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Introduction

To let the agent move funds from the user’s wallet, you create a signer for the agent and add it to the wallet. A signer is just a key that can authorize transactions — the agent gets its own, separate from the user’s. You add the signer in two parts so the user stays in control:
  1. On your server, you create the signer and register it on the wallet as pending. The signer’s secret stays in your backend and never reaches the browser or the agent.
  2. On the client, the user approves the new signer with a one-time email code. Only after this step can the agent sign anything.
Once approved, the agent can transact within the limits you set — and the user can revoke it at any time.

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Prerequisites

  • A user wallet. See Server signer setup if you have not done this yet.
  • A Crossmint server-side API key from the Crossmint Console.
  • The user must be authenticated and signed in to your app.

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Steps

1

Generate and store the signer secret

The server signer is backed by a 32-byte secret. Generate one with the signer secret generator and store it only as a backend environment variable.
CROSSMINT_SERVER_SIDE_API_KEY=sk_staging_...
CROSSMINT_SIGNER_SECRET=your-64-char-hex-secret
Anyone who holds CROSSMINT_SIGNER_SECRET can sign on behalf of every wallet it has been authorized on. Never commit it, never expose it to the client, and rotate it if you suspect leakage.
2

Register the signer from the server

Use a server action (or any server-only handler) to call addSigner in prepareOnly mode. The secret is read from the environment and never sent over the wire.
"use server";

import { createCrossmint, CrossmintWallets } from "@crossmint/wallets-sdk";

export async function prepareServerSigner({ walletAddress }: { walletAddress: string }) {
    const crossmint = createCrossmint({
        apiKey: process.env.CROSSMINT_SERVER_SIDE_API_KEY,
    });
    const wallets = CrossmintWallets.from(crossmint);
    const wallet = await wallets.getWallet(walletAddress, { chain: "base-sepolia" });

    const { locator, signatureId } = await wallet.addSigner(
        { type: "server", secret: process.env.CROSSMINT_SIGNER_SECRET },
        { prepareOnly: true }
    );

    return { locator, signatureId };
}
prepareOnly: true is what makes this safe. Without it, the SDK would auto-approve using whatever signer is active, which on the server is the server signer itself — the user would never see a prompt.
3

Approve from the client

On the client, call the server action and then ask the SDK to approve the pending signature with the user’s recovery signer. This is the step that triggers the email code.
"use client";

import { useWallet, useCrossmintAuth } from "@crossmint/client-sdk-react-ui";
import { prepareServerSigner } from "@/app/actions/add-server-signer";

export function AuthorizeAgent() {
    const { wallet } = useWallet();
    const { user } = useCrossmintAuth();

    const handleAuthorize = async () => {
        if (!wallet || !user?.email) return;

        const { signatureId } = await prepareServerSigner({ walletAddress: wallet.address });

        await wallet.useSigner({ type: "email", email: user.email });
        await wallet.approve({ signatureId });
    };

    return <button onClick={handleAuthorize}>Authorize agent</button>;
}
Once the user completes the verification, the server signer becomes an authorized signer on the wallet. Verify by calling wallet.signers() — you should see an entry with type: "server".

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Revoking the Agent

Once authorized, the agent can be revoked at any time. See Remove Agent Access.

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Common Gotchas

If you omit it, the server signer signs its own authorization and the user never approves. The pending signature step is what gives the user veto power.
Anyone holding the secret can sign for every wallet it has been authorized on. Treat it like a database password — environment variables only, never the client bundle.

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Next Steps

Onramp Funds

Add stablecoin balance to the user’s wallet so the authorized agent can start spending.

Server Signer Deep Dive

Full reference for the server signer, secret rotation, and multi-tenant patterns.