Hi @pierreguillou I recently tried to use fastT5 with on a mT5 model and it worked great (regardless the warning), however you’ll need to have transformers=="4.6.1".
Are you getting any error ? Or are the outputs nonsense?
Note: I changed T5ForConditionalGeneration to MT5ForConditionalGeneration in onnx_exporter.py but I still think it was working without this.