API v2 problem types

Every v2 failure answers RFC 9457 application/problem+json carrying a stable code. Branch on code, never on the prose. Each row below is the page a document's type URI resolves to.

Code	Status	Meaning
`contact_already_confirmed`	409	This contact is already confirmed.
`contact_type_not_creatable`	422	Contacts of this type can no longer be created.
`credential_write_only`	422	The read sentinel cannot be sent as a credential value.
`duplicate_contact`	409	A contact with this type, address, gateway and alert delay already exists.
`duplicate_monitor`	409	A monitor with this url and type already exists on this account.
`duplicate_resource`	409	A resource with this key already exists on this account.
`filter_required`	422	This operation refuses to run without an explicit filter.
`idempotency_key_conflict`	409	This idempotency key is already in use for a different request.
`idempotency_key_required`	400	This operation requires an Idempotency-Key header.
`insufficient_agents`	422	Not enough monitoring locations match the requested selection.
`insufficient_balance`	402	The account balance is too low to deliver this message.
`insufficient_rights`	403	The subaccount does not hold the rights this operation requires.
`internal_error`	500	The request failed because of an unexpected server error.
`interval_below_type_floor`	422	The requested check interval is below this monitor type's floor.
`invalid_alert_delay`	422	The requested alert delay is not one of the supported values.
`invalid_confirmation_code`	422	The confirmation code is not valid.
`invalid_cursor`	422	The pagination cursor is not valid.
`invalid_interval`	422	The requested check interval is not allowed for this account.
`invalid_limit`	422	The requested page size is outside the allowed range.
`invalid_range`	422	The requested time range is not valid.
`invalid_settings`	422	A value inside settings is not valid for this monitor type.
`invalid_token`	401	The access token is missing, malformed or expired.
`invalid_url`	422	The url is not usable for this check.
`ip_not_allowed`	403	The caller's IP address is not on this token's allow-list.
`job_not_cancellable`	409	This job has already reached a terminal state.
`job_not_resumable`	409	This job cannot be resumed.
`malformed_request`	400	The request could not be parsed.
`method_not_allowed`	405	This method is not allowed on this resource.
`missing_scope`	403	The token does not carry the scope this operation requires.
`monitor_type_discontinued`	422	This monitor type is discontinued.
`not_found`	404	The requested resource does not exist.
`package_interval_conflict`	403	No check interval satisfies both this monitor type and the account's plan.
`package_limit`	403	The account's package does not allow this operation.
`payload_too_large`	413	The request payload is larger than this endpoint accepts.
`quota_exceeded`	429	The account's API quota for this scope is exhausted.
`rate_limited`	429	Too many requests - slow down.
`selection_mismatch`	409	The selection changed since it was reported.
`service_unavailable`	503	The service is temporarily unavailable.
`too_many_items`	422	The request carries more items than this operation accepts.
`type_immutable`	422	This resource's type cannot be changed after creation.
`unknown_contact_ref`	422	A subscription references a contact ref that no contacts[] item declares.
`unknown_enum_value`	422	The value is not one this field accepts.
`unknown_event_type`	422	The webhook declares an event type outside the published catalogue.
`unknown_expand`	422	The request asked to expand a value this endpoint does not offer.
`unknown_field`	422	The request asked for a field this row does not publish.
`unknown_parameter`	422	The request carries a query parameter this endpoint does not define.
`unknown_pool`	422	The requested agent pool does not exist.
`unsupported_media_type`	415	The request's media type is not one this endpoint reads.
`unsupported_report_channel`	422	Reports cannot be delivered to a contact of this type.
`upstream_error`	502	A service this request depends on failed.
`url_blacklisted`	403	This url is not accepted for monitoring.
`validation_failed`	422	The request body or query is not valid.

52 registered types. A code that is not on this list can never reach the wire.