Transaction Simulation API

Understand a transactoin before signing it

See exactly what a transaction will do—before it’s signed. Our API runs a full on‑chain simulation and returns clear, USD‑denominated balance changes, so users of custodial platform such as Cobo and Fireblocks can approve or reject transfers with confidence.

Why it matters

• Pre‑sign insights – Know the outcome of any transaction in advance.

• Seamless integration – A lightweight REST endpoint that drops straight into your existing workflow.

• Actionable data – Balance deltas presented in familiar USD terms.

Supported chains Ethereum · BNB Smart Chain · Base · Optimism · Polygon

Get started Request an API key at [e-mail] and start simulating in minutes.

Balance Change

Raw Transaction Simulation

post

Simulate raw transaction and get balance changes

Header parameters

API-KeystringRequired

Body

transactionstringRequired

Raw transaction hex string

Example: 0xf8a91f85017ad70c7582881994c02aaa39b223fe8d0a0e5c4f27ead9083c756cc280b844a9059cbb000000000000000000000000d8da6bf26964af9d7eed9e03e53415d37aa9604500000000000000000000000000000000000000000000000000005af3107a400025a0bbfc1f56c70c58f623a36639f9dd01db632af5bdc1106c16906f6c13b17765cea07a34be3f43332eddbcb9ac8a3b1e261eda25c007283a645d68a8380b19e6602f

Responses

200

Simulation result

application/json

post

HTTPcURLJavaScriptPython

POST /simulation/v1/raw/balancechange HTTP/1.1
Host: api.blocksec.com
API-Key: text
Content-Type: application/json
Accept: */*
Content-Length: 362

{
  "transaction": "0xf8a91f85017ad70c7582881994c02aaa39b223fe8d0a0e5c4f27ead9083c756cc280b844a9059cbb000000000000000000000000d8da6bf26964af9d7eed9e03e53415d37aa9604500000000000000000000000000000000000000000000000000005af3107a400025a0bbfc1f56c70c58f623a36639f9dd01db632af5bdc1106c16906f6c13b17765cea07a34be3f43332eddbcb9ac8a3b1e261eda25c007283a645d68a8380b19e6602f"
}

200

Simulation result

{
  "code": 0,
  "message": "success",
  "data": {
    "status": true,
    "blockNumber": 22467376,
    "txnHash": "0x5466e564db157b73cbd8dd1d688631a3afcf99a35c372009d8c298441fa8d9e0",
    "sender": "0x989e7947a9d9210eec3a41dcf5f536bb1d7726f6",
    "receiver": "0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2",
    "timestamp": "2025-05-12T13:03:23Z",
    "callData": "0xa9059cbb000000000000000000000000d8da6bf26964af9d7eed9e03e53415d37aa9604500000000000000000000000000000000000000000000000000005af3107a4000",
    "gasLimit": 34841,
    "gasPrice": "6355881077",
    "gasUsed": 34482,
    "baseFee": "6029791982",
    "transactionFee": "219163491297114",
    "nonce": 31,
    "value": "0",
    "errorInfo": "",
    "viewLink": "https://app.blocksec.com/explorer/tx/eth/0x5466e564db157b73cbd8dd1d688631a3afcf99a35c372009d8c298441fa8d9e0?event=simulator",
    "balanceChanges": [
      {
        "account": "0x989e7947a9d9210eec3a41dcf5f536bb1d7726f6",
        "assets": [
          {
            "address": "0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2",
            "tokenId": "",
            "rawAmount": "100000000000000",
            "amount": "0.0001",
            "value": "0.255386",
            "iconUrl": "https://assets.blocksec.com/icon/f1cb2d02c471d545bbc7722daf3277fd852d98e63d118771af7047bd8256c516.png",
            "isERC721": false,
            "isERC1155": false,
            "sign": false
          }
        ]
      },
      {
        "account": "0xd8da6bf26964af9d7eed9e03e53415d37aa96045",
        "assets": [
          {
            "address": "0xc02aaa39b223fe8d0a0e5c4f27ead9083c756cc2",
            "tokenId": "",
            "rawAmount": "100000000000000",
            "amount": "0.0001",
            "value": "0.255386",
            "iconUrl": "https://assets.blocksec.com/icon/f1cb2d02c471d545bbc7722daf3277fd852d98e63d118771af7047bd8256c516.png",
            "isERC721": false,
            "isERC1155": false,
            "sign": true
          }
        ]
      }
    ]
  }
}

Custom Transaction Simulation

post

Simulate custom transaction parameters and get balance changes

Header parameters

API-KeystringRequired

Body

chainIdinteger · int64Required

Chain ID for the transaction

Responses

200

Simulation result

application/json

post

HTTPcURLJavaScriptPython

POST /simulation/v1/custom/balancechange HTTP/1.1
Host: api.blocksec.com
API-Key: text
Content-Type: application/json
Accept: */*
Content-Length: 224

{
  "chainId": 1,
  "transaction": {
    "sender": "0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045",
    "receiver": "0x761A8482CE9EBa5baCBc20CDd869D4AA00275F8f",
    "gasLimit": 21000,
    "gasPrice": "20634341671000",
    "value": "9000000000000000",
    "input": "0x"
  }
}

200

Simulation result

{
  "code": 0,
  "message": "",
  "data": {
    "status": true,
    "blockNumber": 22467282,
    "txnHash": "0x80407f7f2c695c82847bdbdecac4070274523bcbc0b9fa3493afb06ee7709813",
    "sender": "0xd8da6bf26964af9d7eed9e03e53415d37aa96045",
    "receiver": "0x761a8482ce9eba5bacbc20cdd869d4aa00275f8f",
    "timestamp": "2025-05-12T12:44:35Z",
    "callData": "0x",
    "gasLimit": 21000,
    "gasPrice": "20634341671000",
    "gasUsed": 21000,
    "baseFee": "7094316180",
    "transactionFee": "433321175091000000",
    "nonce": 1555,
    "value": "9000000000000000",
    "errorInfo": "",
    "viewLink": "https://app.blocksec.com/explorer/tx/eth/0x80407f7f2c695c82847bdbdecac4070274523bcbc0b9fa3493afb06ee7709813?event=simulator",
    "balanceChanges": [
      {
        "account": "0xd8da6bf26964af9d7eed9e03e53415d37aa96045",
        "assets": [
          {
            "address": "0xeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee",
            "tokenId": "",
            "rawAmount": "9000000000000000",
            "amount": "0.009",
            "value": "22.94604",
            "iconUrl": "https://assets.blocksec.com/image/1663669189317-3.png",
            "isERC721": false,
            "isERC1155": false,
            "sign": false
          }
        ]
      },
      {
        "account": "0x761a8482ce9eba5bacbc20cdd869d4aa00275f8f",
        "assets": [
          {
            "address": "0xeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee",
            "tokenId": "",
            "rawAmount": "9000000000000000",
            "amount": "0.009",
            "value": "22.94604",
            "iconUrl": "https://assets.blocksec.com/image/1663669189317-3.png",
            "isERC721": false,
            "isERC1155": false,
            "sign": true
          }
        ]
      }
    ]
  }
}