# Answer a swap’s bonus flip

> Submit a coinflip decision for an eligible order

`PUT /v2/swaps/{id}/coinflip`

Submit a coinflip decision for an eligible order.

**Authentication:** Required: the order’s owner session in `X-Session-ID`.

## Parameters

| Name | In | Type | Required | Description |
| --- | --- | --- | --- | --- |
| `id` | path | string (pattern `^[A-Za-z0-9:_-]{1,128}$`) | Yes | Public order identifier. Not a credential: every read still requires the owner session or the destination proof. |

## Request body

- `round` · integer · **required** — The pending round this decision answers. A stale decision is rejected, never applied to a later round.
- `accept` · boolean · **required** — Whether to take the offered flip. false cashes the bonus out instead. Omission is never treated as a decline — the field is required.
- `clientSeed` · string (max 64, pattern `^[\x20-\x7E]*$`) or null — Your half of the provably-fair seed. The outcome is HMAC(serverSeed, clientSeed), and SHA256(serverSeed) is committed before you choose it. Omit to let the engine use a fallback seed.
- `side` · `"cow"` · `"milk"` or null — Which band wins. It selects the winning range only and never feeds the HMAC, so it cannot change the drawn number.
- `stakeAtomic` · string (atomic integer) or null — Coinflip TABLE only: the bet for this round, in the asset's smallest unit. Proposed, not binding — the engine refuses a bet above your balance, below the minimum, or beyond the pool's exposure cap, and leaves the round open so you can correct it. Ignored by the in-swap bonus, which always stakes the whole delivery.

## Response

`200` returns an object with:

- `round` · number
- `result` · number
- `won` · boolean
- `side` · `"cow"` · `"milk"`
- `multiplier` · number
- `finalMultiplier` · number or null · **required**
- `serverSeed` · string
- `seedHash` · string
- `cashedOut` · boolean
- `revealedUnusedSeed` · string
- `unusedSeedHash` · string
- `nextRound` · object or null
  - `round` · number · **required**
  - `seedHash` · string · **required**
  - `multiplierIfWin` · number · **required**
  - `currentMultiplier` · number · **required**
  - `decisionDeadline` · string
- `policy` · object
  - `edgePercent` · number · **required**
  - `milkWinBelow` · number · **required**
  - `cowWinAbove` · number · **required**
  - `winChancePercent` · number · **required**
  - `maxWalletFractionPercent` · number
  - `maxPayoutAtomic` · string (atomic integer) — Positive base-10 integer in the asset’s atomic unit.

Errors use the [standard error envelope](https://docs.wagyu.xyz/actions-and-retries.md#the-error-envelope): branch on `code`, never on the message.

## Example

<!-- curl -->
```bash
curl -X PUT 'https://api-beta.wagyu.xyz/v2/swaps/<orderId>/coinflip' \
  -H 'x-session-id: <sessionId>' \
  -H 'content-type: application/json' \
  -d '{
    "round": 1,
    "accept": false
  }'
```

<!-- JavaScript -->
```javascript
const res = await fetch('https://api-beta.wagyu.xyz/v2/swaps/<orderId>/coinflip', {
  method: 'PUT',
  headers: {"content-type":"application/json","x-session-id":"<sessionId>"},
  body: JSON.stringify({
    "round": 1,
    "accept": false
  }),
});
const data = await res.json();
```

<!-- Python -->
```python
import requests

res = requests.put('https://api-beta.wagyu.xyz/v2/swaps/<orderId>/coinflip', headers={"x-session-id":"<sessionId>"}, json={
        "round": 1,
        "accept": False
    }, timeout=30)
data = res.json()
```

_No response is captured for this route: calling it creates or changes a real order. The response fields above come from the live contract._

---
Source: https://docs.wagyu.xyz/api-reference/resolve-coinflip · Built from the live Wagyu API on 2026-11-04.