system_dryRun - Polkadot

Example code for the system_dryRun JSON RPC method. Complete guide on how to use system_dryRun JSON RPC in GetBlock Web3 documentation.

This method dry-runs an extrinsic against the current state and returns the encoded result without submitting it. It is used to check whether a transaction would succeed. It may be restricted on public endpoints.

Parameters

Parameter	Type	Required	Description
extrinsic	string	Yes	A signed, SCALE-encoded extrinsic, hex-encoded
at	string	No	Block hash to query at. Defaults to the latest block when omitted

Request

cURL

curl --location --request POST 'https://shared.eu-central-1.getblock.io/<ACCESS-TOKEN>/' \
--header 'Content-Type: application/json' \
--data-raw '{"jsonrpc": "2.0", "method": "system_dryRun", "params": ["0x4d028400d43593c715fdd31c61141abd04a99fd6822c8558854ccde39a5684e7a56da27d01..."], "id": "getblock.io"}'

JavaScript

example.js

const response = await fetch('https://shared.eu-central-1.getblock.io/<ACCESS-TOKEN>/', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({
        jsonrpc: '2.0',
        method: 'system_dryRun',
        params: ["0x4d028400d43593c715fdd31c61141abd04a99fd6822c8558854ccde39a5684e7a56da27d01..."],
        id: 'getblock.io'
    })
});

const data = await response.json();
console.log(data.result);

Python

example.py

import requests

response = requests.post(
    'https://shared.eu-central-1.getblock.io/<ACCESS-TOKEN>/',
    headers={'Content-Type': 'application/json'},
    json={
        'jsonrpc': '2.0',
        'method': 'system_dryRun',
        'params': ["0x4d028400d43593c715fdd31c61141abd04a99fd6822c8558854ccde39a5684e7a56da27d01..."],
        'id': 'getblock.io'
    }
)

print(response.json()['result'])

Response

{
    "id": "getblock.io",
    "jsonrpc": "2.0",
    "result": "0x0100"
}

Response Parameters

Parameter	Type	Description
id	string	Request identifier matching the request
jsonrpc	string	JSON-RPC protocol version ("2.0")
result	string	SCALE-encoded ApplyExtrinsicResult

Use Cases

• Pre-Flight Checks: Verify an extrinsic before submitting it

• Error Prevention: Detect failures without paying fees

• Simulation: Preview the outcome of a transaction

• Wallet UX: Warn users of likely failures

Error Handling

Error Code	Message	Description
-32602	Invalid params	A parameter is missing or has the wrong type or format
-32601	Method not found	The method is not exposed on this endpoint
-32000	Unsafe call	The method may be restricted on public endpoints for safety

Library Integration

The idiomatic way to call Polkadot RPC is the Polkadot.js API (@polkadot/api), which connects over WebSocket and exposes the method as api.rpc.system.dryRun.

polkadot-js.js

import { ApiPromise, WsProvider } from '@polkadot/api';

const provider = new WsProvider('wss://shared.eu-central-1.getblock.io/<ACCESS-TOKEN>/');
const api = await ApiPromise.create({ provider });

const result = await api.rpc.system.dryRun('0x4d028400d43593c715fdd31c61141abd04a99fd6822c8558854ccde39a5684e7a56da27d01...');
console.log(result.toHuman());