toUIMessageStream read Issues

[ghardin1314]

When using with anthropic/sonnet-claude-4 and the ai sdkv5 beta, I am getting an issue parsing the streamResult.toUIMessageStream when using the readUIMessageStream utility. Here is an example:

readUIMessageStream({
      stream: streamResult.toUIMessageStream({
        sendReasoning: true,
        sendSources: true,
      }),
      message: lastMessage,
    })

When I am doing this, the output of the assistant message only gives the step-start part and none of the following parts. This workflow was working fine with the first party anthropic provider, and it can also see the streamResult.fullStream contains all the right data as well