# Solvers

Solvers are a class of originator on Blink — they submit transactions through the standard Ethereum RPC endpoint but receive specialised auction treatment. Specifically:

* Solver transactions are forwarded to builders **without a gas recovery delay**
* Solver transactions are eligible for a dedicated **OFA fan-out** window: Blink holds the transaction until 10 seconds into the pending slot, then fans it out to other originators only if no searcher has placed a bid. This preserves searcher exclusivity while still ensuring coverage.

There are two integration paths available to solvers:

| Path                              | Best for                                                         |
| --------------------------------- | ---------------------------------------------------------------- |
| Raw transaction via RPC endpoint  | Submitting individual solver transactions                        |
| Builder endpoint (`/builder/v1/`) | Submitting and cancelling bundles using Flashbots-compatible API |

***

## Cancelling a Transaction

Solvers can cancel a pending transaction by submitting a **cancel signal** — a zero-value self-transfer with the same nonce as the transaction to cancel.

**A cancel transaction is defined as:**

* `to` — same address as the sender
* `value` — `0`
* `data` — empty

When Blink receives this, it:

1. Detects it is a cancel signal (not a real transaction)
2. Looks up the original transaction by `(sender, nonce)` in cache
3. Removes the original transaction from the auction
4. Forwards `eth_cancelBundle` requests to all active builders for the next 3 blocks

{% hint style="info" %}
The cancel signal itself is never forwarded to builders or stored — only the resulting cancellation is propagated.
{% endhint %}

***

## Builder Endpoint

Blink exposes a **builder-compatible API** at `/builder/v1/<originatorId>` that emulates the standard Flashbots builder interface. This lets solvers use the same bundle submission code they already use with direct-builder integrations.

The endpoint supports the following JSON-RPC methods:

* `eth_sendRawTransaction`
* `eth_sendBundle`
* `eth_cancelBundle`

### eth\_sendBundle

Submit a bundle targeting a specific block. Blink fans the bundle out to all active builders.

```json
{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "eth_sendBundle",
  "params": [
    {
      "txs": [
        "0x...",
        "0x..."
      ],
      "blockNumber": "0x1488DCF",
      "replacementUuid": "d9be8dba-6954-47bf-8ee2-dab3feeb11ee",
      "revertingTxHashes": []
    }
  ]
}
```

| Field               | Type       | Required | Description                                                                                              |
| ------------------- | ---------- | -------- | -------------------------------------------------------------------------------------------------------- |
| `txs`               | `string[]` | Yes      | Array of signed raw transactions (hex-encoded)                                                           |
| `blockNumber`       | `string`   | No       | Target block number (hex). Defaults to current block                                                     |
| `replacementUuid`   | `string`   | No       | Canonical UUID string (RFC 4122); used to cancel or replace this bundle (not an arbitrary opaque string) |
| `revertingTxHashes` | `string[]` | No       | Hashes of transactions in the bundle that are allowed to revert                                          |
| `maxTimestamp`      | `number`   | No       | Latest Unix timestamp at which the bundle is valid                                                       |

**Response:**

```json
{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "bundleHash": "0x..."
  }
}
```

{% hint style="info" %}
Set `?solver=true` as a query parameter to enable solver-mode fan-out for bundles submitted through this endpoint.
{% endhint %}

### eth\_cancelBundle

Cancel a previously submitted bundle by its `replacementUuid`. Blink removes the bundle from its cache and forwards the cancellation to all active builders.

```json
{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "eth_cancelBundle",
  "params": [
    {
      "replacementUuid": "d9be8dba-6954-47bf-8ee2-dab3feeb11ee"
    }
  ]
}
```

**Response:**

```json
{
  "jsonrpc": "2.0",
  "id": 1,
  "result": 200
}
```

{% hint style="warning" %}
`replacementUuid` must match exactly what was set in the original `eth_sendBundle` call. If no matching bundle is found in cache, the cancellation is still forwarded to builders but has no effect on Blink's internal state.
{% endhint %}