This looks really useful! Am I correct that there isn’t an S3 compatible API, just the “fetch” API?

Being able to set an S3 client’s endpoint to proxy traffic straight through this would be quite useful.

Yes, currently it has its own /fetch endpoint that then makes S3 GET(s) internally. One potential gotcha depending on how you are using it, an exact byte "Range" header is always required so that the request can be mapped to page-aligned byte range requests on the S3 object. But with that constraint, it is feasible to add an S3 shim.

It is also possible to stop requiring the header, but I think it would complicate the design around coalescing reads – the layer above foyer would have to track concurrent requests to the same object.