Gateway Circuit Breaker Simulation

v4.2 Sprint D adds pure circuit-breaker state transition simulation.

The circuit-breaker module does not persist state, probe provider health, start timers, or call providers. Callers provide the current state, event, policy, and current time.

Policy Fields

enabled
failure_threshold
success_threshold
open_duration_ms
half_open_max_attempts
tracked_categories
scope
metadata

Supported scopes:

• provider
• model
• provider-model

Supported states:

• closed
• open
• half-open

Transition Model

simulateCircuitBreakerTransition(...) returns:

previous_state
next_state
failure_count
success_count
opened_at
eligible_for_half_open_at
allows_attempt
reasons

Policy and configuration failures are not automatically counted as provider health failures. The tracked category list decides which simulated failures affect the breaker.

Safety Boundary

This is not a runtime health system yet. It is a deterministic transition contract for tests, dry-run explanations, and future gateway design.