How do PoA fees work?

PoA fees are the Privacy on Demand fees you pay in AVAX on Avalanche Fuji C-Chain (43113) for a two-way Inbox message. Your Fuji contract typically calls something like add(itUint64 a, itUint64 b), and the success path returns ctUint64 c in a later callback on Fuji. The same payment model applies to other PodLib operations that take msg.value and callbackFeeLocalWei.

You always pay on Fuji in AVAX. Oracles convert the COTI-bound slice into a remote execution budget; the callback slice stays denominated for Fuji gas.

ABI note: The parameter is named callbackFeeLocalWei in Solidity. On Fuji that means the 18-decimal AVAX subunit (same decimal layout as wei on Ethereum), not ETH. Avalanche gas prices are usually quoted in nAVAX ((10^{-9}) AVAX = (10^{9}) of those subunits).

What you are paying for

A two-way PoD request does work on two chains, in order:

1. COTI (outbound leg) — relay the request and run private compute (for example MPC add).

2. Avalanche Fuji (return leg) — relay the response and run your onlyInbox callback so c is written on Fuji.

If either leg is underfunded, the job can stall as pending even when the other leg would have succeeded (see Async private operations).

callbackFeeLocalWei is a slice of msg.value, not an extra payment on top.

How Fuji AVAX becomes gas-unit budgets

The Inbox fee manager (conceptually) does this when you call sendTwoWayMessage / a PodLib helper:

Symbol	Meaning on Fuji
(V)	msg.value in AVAX subunits
(C)	callbackFeeLocalWei (Fuji / callback slice), (C \le V)
(R)	Remote slice (R = V - C) (funds COTI via oracle)
(G)	Effective gas price of the Fuji tx (subunits per gas), typically tens of nAVAX

Fuji callback budget (gas units):

[ \texttt{callerFee} = \left\lfloor \frac{C}{G} \right\rfloor ]

COTI remote budget (gas units), after oracle scaling:

[ \texttt{targetFee} = \left\lfloor \frac{R}{G} \right\rfloor \times \frac{P_{\mathrm{AVAX}}}{P_{\mathrm{COTI}}} ]

where (P_{\mathrm{AVAX}}) and (P_{\mathrm{COTI}}) are the Fuji Inbox price oracle USD prices for local (AVAX) and remote (COTI) tokens.

Because AVAX is typically much more valuable than COTI per token, a modest AVAX remote slice often converts into a large COTI gas-unit budget. That is expected on Fuji; do not copy Sepolia ETH numbers.

Live conversions and minima come from the deployed Fuji Inbox / fee manager / oracle — always prefer on-chain views over this page’s arithmetic.

Worked example (Avalanche Fuji, pedagogical)

Assumptions for one readable walkthrough (not live oracle quotes):

Input	Value
msg.value	0.02 AVAX
callbackFeeLocalWei	0.008 AVAX (Fuji callback slice)
Remote slice (R)	0.012 AVAX (pays for COTI leg)
Fuji gas price (G)	25 nAVAX
Oracle (P_{\mathrm{AVAX}})	$25 / AVAX
Oracle (P_{\mathrm{COTI}})	$0.017 / COTI

Step A — Split the AVAX payment

Leg	AVAX	Role
Callback (Fuji)	0.008	Relayer response + addCallback on C-Chain
Remote (COTI)	0.012	Relay + private add on COTI
Total	0.02	Attached as msg.value on the Fuji tx

USD value of the remote slice: (0.012 \times 25 = $0.30), or about 17.65 COTI at $0.017 / COTI. That intuition check is optional; the Inbox stores gas-unit budgets, not a COTI balance in your wallet.

Step B — Convert to gas-unit ledgers

Fuji callerFee:

[ \frac{0.008\ \mathrm{AVAX}}{25\ \mathrm{nAVAX}} = 320{,}000\ \text{gas units} ]

Pre-oracle remote units (AVAX paid ÷ Fuji gas price):

[ \frac{0.012\ \mathrm{AVAX}}{25\ \mathrm{nAVAX}} = 480{,}000 ]

COTI targetFee (scale by AVAX/COTI oracle prices):

[ 480{,}000 \times \frac{25}{0.017} \approx 705{,}882{,}353\ \text{gas units} ]

Ledger	Opening balance (gas units)
COTI (targetFee)	705,882,353
Avalanche Fuji (callerFee)	320,000

Step C — Spend the ledgers in order

COTI runs first; Fuji callback runs only after the encrypted result exists.

Step	Where	What happens	Gas used	COTI remaining	Fuji remaining
—	—	Opening balances	—	705,882,353	320,000
1	COTI	Relayers relay request	12,000	705,870,353	320,000
2	COTI	Private add → ciphertext c	492,902	705,377,451	320,000
3	Avalanche Fuji	Miner relays response	8,000	705,377,451	312,000
4	Avalanche Fuji	addCallback stores ctUint64 c	230,000	705,377,451	82,000

Remainders are illustrative. Production refunds / keep policies depend on InboxMiner / InboxFeeManager configuration.

Why underfunding fails differently on each leg

• Too little msg.value overall (or too little remote slice) → COTI relay / private execution may not clear minima → outbound path fails.

• callbackFeeLocalWei too small → COTI may finish, but Fuji never successfully writes c → UI stays pending.

• Gas price matters: budgets are ({\approx},\mathrm{fee}/G). A higher Fuji tx.gasprice for the same AVAX payment yields a smaller gas-unit budget. Estimate with the gas price you will actually use.

How to budget fees in practice on Fuji

1. Prefer the deployed Fuji Inbox view:

   calculateTwoWayFeeRequiredInLocalToken(...)

2. Or estimate from TypeScript with @coti-io/pod-sdk:

const fee = await pod.estimateFee("add", podArgs, {
  forwardGasLimit: 400_000n,
  callBackGasLimit: 250_000n,
  callBackDataSize: 512n,
  gasPrice: (await signer.provider!.getFeeData()).gasPrice!,
});
// fee.totalFee → msg.value
// fee.callBackFee → callbackFeeLocalWei (must be ≤ totalFee)

1. Attach fee.totalFee as msg.value and pass fee.callBackFee as callbackFeeLocalWei (mark that argument with isCallBackFee: true when using PodContract).

2. Confirm the Fuji price oracle and Inbox addresses on Avalanche Fuji.

Where this shows up in Solidity

• Payable add (or other PodLib helpers) with msg.value and callbackFeeLocalWei — see Tutorial: private Adder on Avalanche Fuji.

• Integration model context: Tutorials overview.

• Contract-level detail: SDK Fees, gas, and oracle.

Disclaimer

Oracle prices, gas prices, minimum fee templates, and remainders on this page are pedagogical. They show the Fuji AVAX → dual gas-unit ledger story with arithmetic that matches the Inbox fee-manager shape. They do not replace live Fuji Inbox / oracle configuration for your deployment.