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Base Flashblocks API quickstart

Get started building on Base using Flashblocks

Flashblocks on Base are a transaction preconfirmation feature that provides near-instant transaction feedback by streaming partial block updates every 200 milliseconds. This significantly reduces the effective block time from Base's standard 2 seconds to 200 milliseconds, a 10x improvement.

Flashblocks is great for developers who demand instant UX such as decentralized exchanges, onchain gaming, and other high-frequency applications.

Flashblocks is currently supported on both Base testnet and mainnet and can be accessed using your existing Alchemy Base RPC.

NetworkFlashblocks Availability
Base Sepolia✅
Base Mainnet✅

Use the pending tag to retrieve the latest Flashblock:

curl https://base-sepolia.g.alchemy.com/v2/<YOUR_API_KEY> -X POST -H "Content-Type: application/json" -d '{"jsonrpc":"2.0","method":"eth_getBlockByNumber","params":["pending",true],"id":1}'

Example Response

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "number": "0x1234",
    "hash": "0x...",
    "transactions": [...]
  }
}

Use the existing receipt RPC to get preconfirmed receipts:

curl https://base-sepolia.g.alchemy.com/v2/<YOUR_API_KEY> -X POST -H "Content-Type: application/json" -d '{"jsonrpc":"2.0","method":"eth_getTransactionReceipt","params":["0x..."],"id":1}'

Example Response

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "transactionHash": "0x...",
    "blockNumber": "0x1234",
    "status": "0x1"
  }
}

Use the pending tag to get the address balance in the latest Flashblock:

curl https://base-sepolia.g.alchemy.com/v2/<YOUR_API_KEY> -X POST -H "Content-Type: application/json" -d '{"jsonrpc":"2.0","method":"eth_getBalance","params":["0x...","pending"],"id":1}'

Example Response

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": "0x0234"
}

Use the pending tag to get the address nonce in the latest Flashblock:

curl https://base-sepolia.g.alchemy.com/v2/<YOUR_API_KEY> -X POST -H "Content-Type: application/json" -d '{"jsonrpc":"2.0","method":"eth_getTransactionCount","params":["0x...","pending"],"id":1}'

Example Response

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": "0x1b" // 27 transactions
}

Use the existing get transaction by hash RPC to get preconfirmed transactions:

curl https://base-sepolia.g.alchemy.com/v2/<YOUR_API_KEY> -X POST -H "Content-Type: application/json" -d '{"jsonrpc":"2.0","method":"eth_getTransactionByHash","params":["0x..."],"id":1}'

Example Response

{
  "type": "0x2",
  "chainId": "...",
  "nonce": "...",
  ...
}

Use the pending tag to execute a smart contract call against the latest Flashblock:

curl https://base-sepolia.g.alchemy.com/v2/<YOUR_API_KEY> -X POST -H "Content-Type: application/json" -d '{"jsonrpc":"2.0","method":"eth_call","params":[{"to":"0x...","data":"0x..."},"pending"],"id":1}'

Example Response

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": "0x0000000000000000000000000000000000000000000000000000000000000001"
}

Use the pending tag to simulate transactions against the latest Flashblock:

curl https://base-sepolia.g.alchemy.com/v2/<YOUR_API_KEY> -X POST -H "Content-Type: application/json" -d '{"jsonrpc":"2.0","method":"eth_simulateV1","params":[{"blockStateCalls":[{"calls":[{"to":"0x...","data":"0x..."}],"stateOverrides":{}}],"traceTransfers":true,"validation":true},"pending"],"id":1}'

Example Response

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": [
    {
      "calls": [
        {
          "status": "0x1",
          "gasUsed": "0x5208",
          "returnData": "0x"
        }
      ]
    }
  ]
}

Use the pending tag to estimate gas against the latest Flashblock:

curl https://base-sepolia.g.alchemy.com/v2/<YOUR_API_KEY> -X POST -H "Content-Type: application/json" -d '{"jsonrpc":"2.0","method":"eth_estimateGas","params":[{"to":"0x...","data":"0x..."},"pending"],"id":1}'

Example Response

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": "0x5208"
}

Use the pending tag for toBlock to retrieve logs from the latest Flashblock:

curl https://base-sepolia.g.alchemy.com/v2/<YOUR_API_KEY> -X POST -H "Content-Type: application/json" -d '{"jsonrpc":"2.0","method":"eth_getLogs","params":[{"fromBlock":"latest","toBlock":"pending","address":"0x...","topics":["0x..."]}],"id":1}'

Example Response

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": [
    {
      "address": "0x...",
      "topics": ["0x..."],
      "data": "0x...",
      "blockNumber": "0x1234",
      "transactionHash": "0x...",
      "transactionIndex": "0x0",
      "blockHash": "0x...",
      "logIndex": "0x0",
      "removed": false
    }
  ]
}

Flashblocks-enabled WebSocket subscriptions stream preconfirmed data at approximately 200ms intervals via eth_subscribe. Standard subscription types (newHeads, logs, newPendingTransactions) are also supported.

Connect using your Base WebSocket endpoint:

  • wss://base-mainnet.g.alchemy.com/v2/<YOUR_API_KEY>
  • wss://base-sepolia.g.alchemy.com/v2/<YOUR_API_KEY>

Flashblocks subscription types

newFlashblockTransactions

Receive each transaction as it becomes preconfirmed into a Flashblock. Pass true as the second parameter to include full transaction objects and associated logs.

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "eth_subscribe",
  "params": ["newFlashblockTransactions"]
}

With full transaction data:

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "eth_subscribe",
  "params": ["newFlashblockTransactions", true]
}
pendingLogs

Monitor contract events from preconfirmed transactions with sub-block latency. You can optionally filter by contract address and topics.

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "eth_subscribe",
  "params": [
    "pendingLogs",
    {
      "address": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
      "topics": [
        "0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef"
      ]
    }
  ]
}

The address field accepts a single address or an array of addresses, and topics follows the same format as eth_getLogs.

newFlashblocks

Receive full block state updates as each Flashblock is constructed, with accumulated preconfirmed state for the block in progress.

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "eth_subscribe",
  "params": ["newFlashblocks"]
}

Subscription response

After subscribing, you receive a subscription ID:

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": "0x1887ec8b9589ccad00000000000532da"
}

Events are then delivered through eth_subscription notifications:

{
  "jsonrpc": "2.0",
  "method": "eth_subscription",
  "params": {
    "subscription": "0x1887ec8b9589ccad00000000000532da",
    "result": { ... }
  }
}

eth_unsubscribe

Cancel any active subscription using the subscription ID:

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "eth_unsubscribe",
  "params": ["0x1887ec8b9589ccad00000000000532da"]
}

For more details, see the Base Flashblocks API reference and the Base Flashblocks documentation.

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