Would be useful to be able to output ESM, cdk PR dropped 2 min ago https://github.com/aws/aws-cdk/pull/18346

Do you know if lambda requires it to be mjs file extension?

it probably is just like node in that it assumes cjs unless you have a package.json with type:module or a .mjs extension I believe

^ this. Probably would be good to be explicit but either works.