Structured output from GPT — function calling vs external parsing?

[JSONFIRST]

Two approaches for getting structured data from GPT:

Option A — OpenAI function calling / tool use: Let GPT decide when to call a tool and with what params. Flexible but can hallucinate parameters.

Option B — External intent parsing (JSONFIRST): Parse the user input before the GPT call. Return a validated JDON object. Then GPT gets a clean, typed context.

Which approach do you use? What are the trade-offs you've hit?