# Stable Pools

Stable pools are Biscotti's second AMM: two-coin **StableSwap** pools built
for assets that should trade near parity — stablecoin pairs like USDC/EURC,
or wrapped/liquid-staked variants of the same asset.

## Why a different curve?

A constant-product AMM (`x·y = k`) always charges slippage, even when both
assets are worth $1. The StableSwap invariant blends a constant-*sum* curve
(zero slippage) with a constant-product curve (guaranteed liquidity at any
price):

```
A·n^n·Σxᵢ + D = A·D·n^n + D^(n+1) / (n^n·Πxᵢ)
```

* `D` — the invariant: total value of the pool when balanced
* `A` — the **amplification coefficient**: how strongly the pool behaves like
  a constant-sum curve near the peg
* `n = 2` — number of coins in a Biscotti pool

While the pool is balanced, swaps execute almost 1:1. If one side is drained,
the curve smoothly degrades toward constant-product pricing, so the pool never
runs out of either asset.

```
price
  │  constant-product (x·y=k)
  │      ╲
  │       ╲            StableSwap (high A)
  │        ╲       ────────────────────
 1├─────────╲─────╱────────────────────╲─────
  │          ╲___╱                      ╲___
  │
  └──────────────────────────────────────────▶
                 pool balance ratio
```

### Amplification in practice

Biscotti's deployed pools use `A = 200`. Higher `A` = flatter curve = lower
slippage near the peg, but sharper price movement if the pool becomes very
imbalanced. `A` can be ramped gradually by governance
(`ramp_A`), never jumped instantly.

## Fees

| Parameter | Value (USDC/EURC pool) | Notes |
| --- | --- | --- |
| Swap fee | `4000000` = **0.04%** | Denominator `1e10`; max 0.5% |
| Admin fee | `5000000000` = **50% of the swap fee** | Protocol's share; remainder compounds for LPs |

Effective LP earnings: 0.02% of every swap compounds directly into the pool
balance (there is no separate claim step for stable-pool fees).

## LP tokens

Each pool has a companion `StableSwapLP` ERC-20. Unlike
[concentrated liquidity](/concentrated-liquidity) NFTs, stable LP tokens are
fungible — deposit any mix of the two coins and receive LP tokens
proportional to the value you added.

Stake the LP token in a [classic farm](/farms/classic-farms) to earn BSCT on
top of swap fees.

## Deployed pools — ARC Testnet

| Pool | Swap contract | LP token |
| --- | --- | --- |
| USDC/EURC | `0xBB9d51F97CA9553A0D1463c88173603a140E0E3f` | `0x225057406e3C7D3f3196EC13fC82ed04E2D647Dd` |

Infrastructure addresses (factory, router, info helper) are in the
[Contract Reference](/stable-pools/contracts).

## Learn more

* [Providing Liquidity](/stable-pools/providing-liquidity) — deposit, withdraw
  and the virtual price
* [Creating a Pool](/stable-pools/creating-a-pool) — how new pairs are added
* [Contract Reference](/stable-pools/contracts) — full on-chain API