Error codes

The engine produces stable error codes for every failure mode. UI / CLI / tests assert on these.

Code	Class	Retryable	Common cause
E_SCHEMA_INVALID	Schema	No	Malformed YAML or schema version mismatch
E_CYCLE	Schema	No	Circular dependency in chain
E_REF_UNDEFINED	Schema	No	{{X.y}} refers to non-existent producer
E_VAR_UNRESOLVED	Resolution	No	Required variable couldn’t be substituted
E_NETWORK_TIMEOUT	Network	Yes	Read or connect timeout
E_NETWORK_DNS	Network	Yes	DNS resolution failed
E_NETWORK_TLS	Network	No	TLS handshake failure
E_HTTP_5XX	HTTP	Yes	Server-side 5xx
E_HTTP_4XX	HTTP	No	Client-side 4xx
E_STATUS_MISMATCH	HTTP	No	Status code didn’t match expect_status
E_SESSION_REFRESH_FAILED	Auth	No	Auth flow returned an error
E_HOOK_FAILURE	Hook	No	JS pre/post hook threw or timed out
E_EXTRACTION_FAILED	Extraction	No	JSONPath didn’t match the response
E_RESPONSE_PARSE	Extraction	No	Response declared JSON but didn’t parse
E_CANCELLED	Run	No	User cancelled

Full taxonomy is in Engine requirement §5.