How to build a custom dynamic calldata builder

Create the dynamic calldata service

First, you will need to create a hosted api that can be called from the automations service using REST. This api should accept a POST request with a JSON body that contains the necessary information to generate the calldata. The response should be a JSON object that contains the calldata.

You can find an example of such a service in the reference implementation (opens in a new tab).

Create the calldata encoding logic

Next, you will need to create the logic that encodes the calldata. This logic should be able to take the JSON object from the response of the dynamic calldata service and encode it into a hex string that can be used as the calldata for the automation execution. An example is below:

import { encodeFunctionData } from "viem";
 
app.post("/call-data-builder", (req: Request, res: Response) => {
  const data = req.body;
 
  const calldata = encodeFunctionData({
    functionName: "setSomeState",
    abi: [
      {
        type: "function",
        name: "setSomeState",
        inputs: [{ type: "uint256" }],
      },
    ],
    args: [BigInt(data.someState)],
  });
 
  // return the calldata as a hex string
});

Host the dynamic calldata builder

Finally, you will need to host the dynamic calldata builder. This can be done using a service like Vercel or Netlify. Once the service is hosted, you can use the url as the dynamic calldata builder in the automation creation.

Create the automation

Now, you can create an automation. For a full tutorial, check out the tutorial on creating an automated transfer. For the purposes of this guide, the important part is that you pass the correct actions to the automations, as seen below:

const actions = [
  {
    type: "dynamic",
    target: "0x123...",
    value: 0,
    callDataBuilderUrl: "https://your-dynamic-calldata-builder-url.com",
    functionSelector: "0x5bf67475", // setSomeState(uint256)
    params: {
      static: {
        someState: 100,
      },
    },
  },
];