OpenAPI / REST mapping
The OpenAPI mapping is a REST-style surface for the OPC UA service set: POST /<service> with the bare service request as JSON. The OPC Foundation publishes normative descriptions of it - opc.ua.openapi.allservices.json, opc.ua.openapi.sessionless.json and the matching JSON Schema files - and servers advertise it as a discovery-only twin endpoint.
What a call looks like
POST to a path named after the service, with the plain request structure as the body: /read carries a bare ReadRequest, /browse a BrowseRequest. At the HTTPS layer there is no {UaTypeId, UaBody} envelope. The content type selects the JSON flavour with encoding=compact or encoding=verbose. Responses are the corresponding response structures, so the service semantics are unchanged - only the framing is new.
Sessionless access
The sessionless OpenAPI description is the interesting half: it defines service invocation without CreateSession and ActivateSession, carrying identity in a bearer token instead. That is what makes stateless, horizontally scaled REST front ends possible, and it is why the reference stack has a sessionless identity provider pipeline. It also means no session state, so no continuation points and no session-scoped node registrations.
Discovery-only twin endpoints
Servers emit the OpenAPI endpoint as an additional EndpointDescription alongside each SecurityMode None HTTPS or WebSocket binary endpoint, so a discovery client finds the REST route without anyone hardcoding a URL. The twin exists in discovery results but is not intended for the binary client path - which is worth knowing when an endpoint list contains more entries than the server appears to support.
Example
A read over the REST surface
POST https://server:4843/read
Content-Type: application/json; encoding=compact
{
"NodesToRead": [
{ "NodeId": "ns=0;i=2258", "AttributeId": 13 }
],
"TimestampsToReturn": 2
}
Response: a ReadResponse structure. AttributeId 13 is Value; TimestampsToReturn 2 is Both.Defined in: Part 6 - Mappings, Annex G, and the OpenAPI and JSON Schema files published in the UA-Nodeset schema directory.
Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.