402Guard
A policy and finance-operations guardrail for x402-enabled autonomous machine payments.
Overview & Research Motivation
402Guard is a safety and finance-operations layer for x402 payments and subscriptions. x402 defines how a payment can happen; 402Guard adds the policy decision that determines when an autonomous client should not authorize it.
The prototype wraps Axios/fetch-style requests, intercepts HTTP 402 responses, and evaluates USD-denominated budgets, rate limits, and per-agent, per-service, and per-endpoint rules before payment. Explicit allow/block decisions feed usage attribution, audit records, subscription tracking, and invoice generation.
The Core Systems Problem
- Autonomous clients can otherwise authorize runaway 402 payments without explicit budgets, service scopes, endpoint limits, or a durable finance-operations record.
My Specific Technical Contributions
- Built the guarded Axios/fetch request layer and pre-payment policy evaluation path for intercepted HTTP 402 flows.
- Implemented USD-denominated budget and rate-limit controls with explicit allow/block decisions and audit logging.
- Connected Avalanche subscription state to off-chain usage tracking, spend attribution, and invoice/audit generation in the prototype.
System Architecture & Verification Pipeline
Client SDK & Interceptors
Guarded Axios/fetch-style requests detect 402 payment challenges and route them through policy evaluation before authorization.
Off-Chain Policy Engine
Fast granular enforcement evaluates USD budgets, rate limits, and agent, service, and endpoint scopes with auditable allow/block outcomes.
Subscription & Finance Operations
Avalanche provides on-chain subscription truth while the usage store supports analytics, spend attribution, invoices, and audit trails.
Technical Stack & Tools
Known Limitations
- Production middleware packaging, broader wallet support, and expanded alerting remain follow-on work beyond the demonstrated prototype.