Interesting to see budget enforcement paired with x402. We've been building in the same space — Apiosk (https://apiosk.com) approaches it from the server side: a gateway that enforces per-request x402 payments so API providers can monetize without accounts or keys.
Your budget SDK + Apiosk would be a natural combo — the agent has a spending ceiling (AgentBudget) and the APIs it calls use x402 for micropayments (Apiosk handles gateway/verification). Have you thought about hooks for x402-aware budget tracking where the ledger automatically records on-chain settlements?