Properly handle client function/tool calls that appear partway through the response stream.

[rajdtta]

The Issue

Currently, if a LLM response starts with regular content and then calls a tool or function partway through, the entire response will still be put into the message content field rather than populating the appropriate function_call or tool_calls field of the message (which will trigger the client callbacks to handle the function/tool calls).

Diagnosis / Root Cause

When l looked at the underlying implementation, I saw that callChatApi() was only checking whether the streamed response started with the function/tool call prefix (and not whether they were present midway though).

Proposed Solution

I added some extra logic to detect whether an intermediate chunk starts with the tool/function call prefix.

By default, all streamed chunks will populate the content field. The moment a function/tool call prefix is detected, a flag is set to parse the remainder of the message as either a function or tool call.

This change seems to solve the issue entirely, and I'm no longer able to replicate the original bug on my local environment despite my best efforts.

[rajdtta]

Hey @lgrammel,

Apologies if you're not the right person to tag for this, but I was wondering when someone would be able to take a look through (and hopefully approve) the PR.

[lgrammel]

Thanks for the PR. We are planning to switch to the StreamData protocol in the next major release and remove this part of the code: https://github.com/vercel/ai/pull/1192

In the meantime, you can already use experimental_streamData: true to activate the stream data protocol which should not have these issues.