/harness/src/monolith.rs
@@ -1,5 +1,5 @@
 use std::{
-    collections::HashMap,
+    collections::{HashMap, HashSet},
     net::{IpAddr, Ipv6Addr, SocketAddr},
     pin::Pin,
     sync::{
@@ -14,7 +14,7 @@ use http_body_util::{BodyExt, Full};
 use hyper::{body::Incoming as IncomingBody, Request};
 use hyper::{service::Service, Response};
 
-use ott_balancer_protocol::{monolith::*, RoomName};
+use ott_balancer_protocol::{monolith::*, ClientId, RoomName};
 use tokio::{net::TcpListener, sync::Notify};
 use tracing::warn;
 use tungstenite::Message;
@@ -52,6 +52,7 @@ pub(crate) struct MonolithState {
     response_mocks: HashMap<String, (MockRespParts, Bytes)>,
     rooms: HashMap<RoomName, RoomMetadata>,
     room_load_epoch: Arc<AtomicU32>,
+    clients: HashSet<ClientId>,
 }
 
 impl Monolith {
@@ -103,7 +104,21 @@ impl Monolith {
                                 match msg {
                                     Ok(msg) => {
                                         println!("monolith: incoming msg: {}", msg);
-                                        state.lock().unwrap().received_raw.push(msg);
+                                        let mut state = state.lock().unwrap();
+                                        state.received_raw.push(msg.clone());
+                                        // TODO: there's a better way to generalize this
+                                        if let Message::Text(m) = msg {
+                                            let msg: MsgB2M = serde_json::from_str(&m).unwrap();
+                                            match msg {
+                                                MsgB2M::Join(join) => {
+                                                    state.clients.insert(join.client);
+                                                },
+                                                MsgB2M::Leave(leave) => {
+                                                    state.clients.remove(&leave.client);
+                                                },
+                                                _ => {},
+                                            }
+                                        }
                                         _notif_recv.notify_one();
                                     },
                                     Err(e) => {
@@ -178,6 +193,10 @@ impl Monolith {
         }
     }
 
+    pub fn clients(&self) -> HashSet<ClientId> {
+        self.state.lock().unwrap().clients.clone()
+    }
+
     /// Tell the provider to add this monolith to the list of available monoliths.
     pub async fn show(&mut self) {
         println!("showing monolith");
@@ -371,3 +390,27 @@ pub struct MockRequest {
     pub headers: hyper::HeaderMap,
     pub body: Bytes,
 }
+
+#[cfg(test)]
+mod tests {
+    use test_context::test_context;
+
+    use crate::{Client, Monolith, TestRunner};
+
+    #[test_context(TestRunner)]
+    #[tokio::test]
+    async fn should_track_clients(ctx: &mut TestRunner) {
+        let mut m = Monolith::new(ctx).await.unwrap();
+        m.show().await;
+
+        let mut c1 = Client::new(ctx).unwrap();
+        c1.join("foo").await;
+
+        m.wait_recv().await;
+        assert_eq!(m.clients().len(), 1);
+
+        c1.disconnect().await;
+        m.wait_recv().await;
+        assert_eq!(m.clients().len(), 0);
+    }
+}