# Fast by Circular — API

> Land your Solana transactions in the current slot, 0-slot delay, and earn back 65% of the value they leak, paid in SOL.

Source: https://landfast.io · Docs: https://docs.circular.fi/api/fast

## Endpoint

Drop-in replacement for any Solana RPC `sendTransaction`. Swap the URL, add the
key header; nothing else changes in your code.

- HTTPS: `https://fast.circular.fi/transactions`
- gRPC: `fast.circular.fi:443` (regional engines only, not the global entry point)

Plain HTTP is also accepted. It skips the TLS handshake and is the faster of the
two, at the cost of sending your key unencrypted.

## Authentication

Pass your API key in the `x-api-key` header. Keys are created on https://circular.fi.

## Regions

| Region | Host |
|---|---|
| Global | `fast.circular.fi` |
| Frankfurt | `fra.fast.circular.fi` |
| Amsterdam | `ams.fast.circular.fi` |
| New York | `ny.fast.circular.fi` |

Targeting a region skips the routing to the nearest engine. The global entry
point picks the closest one for you.

## Request

```bash
curl -X POST https://fast.circular.fi/transactions \
  -H 'Content-Type: application/json' \
  -H 'x-api-key: YOUR_API_KEY' \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "sendTransaction",
    "params": [
      "BASE64_OR_BASE58_ENCODED_TRANSACTION",
      { "encoding": "base64", "cashbackAddress": "YOUR_SOLANA_ADDRESS" }
    ]
  }'
```

Base58 or Base64, whatever your SDK already produces.

## cashbackAddress

Optional but recommended: without it there is nowhere to send your share of the
MEV. Any Solana address you control works, including one different from the
signer.

Full reference: https://docs.circular.fi/api/fast