Agents and harnesses
Prompt Studio has no coding agent of its own. Harness extensions connect it to agents you install, such as Claude Code, Codex, and OpenCode.
Harnesses
A harness is a contribution from an extension. It knows how to start an agent, send it messages, resume it, and stop it. It turns the agent’s output into a conversation that Prompt Studio can show and save.
Each harness has a full ID made of the extension ID and its own ID, such as pstdio.harness-claude-code.harness.claude-code. The pst command also accepts short names such as claude-code.
A harness does not install the agent. Prompt Studio checks whether the agent’s command is installed each time it lists harnesses. pst agents list shows the result. You sign in to the agent with the agent’s own tools.
Anyone can write a harness for another agent with the same extension API. Start with Write an extension. The Codex harness source is a complete example.
Sessions
A session is one conversation with an agent in one workspace. The work is split like this:
| Prompt Studio owns | The harness owns |
|---|---|
| Starting, queueing, and stopping sessions | Talking to the agent in its own protocol |
| Which workspace the session uses | The agent’s own session ID and transcript format |
| Approval requests and your answers | Turning agent output into conversation updates |
| The saved conversation | Reporting when the agent finished, failed, or stopped |
A follow-up continues the same session using the saved conversation and the harness’s resume support. The saved conversation is Prompt Studio’s record of the session.
Queued work survives a runtime restart. Already-running work reconnects only when the harness supports reattachment. Otherwise, the session becomes disconnected; send a follow-up to resume it if the agent’s session is still available. A restart does not guarantee that an interrupted task finishes.
A session finishing only means the agent stopped. It does not prove that the work is correct. Tools such as Planner add their own review steps for that.
Where the agent runs
A local agent runs as a process on your computer, in the workspace folder. A remote-capable harness can run the agent on another machine through a remote workspace. Prompt Studio never falls back to a local folder for a remote workspace. See Local and remote work.
Skills
Extensions can ship skills: instruction files that teach an agent how to use a tool. pst agents setup <agent-id> copies the skills of your enabled extensions to the place where that agent reads them. Prompt Studio does not overwrite a skill you already have with the same name.