diff --git a/src/web/api.rs b/src/web/api.rs
index c40f16cc..78301414 100644
--- a/src/web/api.rs
+++ b/src/web/api.rs
@@ -194,11 +194,12 @@ async fn handle_api_syncback(
         }
     };
 
-    if syncback_request.protocol_version != PROTOCOL_VERSION {
+    let client_protocol = syncback_request.protocol_version as u64;
+    if client_protocol != PROTOCOL_VERSION {
         return msgpack(
             ErrorResponse::bad_request(format!(
                 "Protocol version mismatch: expected {}, got {}",
-                PROTOCOL_VERSION, syncback_request.protocol_version
+                PROTOCOL_VERSION, client_protocol
             )),
             StatusCode::BAD_REQUEST,
         );
@@ -219,7 +220,8 @@ async fn handle_api_syncback(
         );
     }
 
-    if let Some(place_id) = syncback_request.place_id {
+    if let Some(place_id_f64) = syncback_request.place_id {
+        let place_id = place_id_f64 as u64;
         if let Some(expected) = service.serve_session.serve_place_ids() {
             if !expected.contains(&place_id) {
                 return msgpack(
diff --git a/src/web/interface.rs b/src/web/interface.rs
index 4fa7b796..2f0a27fb 100644
--- a/src/web/interface.rs
+++ b/src/web/interface.rs
@@ -37,12 +37,15 @@ pub struct ServiceChunk {
 }
 
 /// Incoming request from the plugin for live syncback.
+/// Numeric fields use `f64` because Luau has no integer type -- all numbers
+/// are IEEE 754 doubles, and large values (like place IDs) get encoded as
+/// msgpack float64 which rmp_serde cannot coerce into u64.
 #[derive(Debug, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct SyncbackRequest {
-    pub protocol_version: u64,
+    pub protocol_version: f64,
     pub server_version: String,
-    pub place_id: Option<u64>,
+    pub place_id: Option<f64>,
     pub services: Vec<ServiceChunk>,
 }