Claude agreed to pay in a lookalike USDC token

A test of five hard x402 payment cases found that Claude Sonnet and Haiku, given no extra instructions, agreed to pay using a token contract differing from real USDC on Base by two swapped digits. Sonnet called the address the canonical USDC contract, and Haiku approved paying for a product its agent never requested. With a loaded skill, the models refused and named the wrong field.

• Without instructions, both models approved payment in a fake USDC contract on Base
• Sonnet called the lookalike token address the canonical USDC contract
• Haiku approved paying for a product its agent never requested
• With a loaded skill, all answers became refusals naming the wrong field