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Meta-transactions (NEP-366) allow a user to sign a transaction off-chain, which is then submitted and paid for by a separate “Relayer” account.

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How it Works

  1. User (Client): Constructs a transaction and calls .delegate(). This returns a typed object (for UI/validation) and a serialized string (for transport).
  2. Transport: The user sends the serialized string (payload) to your backend.
  3. Relayer (Server): Your server decodes the payload back into an object, validates it, wraps it, and submits it.

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1. Client Side: Signing the Action

On the frontend, use the .delegate() method. This works exactly like .send(), but implies no network activity and no gas cost for the signer. It returns two properties:
  • signedDelegateAction: The raw JS object. Useful for debugging or showing details in your UI.
  • payload: A Base64-encoded string. This is what you send to the server.
// 1. Delegate (builds and signs the transaction off-chain)
const { signedDelegateAction, payload } = userNear
  .transaction("user.near")
  .functionCall("game.near", "move", { x: 1, y: 2 })
  .delegate()

// Inspect locally (Typed Object)
console.log("User signed for:", signedDelegateAction.delegateAction.receiverId)

// 3. Send the PAYLOAD to your API (Base64 String)
await fetch("https://api.mygame.com/relay", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ payload }),
})

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2. Server Side: The Relayer

The server receives the Base64 string (payload). It must decode this string back into a typed object to inspect it before submitting. Use near-kit’s decodeSignedDelegateAction helper for this.

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Example: Express.js Route

import { Near, decodeSignedDelegateAction } from "near-kit"

const relayer = new Near({
  network: "testnet",
  privateKey: process.env.RELAYER_KEY,
})

app.post("/relay", async (req, res) => {
  try {
    const { payload } = req.body

    // 1. Decode the Base64 string back into a Typed Object
    const userAction = decodeSignedDelegateAction(payload)

    // 2. SECURITY: Inspect the object before paying for it!
    // access the inner details via .delegateAction
    const innerAction = userAction.delegateAction

    // Example Check: Only allow calls to "game.near"
    if (innerAction.receiverId !== "game.near") {
      return res.status(400).send("Invalid target contract")
    }

    // 3. Submit to the Network
    // The relayer wraps the user's action in its own transaction
    const result = await relayer
      .transaction("relayer.near")
      .signedDelegateAction(userAction)
      .send()

    res.json({ hash: result.transaction.hash })
  } catch (e) {
    console.error(e)
    res.status(500).send("Relay failed")
  }
})

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Security Checklist

Running a relayer makes you a target.
  1. Whitelist Receivers: Always check userAction.delegateAction.receiverId.
  2. Whitelist Methods: Inspect userAction.delegateAction.actions to ensure users are only calling allowed methods (e.g., “move” vs “withdraw”).
  3. Rate Limiting: Rate limit your API endpoint to prevent draining your relayer’s funds.