diff --git a/crates/native-sidecar/src/execution/javascript/rpc.rs b/crates/native-sidecar/src/execution/javascript/rpc.rs
index 047e8ad321..d57f3db8bf 100644
--- a/crates/native-sidecar/src/execution/javascript/rpc.rs
+++ b/crates/native-sidecar/src/execution/javascript/rpc.rs
@@ -225,12 +225,31 @@ pub(crate) fn deferred_kernel_wait_request_for_process(
         return Ok(None);
     }
     let fd = javascript_sync_rpc_arg_u32(&request.args, 0, "filesystem write fd")?;
+    // Mapped host fds live in the per-process map, NOT the kernel fd table, so
+    // stat'ing one here raised `EBADF: bad file descriptor 1000000000` on EVERY
+    // guest file write. That error escaped the process-event pump: it used to
+    // kill the whole sidecar (LT-011 cross-tenant DoS) and, once the pump
+    // stopped propagating it, left the guest's sync-RPC response undelivered
+    // until the 31 s bridge deadline (LT-024). A mapped host fd is host-backed
+    // and can never be a kernel pipe, so deferral never applies to it.
+    //
+    // The ownership lookup is authoritative and is preferred over a numeric
+    // `fd >= MAPPED_HOST_FD_START` range test: an fd in that range that this
+    // process does NOT own should still fall through to the kernel and be
+    // rejected there, rather than being silently treated as host-backed.
     if process.mapped_host_fd(fd).is_some() {
         return Ok(None);
     }
-    let stat = kernel
-        .fd_stat(EXECUTION_DRIVER_NAME, process.kernel_pid, fd)
-        .map_err(kernel_error)?;
+    let stat = match kernel.fd_stat(EXECUTION_DRIVER_NAME, process.kernel_pid, fd) {
+        Ok(stat) => stat,
+        // Defense in depth for the whole class: any fd this kernel does not own
+        // is by definition not a kernel pipe, so it is not deferrable. Report
+        // "no deferral" rather than failing the write with an unrelated errno —
+        // the normal write path re-checks ownership and raises the correct
+        // errno there, so the loud failure is preserved, not swallowed.
+        Err(error) if error.code() == "EBADF" => return Ok(None),
+        Err(error) => return Err(kernel_error(error)),
+    };
     if stat.filetype != agentos_kernel::fd_table::FILETYPE_PIPE {
         return Ok(None);
     }