SpendGuard - Autonomous API Spend Control via x402

Modern AI agents and cloud services increasingly rely on third-party APIs, yet cost control is still built on subscriptions, API keys, and post-paid billing. This creates a fundamental mismatch: machines can act autonomously, but spending cannot.SS SpendGuard introduces a gateway-based approach to API cost control using HTTP 402 (Payment Required) and the x402 payment pattern. Instead of trusting API keys or monthly limits, each request is evaluated against explicit budget and policy rules before execution. When a request exceeds policy or lacks payment, SpendGuard responds with a 402 boundary that includes payment instructions (USDC, amount, reference, expiry). Once payment is provided, the same request is allowed to proceed. This enables true pay-per-request access, suitable for autonomous agents, internal tools, and machine-to-machine commerce. SpendGuard is designed for scenarios where trustless enforcement, predictable spend, and granular control matter more than traditional subscriptions - such as AI agents, micro-services, and experimental workloads. This demo showcases a working x402 boundary implemented as a Next.js API route, demonstrating how real payment-gated APIs can be built today.