Simulate: which policy would match a message (real or synthetic), with the active policies or a draft

Read-only. message is a received message id or a synthetic message. Without policy/policyId the ACTIVE policies are evaluated (a paused one is not); policy tests an unsaved draft; policyId tests one saved policy even if paused. On a received message the key needs read:inbound too (it reads the mailbox). Every simulate call — synthetic or not — counts against the per-key inbound read limit (120/min by default): over it, 429 rate_limited with Retry-After: 60. That is longer than the 30 s the official SDKs wait by default, so they do NOT retry it: the 429 reaches your code, with the Retry-After — wait and call again, or raise the SDK retry-after cap. The answer includes what the engine SAW. The request body is capped at 256 KB (413). Scope read:inbound-policies.