Short answer
ACP, Zed's Agent Client Protocol, is an open standard for connecting a code editor to a coding agent: a JSON-RPC contract so any supporting editor can drive any supporting agent, with the editor as the client. UHP, the Unified Harness Protocol, is an open standard for a related but broader edge: an application, not only an editor, invoking and controlling the harness that runs an agent, covering tasks, sessions, streaming, files, and cancellation. These edges are closer than most protocol pairs: both are about a client invoking an agent. The difference is scope and setting, editor integration versus a product-backend harness contract, and in some architectures they compose rather than compete. UHP is a newer open standard with a versioned specification dated 2026-08-11, a reference implementation, a conformance suite, and one independent client implementation.
- ACP connects a code editor to an agent; UHP connects an application to the harness that runs an agent. The edges are related, and the difference is scope: editor integration versus a product-backend harness contract.
- ACP is an established open standard for the agent-editor boundary, supported across many editors and agents. UHP is a newer open standard for the application-to-harness boundary, with a versioned specification, reference implementation, conformance suite, and one independent client implementation.
- Whether they compose or overlap depends on the architecture; this page describes the scope difference rather than declaring they never meet. Facts verified 2026-08-27.
Related edges, different scope
ACP and UHP are closer than most protocol pairs, so honesty about scope matters more than a clean layer story. ACP, Zed's Agent Client Protocol, is an open standard for connecting a code editor to a coding agent over JSON-RPC, with the editor as the client, so any supporting editor can work with any supporting agent. UHP, the Unified Harness Protocol, is an open standard for a broader client edge: an application, not just an editor, invoking and controlling the harness that runs an agent, covering tasks, sessions, streaming, files, and cancellation, with session-scoped file namespaces and product-facing outputs in view. Both describe a client invoking an agent; the difference is the setting and scope, editor integration versus a product-backend harness contract. All facts here are from public materials, verified 2026-08-27.
The questions each open standard answers
Because the edges are related, read these as scope differences, not a hard layer split.
- The client
- ACP: a code editor drives the agent. UHP: an application invokes the harness that runs the agent.
- The setting
- ACP: editor integration, so a developer works with an agent in their editor. UHP: a product backend invoking a harness, with session-scoped files and product-facing outputs.
- What it carries
- ACP: editor-to-agent interaction over JSON-RPC. UHP: tasks, sessions, streaming, files, and cancellation for a run.
ACP and UHP, by client and scope
This maps each open standard's client and scope; the edges are related, so it is not a clean either-or and not a scorecard. Facts as of 2026-08-27.
| Dimension | ACP | UHP |
|---|---|---|
| The client | A code editor | An application, of which an editor is one kind |
| Primary setting | Editor integration for a developer working with an agent | A product backend invoking the harness that runs the agent |
| What it carries | Editor-to-agent interaction over JSON-RPC | Tasks, sessions, streaming, files, and cancellation, with files scoped to a session container |
| Maturity | Established open standard, broad editor and agent support | Newer open standard; versioned spec, reference implementation, conformance suite, and one independent client implementation |
| How they might compose | An editor drives an agent directly | A backend invokes a harness; an editor could still use ACP at its own edge in the same system |
When each open standard is the one you reach for
- You are integrating an agent into a code editor
- That is ACP's scope: an established open standard so an editor and an agent can work together over a common contract. It is built for the editor as the client.
- You are invoking a harness from a product backend
- That is UHP's scope: an application invoking and controlling the harness that runs an agent, with tasks, sessions, streaming, files, cancellation, and session-scoped file containers, under an open standard with a versioned specification.
- You have both an editor and a product backend
- For a product backend, UHP is the application-to-harness contract you build on: your application invokes and controls the harness over UHP, while ACP stays at the editor's edge.
FAQ
- Which ACP does this page mean?
- This page means the Agent Client Protocol, Zed's open standard for the editor-to-agent edge. A different protocol also abbreviated ACP, IBM's Agent Communication Protocol for agent-to-agent messaging, merged into A2A under the Linux Foundation in 2025 and is compared separately on UHP vs A2A.
- Are UHP and ACP competitors?
- They are related but scoped differently. ACP, Zed's Agent Client Protocol, connects a code editor to an agent; UHP connects an application to the harness that runs an agent. In some architectures they compose, an editor using ACP and a backend using UHP; whether they overlap depends on the setting.
- What is the difference between ACP and UHP?
- ACP's client is a code editor, and its scope is editor-to-agent integration over JSON-RPC. UHP's client is an application, and its scope is invoking and controlling the harness that runs an agent, covering tasks, sessions, streaming, files, cancellation, and session-scoped file containers for a product backend.
- Is UHP an open standard?
- Yes. The Unified Harness Protocol is an open standard. As of 2026-08-27, its public materials include a versioned specification dated 2026-08-11, a reference implementation, and a conformance suite, and the independent adoption tracker lists one independent client implementation, SuperQode.