What is an x402 Circuit Breaker?

The circuit breaker pattern, borrowed from electrical engineering and popularised in software by Michael Nygard, is essential for x402 because every payment failure costs real money. In traditional APIs, a retry loop wastes bandwidth. In x402, a retry loop drains wallets.

PolicyLayer's x402 implementation trips the circuit breaker after 10 consecutive policy violations, automatically pausing all x402 payments. This protects against:

• Infinite payment loops — agent pays for a resource, gets an error, retries, pays again, loops indefinitely
• Cascading endpoint failures — a compromised endpoint starts returning 402 for every request, draining the agent across multiple requests
• Prompt injection attacks — a malicious endpoint injects instructions causing the agent to pay inflated amounts or to repeatedly call expensive endpoints
• Amount hallucination cascades — an agent consistently mis-parsing payment amounts due to a bug, overpaying on every request

The circuit breaker has three states: closed (payments flowing normally), open (all payments blocked), and half-open (allowing test payments to determine if the issue is resolved). The transition from closed to open happens automatically when violation thresholds are breached.

The $47,000 GetOnStack incident — where a multi-agent system looped for 11 days undetected — demonstrates why circuit breakers are non-negotiable. With x402's irreversible on-chain payments, there are no chargebacks to recover from a spending loop.

PolicyLayer's circuit breaker trips after 10 consecutive x402 policy violations and blocks all payments until manually reset or after a configurable cooldown period. When the circuit opens, operators receive immediate webhook notifications with details of the violations that triggered it.