Crossmint Smart Wallets

⏱️ 3 min read

Overview

This guide points to a reference repository that shows you how to integrate DeFindex vaults using Crossmint smart wallets — enabling fully automated, server-side deposits, withdrawals, and cross-chain bridging with zero user interaction.

The pattern uses an EVM private key registered as adminSigner to control both an ERC-4337 smart wallet on Base and a Stellar smart wallet. All Defindex vault interactions go through Crossmint's REST API as Soroban contract-call transactions — no manual XDR construction required.

Repository

defindex-io/crossmint-defindex-guide

What the Repository Covers

Topic	Description
Crossmint setup	Server API key (sk_), wallet email, staging vs production
EVM wallet	ERC-4337 smart wallet on Base with external-wallet adminSigner
Stellar wallet	Stellar smart wallet with auto-XLM funding, Soroban contract-call signing
Deposit	contract-call via Crossmint REST → base64 XDR approval → poll
Withdraw	Withdraw by amount or by shares (% redemption)
Bridge	Base USDC → Stellar → Defindex vault via Sodax
Gotchas	9 documented edge cases with root causes and fixes

Architecture at a Glance

Your Server (EVM Private Key as adminSigner)
       │
       ▼
Crossmint REST API (api/2025-06-09)
  ├── EVM Smart Wallet (ERC-4337, Base)
  │     POST /transactions → sign hex bytes → POST /approvals → onChain.txId
  └── Stellar Smart Wallet
        POST /transactions (contract-call) → sign base64 XDR → POST /approvals → onChain.txId

Sodax Bridge
  └── Base USDC → Stellar USDC (via Sonic hub, no Horizon polling needed)

Defindex Vault (Soroban)
  ├── method: deposit         → amounts_desired, amounts_min, from, invest
  ├── method: withdraw        → amounts_to_withdraw, from
  └── method: withdraw_shares → shares_amount, from

All vault operations follow the same pattern:

1. POST to Crossmint REST → create contract-call transaction

2. Response is awaiting-approval with a base64-encoded XDR message

3. Sign with keypair.sign(Buffer.from(message, "base64")) using STELLAR_SERVER_KEY

4. POST signature to /approvals

5. Poll until onChain.txId is returned

Quick Start

git clone https://github.com/defindex-io/crossmint-defindex-guide
cd crossmint-defindex-guide
pnpm install
cp .env.example .env
# Fill in: CROSSMINT_SERVER_API_KEY (sk_...), CROSSMINT_WALLET_EMAIL,
#          EVM_PRIVATE_KEY, STELLAR_SERVER_KEY

CROSSMINT_ENV=staging pnpm example:deposit          # Deposit into testnet vault
CROSSMINT_ENV=staging pnpm example:withdraw         # Withdraw by amount (testnet)
CROSSMINT_ENV=staging pnpm example:withdraw-shares  # Withdraw by shares (testnet)
CROSSMINT_ENV=production pnpm example:bridge        # Base USDC → Stellar → Defindex vault (mainnet)

Prerequisites

• Crossmint account with a server API key (must start with sk_, not ck_)

• EVM_PRIVATE_KEY — becomes the adminSigner of the EVM smart wallet on Base

• STELLAR_SERVER_KEY — Stellar ed25519 secret key, becomes the adminSigner of the Stellar wallet

• Defindex API key — request access on Discord

Additional Resources

• Crossmint Documentation

• Defindex API Reference

• Sodax Bridge Documentation