diff --git a/codegen/genfacet b/codegen/genfacet
index 062683b..6b2c606 100755
--- a/codegen/genfacet
+++ b/codegen/genfacet
@@ -455,9 +455,6 @@ def main():
     parser.add_argument('--input', required=True, help='input IDL JSON5 file')
     # --facet-dir: only with mode=implementation
     parser.add_argument('--facet-dir', required=False, help='base dir for facet json')
-    # --output-impl-hpp: putting this in .json5, will be able to drop this.
-    parser.add_argument('--output-impl-hpp', required=True, help='.hpp detail subdir')
-    parser.add_argument('--output-hpp', required=True, help='.hpp output directory')
     parser.add_argument('--output-cpp', required=False, help='.cpp output directory')
 
     args = parser.parse_args()
@@ -465,12 +462,23 @@ def main():
     idl_fname = args.input
     idl = load_idl(idl_fname)
 
-    output_hpp_dir = Path(args.output_hpp)
+    # output_hpp_dir: prefer IDL, fall back to CLI
+    if 'output_hpp_dir' in idl:
+        output_hpp_dir = Path(idl['output_hpp_dir'])
+    elif args.output_hpp:
+        output_hpp_dir = Path(args.output_hpp)
+    else:
+        parser.error("--output-hpp required when .json5 lacks output_hpp_dir")
     output_hpp_dir.mkdir(parents=False, exist_ok=True)
 
-    # TODO: output_impl_hpp_subdir: use idl['detail_subdir'] instead
-    output_impl_hpp_subdir = Path(args.output_impl_hpp)
-    output_impl_hpp_dir = Path(args.output_hpp) / output_impl_hpp_subdir
+    # output_impl_hpp_subdir: prefer IDL, fall back to CLI
+    if 'output_impl_subdir' in idl:
+        output_impl_hpp_subdir = Path(idl['output_impl_subdir'])
+    elif args.output_impl_hpp:
+        output_impl_hpp_subdir = Path(args.output_impl_hpp)
+    else:
+        parser.error("--output-impl-hpp required when .json5 lacks output_impl_subdir")
+    output_impl_hpp_dir = output_hpp_dir / output_impl_hpp_subdir
     output_impl_hpp_dir.mkdir(parents=False, exist_ok=True)
 
     if 'output_cpp_dir' in idl: