akka-stream-json

Unfortunately, I'm not aware of any Scala libraries which support stream-based parsing of JSON. It seems to me that some support for this is available in Google Gson, but I'm not entirely sure it can properly handle "broken" input.

What you can do, however, is to collect JSON documents in a streaming fashion, similarly to what Framing.delimiter does. This is very similar to the alternative you have mentioned, but it is not using fold(); if you do go this way, you would probably need to mimic what Framing.delimiter does but instead of looking for a single delimiter, you will need to balance curly braces (and optionally brackets, if top-level arrays are possible), buffering the intermediate data, until the entire document comes through, which you would emit as a single chunk suitable for parsing.

Just as a side note, an appropriate interface for a streaming JSON parser suitable to be used in Akka Streams could look like this: