Playground

The same intent→action request, run through a 12M Fern model and through GPT-4o, side by side. Watch the structured call, the latency, and the cost for each.

Turn scheduling requests into calendar actions. Book events, check your agenda, cancel by id, and find a free slot.

add a kickoff Thursday at 1pm, invite Wei

book a retro next Friday at 5pm with Sofia

set up a check-in with Marcus on Tuesday 12:30

what's on my calendar Saturday

show my schedule for next Thursday

do I have anything 2026-08-15

delete event 612

cancel the meeting with id 925

find a free 75 minute slot Saturday

any 105 min gap next Friday?

find time for a 50 minute meeting 2026-08-15

Methodology

Both models receive the identical tool schema. GPT-4o runs via the OpenAI API at published rates ($2.50 / $10 per 1M input / output tokens) - its cost includes re-sending the whole tool spec on every call. Fern runs on a single RTX PRO 6000 with no per-call API fee; it sends a registered tool_set_id instead of the full schema. Latency is wall-clock end-to-end. “Match” means the predicted call exactly matched tool name and arguments; “right tool” means the correct tool was chosen but argument(s) differs. Fern always runs live; because GPT-4o is deterministic here (temperature 0) and returns the same answer for a given preset, its result is cached and refreshed periodically. The cost shown is a representative per-call figure.