PayGuard is an open-source security SDK created to ensure financial safety and autonomous policy enforcement for AI agent transactions. This SDK functions as an automated guardrail, enforcing spending limits and verifying idempotency. It also conducts comprehensive risk evaluations before any purchase is executed by an AI agent. PayGuard aligns with the "Slow and Steady" theme by enabling users to set financial policies once and then allowing the SDK to automatically manage risk checks, policy enforcement, and spend caps without human intervention. This eliminates the need for manual oversight of AI agent spending, which often defeats the purpose of automation. Open innovation is crucial for PayGuard, as it ensures data privacy by running financial evaluation logic and policy checks locally or on self-hosted infrastructure. This approach keeps sensitive transaction metadata off third-party servers. Moreover, PayGuard offers zero API lock-in and cost, integrating seamlessly with open-weight local models and eliminating per-request costs for agent evaluation logic. The open-source nature of PayGuard also provides customizable guardrails, allowing developers to fine-tune risk models, swap inference backends, or adjust rule sets directly in the code.