diff --git a/crates/fbuild-daemon/src/handlers/operations/deploy.rs b/crates/fbuild-daemon/src/handlers/operations/deploy.rs
index 0ce7d646..984a5d1d 100644
--- a/crates/fbuild-daemon/src/handlers/operations/deploy.rs
+++ b/crates/fbuild-daemon/src/handlers/operations/deploy.rs
@@ -1228,6 +1228,19 @@ pub async fn deploy(
         };
 
         ctx.serial_manager.detach_reader(&monitor_port, &request_id);
+        // Detaching the reader alone leaves the session -- and its OS serial
+        // handle -- open forever, so the next client gets EBUSY on a board
+        // that is enumerated and perfectly healthy. The monitor and WebSocket
+        // cleanup paths both schedule the physical close here; this one did
+        // not, which is why the leak only showed up after a deploy.
+        // See FastLED/fbuild#1426.
+        if !ctx.serial_manager.has_clients(&monitor_port) {
+            ctx.serial_manager.close_port_after_grace_if_idle(
+                &monitor_port,
+                &request_id,
+                std::time::Duration::from_secs(2),
+            );
+        }
 
         return match monitor_result {
             MonitorOutcome::Success(msg) => (