--- a/plugins/gnat_json/gnat_ast_to_ptree.ml	2026-02-19 17:43:53.213295946 +0100
+++ b/plugins/gnat_json/gnat_ast_to_ptree.ml	2026-02-19 17:44:30.607845873 +0100
@@ -1389,7 +1389,7 @@
   close_out out_channel
 
 let read_channel env path filename c =
-  let json = Yojson.Safe.from_channel c in
+  let json = (Yojson.Safe.from_channel c :> Gnat_ast.From_json.t) in
   let gnat_file =
     let open Gnat_ast in
     try From_json.file_from_json json