diff --git a/src/room/state.rs b/src/room/state.rs
index 4817732..c987eaf 100644
--- a/src/room/state.rs
+++ b/src/room/state.rs
@@ -157,6 +157,11 @@ const NO_ACTIVE_TURN_ERROR_CODE: i64 = -32002;
 /// clients can render "queue full" rather than generic turn serialization.
 const QUEUE_FULL_ERROR_CODE: i64 = -32003;
 
+/// JSON-RPC error code returned when a mux-owned queue item no longer
+/// exists. Kept distinct from invalid params so clients can distinguish
+/// malformed input from a remove/submit race.
+const QUEUE_ITEM_NOT_FOUND_ERROR_CODE: i64 = -32004;
+
 /// Standard JSON-RPC invalid params code used when an amux control request
 /// is missing its text/session payload.
 const INVALID_PARAMS_ERROR_CODE: i64 = -32602;
@@ -2033,7 +2038,7 @@ impl RoomInner {
             self.send_error_response(
                 peer_id,
                 req.id,
-                INVALID_PARAMS_ERROR_CODE,
+                QUEUE_ITEM_NOT_FOUND_ERROR_CODE,
                 "queue item not found",
             );
             return None;
diff --git a/tests/server.rs b/tests/server.rs
index 398f4cb..6de78ed 100644
--- a/tests/server.rs
+++ b/tests/server.rs
@@ -2816,6 +2816,30 @@ async fn amux_unqueue_prompt_removes_pending_item_and_replays_removal() {
     let _ = ws_c.send(ClientMsg::Close(None)).await;
 }
 
+#[tokio::test]
+async fn amux_unqueue_prompt_missing_item_uses_queue_not_found_error() {
+    let (addr, _) = spawn_server_with_mock_env(&[]).await;
+    let url = format!("ws://{addr}/acp?room=busy-fixes&peer_id=A");
+
+    let (mut ws, _) = tokio_tungstenite::connect_async(url).await.unwrap();
+    let _ = ws_request(&mut ws, r#"{"jsonrpc":"2.0","id":1,"method":"initialize"}"#).await;
+
+    let response = ws_request(
+        &mut ws,
+        r#"{"jsonrpc":"2.0","id":300,"method":"amux/unqueue_prompt","params":{"queueItemId":"q-missing"}}"#,
+    )
+    .await;
+
+    assert_eq!(response["id"], serde_json::json!(300));
+    assert_eq!(response["error"]["code"], serde_json::json!(-32004));
+    assert_eq!(
+        response["error"]["message"],
+        serde_json::json!("queue item not found")
+    );
+
+    let _ = ws.send(ClientMsg::Close(None)).await;
+}
+
 #[tokio::test]
 async fn amux_disconnected_queue_owner_persists_without_becoming_driver() {
     let (addr, _) = spawn_server_with_mock_env(&[("MOCK_ACP_PROMPT_DELAY_MS", "500")]).await;