TextMessageContent Delta: Plain Text or JSON Data?

[SInXW]

Does the delta field in the TextMessageContent event always contain plain text, or can it include JSON data, as in ToolCallArgs? In my scenario, the content includes metadata, so the delta would be JSON-formatted data.
For example, I expect the frontend to perform simple text processing, such as controlling font size and color. The complete data would be:

{"color":"red","size":1,"text":"hello world"}

and the data stream would be:

{"delta":"{\"color\":\"red\",\"size\":1,\"text\":\""}
{"delta":"hello"}
{"delta":"world"}
{"delta":"\"}"}

If the delta field can be JSON fragments, the challenge is that the frontend cannot easily distinguish whether TextMessageContent contains plain text or JSON data. In contrast, ToolCallArgs allows differentiation based on the toolCallName field in ToolCallStart.