FX fees and rounding

OpenPoly settles with integer units only.

Units

• credits-equivalent: integer, 10000 = 1 USDT
• shares: integer micro-shares, 1000000 = 1 share
• IDR: integer minor unit, exponent 0
• FX rate: IDR per 1 USDT
• BPS denominator: 10000

No floating settlement values are persisted.

FX freshness

• target freshness: <= 30 seconds
• hard stale threshold: 120 seconds
• stale FX beyond hard threshold -> execution fails before wallet mutation

Fee model

• fx_fee_bps
• operator_fee_bps
• total_fee_bps = fx_fee_bps + operator_fee_bps

Rules:

• fee values are non-negative integers
• total fee must stay < 10000
• buy applies fee before AMM quote
• sell and redemption apply fee after credits-equivalent proceeds known

Buy formula

Given:

• amount_minor
• idr_per_usdt
• total_fee_bps

net_bps = 10000 - total_fee_bps
credits_equivalent = floor(amount_minor * net_bps * 10000 / (10000 * idr_per_usdt))

Debit amount remains original amount_minor.

Sell formula

Given AMM credits-equivalent proceeds:

net_bps = 10000 - total_fee_bps
external_amount_minor = floor(credits * idr_per_usdt * net_bps / (10000 * 10000))

Redemption formula

credits_equivalent = floor(shares_micro * 10000 / 1000000)
net_bps = 10000 - total_fee_bps
external_amount_minor = floor(credits_equivalent * idr_per_usdt * net_bps / (10000 * 10000))

Rounding rule

Always floor final integer conversion. Never round up payout or buying power.

Quote safety

Execution rejects when:

• quote ID missing or changed
• FX snapshot stale beyond hard threshold
• final execution worse than accepted slippage

Why this matters for operators

• wallet side always sees integer IDR minor amounts
• webhook and reporting APIs expose already-rounded final values
• operator reconciliation should compare integer amounts only