Errors

Every error is JSON with a stable code field — safe to switch on.

Shape

RESPONSE 422

{
  "error":   "invalid_inputs",
  "details": [
    {
      "field":    "country",
      "code":     "not_in_enum",
      "expected": ["US", "MX"]
    }
  ]
}

Status codes

Status	Code	Meaning
400	invalid_payload	Body could not be parsed as JSON.
401	unauthorized	Token missing, malformed, or revoked.
401	invalid_signature	HMAC verification failed for a trigger.
404	not_found	Process, instance, or callback does not exist.
409	callback_already_resolved	One-shot webhook callback was already received.
422	invalid_inputs	Inputs failed schema validation. details lists each violation.
422	invalid_definition	Process YAML failed schema or syntax check.
422	invalid_transition	Tried to act on a terminal instance/step.
429	rate_limited	Too many requests. Retry-After header advises a wait.
500	trigger_misconfigured	Engine env var for HMAC secret is unset.

Retries

The engine itself retries failed automated steps with exponential backoff (max 3 attempts by default). The API does not retry your inbound calls — wrap idempotent calls (POST /start, POST /submit) yourself.