UHP turns the choice of agent harness into a field in the request

The discovery call is a GET to /v1/harnesses. The work goes to /v1/responses as a single POST carrying the prompt, the model name "claude-sonnet-4.6" and a harness_id inside metadata ^[14]. The model and the runtime that drives it are fields in the same body ^[20].

By the specification's own account, products have been answering the driving questions once per harness, and UHP answers them once ^[5]. The document sets out where the disagreement sits. "Each one already knows how to do the work; what none of them agree on is how a product should drive one: how to start a task, follow its progress, continue the conversation, cancel it, get the files it produced, and understand why it failed," the specification says ^[4].

That list is the difference between the two kinds of endpoint. A model API gives you a turn: messages in, tokens out, tools you have to run yourself ^[6]. A UHP server gives you a task, run by an agent that uses its own tools, keeps its own session and hands back results and files ^[6]. "The unit of exchange is a job, not a completion," the document says ^[7].

UHP's task surface is deliberately shaped like the OpenAI Responses API, and a conformant server must accept the subset of that request body described in the Tasks section and emit the streaming event vocabulary ^[11]. The document says existing SDKs, streaming parsers and UI components work against a UHP server with no changes ^[12]. Extensions are confined to additive places: metadata, a small number of extra request fields, new object types, and never a change to the meaning of an existing field, so a client that ignores every extension still gets a working task ^[13].

What the client gets back is narrower than the diagram suggests. The specification defines portability by the conformance suite, and a client written against the spec works with any server that passes it, whoever built that server ^[10]. The fan-out diagram names six harnesses (Codex, Claude Code, Hermes, DeepSeek Harness, Gemini CLI and Pi) and leaves a slot for the harness that ships next ^[16][19]. Under one wire format, those six still run a request differently. The spec's own sentence grants that each harness already knows how to run a task ^[4]; what UHP standardizes is how an application drives one ^[1]. A team that swaps harnesses behind an unchanged endpoint still has to re-run its own evaluations.

Anyone implementing the server side is told to read the architecture document and pick a conformance class, generate types from schema/uhp-2026-09-12.openapi.yaml, and run the conformance suite ^[17]. The count of servers and clients built against the specification so far sits on a separate examples page ^[10].