Extend support for useFileOutput to stream

[mattt]

No description provided.

[aron]

I think we want to explicitly match that data consists of only a valid data uri. There is a reasonable chance that a language model might emit a data: line that starts with "data:" but less so that it will emit a line that consists only of a well formed data-uri.

🤔 That said, I did think today after reading this post on other AI apis that we should move to structured outputs.

Perhaps the file stream should emit JSON.

data: {"type": "url", value: "data://..."}

And in future we refactor the text streaming interface to do the same:

data: {"type": "string", "data: and some more text"}

[mattt]

I think we want to explicitly match that data consists of only a valid data uri. There is a reasonable chance that a language model might emit a data: line that starts with "data:" but less so that it will emit a line that consists only of a well formed data-uri.

That's a good callout. The trick is finding a good way to validate without parsing the whole thing and throwing away the result. I think we can still read lazily if we use a regex to apply some heuristics about its first chunk of content.

🤔 That said, I did think today after reading this post on other AI apis that we should move to structured outputs.

Perhaps the file stream should emit JSON.

data: {"type": "url", value: "data://..."}

And in future we refactor the text streaming interface to do the same:

data: {"type": "string", "data: and some more text"}

I see the advantages of typed outputs, but also quite like the experience we have now of emitting raw tokens. In any case, structured outputs would be a backwards-incompatible change, so we'd have to be clever about a migration.

One way we could support both would be keeping Accept: text/event-stream as-is, but adding support for Accept: text/event-stream+json. The client libraries could start sending that as needed to opt into structured outputs.