diff --git a/client/src/handlers/room_join_request_handler.rs b/client/src/handlers/room_join_request_handler.rs
index c5acccf..6a53825 100644
--- a/client/src/handlers/room_join_request_handler.rs
+++ b/client/src/handlers/room_join_request_handler.rs
@@ -3,11 +3,42 @@ use solstice_proto::server::RoomJoinRequest;
 use solstice_proto::ServerRequest;
 
 use crate::context::Context;
+use crate::control;
 use crate::message_handler::MessageHandler;
+use crate::room::RoomError;
 
 #[derive(Debug, Default)]
 pub struct RoomJoinRequestHandler;
 
+fn start_joining(context: &Context, room_name: &str) -> Result<(), RoomError> {
+  let mut guard = context.state.lock();
+  let result = guard.rooms.start_joining(room_name);
+
+  if let Err(RoomError::MembershipChangeInvalid(_, _)) = result {
+    // This `expect()` should never fail since the error was not
+    // `RoomNotFound` but `MembershipChangeInvalid`.
+    let room = guard
+      .rooms
+      .get_strict(room_name)
+      .expect("querying room state");
+
+    let response =
+      control::Response::RoomJoinResponse(control::RoomJoinResponse {
+        room_name: room_name.to_string(),
+        room: room.clone(),
+      });
+
+    if let Err(err) = context.control_response_tx.blocking_send(response) {
+      error!(
+        "Failed to send RoomJoinResponse for room {}: {}",
+        room_name, err
+      );
+    }
+  }
+
+  result
+}
+
 impl MessageHandler for RoomJoinRequestHandler {
   type Message = String;
 
@@ -16,14 +47,7 @@ impl MessageHandler for RoomJoinRequestHandler {
     context: &Context,
     room_name: &Self::Message,
   ) -> anyhow::Result<()> {
-    {
-      // TODO: Send response immediately if we already joined.
-      let mut guard = context.state.lock();
-      guard
-        .rooms
-        .start_joining(room_name)
-        .context("joining room")?;
-    }
+    start_joining(context, room_name).context("joining room")?;
     context
       .server_request_tx
       .blocking_send(ServerRequest::RoomJoinRequest(RoomJoinRequest {
@@ -41,12 +65,13 @@ impl MessageHandler for RoomJoinRequestHandler {
 #[cfg(test)]
 mod tests {
   use anyhow::Context;
-  use solstice_proto::server::{RoomJoinRequest, RoomListResponse};
+  use solstice_proto::server::RoomJoinRequest;
   use solstice_proto::ServerRequest;
 
-  use crate::context::ContextBundle;
+  use crate::context::{ContextBundle, ContextOptions};
+  use crate::control;
   use crate::message_handler::MessageHandler;
-  use crate::room::Membership;
+  use crate::room::{Membership, Room, Visibility};
 
   use super::RoomJoinRequestHandler;
 
@@ -69,20 +94,48 @@ mod tests {
   }
 
   #[test]
-  fn run_success() -> anyhow::Result<()> {
-    let mut bundle = ContextBundle::default();
+  fn run_already_joined_responds_immediately() -> anyhow::Result<()> {
+    let mut room = Room::new(Visibility::Public, 3);
+    room.membership = Membership::Member;
 
-    bundle
-      .context
-      .state
-      .lock()
+    let mut options = ContextOptions::default();
+    options
+      .initial_state
       .rooms
-      .set_room_list(RoomListResponse {
-        rooms: vec![("bleep".to_string(), 3)],
-        owned_private_rooms: vec![],
-        other_private_rooms: vec![],
-        operated_private_room_names: vec![],
-      });
+      .insert("bleep".to_string(), room.clone());
+    let mut bundle = ContextBundle::new(options);
+
+    RoomJoinRequestHandler::default()
+      .run(&bundle.context, &"bleep".to_string())
+      .unwrap_err();
+
+    // Room state has not changed.
+    assert_eq!(
+      bundle.context.state.lock().rooms.get_room_list(),
+      vec![("bleep".to_string(), room.clone())]
+    );
+
+    assert_eq!(
+      bundle.control_response_rx.blocking_recv(),
+      Some(control::Response::RoomJoinResponse(
+        control::RoomJoinResponse {
+          room_name: "bleep".to_string(),
+          room,
+        }
+      ))
+    );
+
+    Ok(())
+  }
+
+  #[test]
+  fn run_success() -> anyhow::Result<()> {
+    let mut options = ContextOptions::default();
+    options
+      .initial_state
+      .rooms
+      .insert("bleep".to_string(), Room::new(Visibility::Public, 3));
+    let mut bundle = ContextBundle::new(options);
 
     RoomJoinRequestHandler::default()
       .run(&bundle.context, &"bleep".to_string())