File system issue for api / trying to fs.readFileSync dynamically

The included_files configuration would work as long as the directory structure of the Netlify Functions as the folder you’re including is correct. For example in your code, you’ve set the functions directory as functions so I’m assuming you’re directory structure is:

|-- functions/
  |-- someFile.js // here you're accessing  `../tokendata/${id}`
|-- tokendata/
  |-- 12345.json
|-- netlify.toml

For a file structure like this, it should work as you’ve described. But, if you say your files were 50 MB in size, there’s a slight chance you’d run into AWS Lambda limit of 50 MB size for function bundle. Having so many files could also potentially cause Lambda file descriptor issue.