Limits, units and budgets

Units

One unit is 500,000 engine nodes. A request's depth is turned into a node budget (about 1.6× more per ply), the engine runs go depth D nodes N, and you are billed the budget, never more. Multi-PV multiplies the budget by the number of lines; bestmove halves it; a cached position is free.

Depth	10	12	14	16	18	20	22	24	26	28	30
Units, 1 line	1	1	2	4	9	22	57	145	369	945	2,418

Per plan

	Free	Hobby	Pro
Units	2,000 per day, resets 00:00 UTC	100,000 per month, then $0.20 per 1,000	1,000,000 per month, then $0.12 per 1,000
Max depth	14	22	30
Burst / sustained	5 rps / 60 rpm	10 rps / 300 rpm	50 rps / 1,200 rpm
Concurrent async jobs	2	5	50

Caps you set

Key budget: budget_units or budget_usd on any key, hard stop with 402 raise_budget. Resets monthly.
Account spend cap: stops overage across all keys; emails at 80% and 100%.
Usage alerts: email or webhook at a threshold you choose.

Headers on every response

X-Cost-Units         units charged for this request (0 on a cache hit)
X-RateLimit-Remaining  burst tokens left
X-Quota-Remaining      units left today (Free) or this month (paid)
X-Budget-Remaining     units left on the key's budget, if set
X-Engine               build that answered, e.g. stockfish-17-wasm
Retry-After            seconds, on 429 and 503