> ## Documentation Index
> Fetch the complete documentation index at: https://corridor.udokaam.dev/llms.txt
> Use this file to discover all available pages before exploring further.
# Tempo: the settlement hub
> Why Tempo is the hub, and exactly how Corridor uses it: pathUSD as the unit of account, TIP-20 transferWithMemo with indexed memos, gas paid in stablecoins through the Fee Manager, and a log observer with no indexer.
Tempo is a payments-first chain, incubated by Stripe and Paradigm. Corridor holds every customer balance on it, settles every flow through it, and reconciles against its logs.
## Why Tempo is the hub
TIP-20 `transferWithMemo` takes a `bytes32` memo and emits it as an **indexed topic** of `TransferWithMemo`. The 32-byte Corridor Reference fits exactly, and reconciliation filters logs by reference with plain `eth_getLogs`.
There is no native gas token. Vaults pay fees in pathUSD, the asset they already hold, through the Fee AMM. No ETH float to manage per vault.
About 0.6 s to finality, so the hub is never the slow step in a payment.
pathUSD is Tempo's quote token, issued by Bridge (a Stripe company). Every USD TIP-20 routes through it on the enshrined stablecoin DEX.
## pathUSD as the unit of account
Every stablecoin on Tempo is a TIP-20, including USDC.e (Stargate-bridged USDC). Corridor treats **pathUSD** as its unit of account and accepts any USD TIP-20 at the edges:
```text theme={"dark"}
Ingress (any USD TIP-20) Core (Tempo) Egress
USDC.e / USDT0 / pathUSD ──DEX──▶ pathUSD vaults + ledger ──Across──▶ Base USDC ──▶ partner
```
pathUSD is supported by Across (`pathUSD (4217) → USDC` on Base and others), issued by the Moderato faucet so testnet matches production, and accepted by validators as a fee token. Its circulating supply is still small, so each corridor carries a `settlementAsset` policy: when a ticket exceeds pathUSD depth, the corridor settles in USDC.e instead. Switching is configuration, not a migration. See [ADR 0001](/decisions/0001-pathusd-settlement-unit).
## Client
Corridor uses viem's built-in Tempo support (`viem/tempo`):
```ts theme={"dark"}
import { createClient, Actions, Addresses } from "viem/tempo";
const client = createClient({
account: privateKeyToAccount(key),
testnet: network === "moderato",
feeToken: Addresses.pathUsd, // pay gas in pathUSD
transport: http(rpcUrl, { timeout: 30_000, retryCount: 5 }),
});
```
| Network | Chain id | RPC | Explorer |
| - | - | - | - |
| Moderato testnet | 42431 | `https://rpc.moderato.tempo.xyz` | `explore.testnet.tempo.xyz` |
| Mainnet | 4217 | `https://rpc.tempo.xyz` | `explore.tempo.xyz` |
The Moderato faucet funds an address through the RPC method `tempo_fundAddress`.
## Moving value
```ts theme={"dark"}
const obs = await transferWithReference(client, {
asset: "pathUSD",
to: escrow,
amount: 1_000_000_000n, // 1,000 pathUSD (6 decimals)
reference: ref, // 32-byte Corridor Reference
});
// → the Observation for the resulting TransferWithMemo log, ready to book
```
Legs call `tempoMoveOnce`: before sending, they search for an identical movement (same memo, parties and amount) since the saga's start block. A retry after a crash between send and bookkeeping finds the transfer instead of sending it twice.
## Observing
`TempoObserver` watches the vault addresses from a stored block cursor:
* `TransferWithMemo` logs **in and out** of each vault (two queries, because indexed-argument filters are ORed within an array).
* Optionally, **unreferenced** movements: plain inbound `Transfer`s (faucet mints, owner top-ups, deposits without a memo) and **fee transfers** to the Fee Manager.
## Gas accounting
Every Tempo transaction pays its fee as a TIP-20 `Transfer` from the fee payer to the Fee Manager at `0xfeec000000000000000000000000000000000000`, logged in the same receipt. On a real 2.000000 pathUSD transfer the vault's balance fell by 2.000098: 98 base units were the fee, equal to `gasUsed × effectiveGasPrice ÷ 10¹²`, rounded up. Corridor books each fee a vault pays to `expense:gas`, keyed by that log, so the treasury's on-chain balance equals the ledger to the last unit. See [Reconciliation](/architecture/reconciliation#vault-accounting-from-the-chain).
## Evidence
* Memo transfer found by indexed topic and booked once: `pnpm tempo:smoke`.
* Payout to a Shenzhen supplier with a decodable reference: [0x671161f6…](https://explore.testnet.tempo.xyz/tx/0x671161f61c70a0b5b33c6c8de4221c938d86c310958725146ea9f21517957b5e).
* Treasury on-chain = ledger: 1,000,051.994555 pathUSD, 10 fees booked from the chain.
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