A proposed standard for agentic harness extensions. Working codename: AHXP (Agentic Harness Extension Protocol).
Agentic LLM harnesses are used well beyond software engineering: for research, data analysis, customer support, workflow automation, content creation, operations, and general personal assistance. They range from coding-focused tools (Claude Code, Codex CLI, Gemini CLI, Copilot CLI, Cursor, Cline, Aider) to domain-agnostic frameworks (LangChain, the OpenAI Agents SDK, Pydantic AI, Semantic Kernel, the Vercel AI SDK) and general-purpose assistants. Standards have appeared for what an agent can access (MCP), what it knows (Skills), and how it connects to editors, other agents, and user interfaces (ACP, A2A, AG-UI).
No standard yet covers the internal extension points of the harness itself: the lifecycle hooks, the context, model, tool, and policy providers, and the presentation surfaces inside the agent loop. Those extension points do not depend on the domain, since the loop is the same whether the agent writes code, drafts reports, or answers support tickets. This repository works toward that standard.
The goal is to turn the ecosystem's M-by-N integration problem, where M harnesses and N extensions each need bespoke glue, into M plus N: implement AHXP once per harness, and target it once per extension.
This is not a blank-page effort. The four major command-line harnesses (Claude
Code, Codex CLI, Gemini CLI, and Copilot CLI) have already converged,
independently, on roughly 85% of a hook standard: the same transport (JSON on stdin
and stdout, with exit code 2 as a block), overlapping event names, and an
allow/deny/ask/block decision vocabulary. Copilot CLI even ships a VS
Code-compatible mode that accepts Claude Code's format directly. AHXP's job is to
write that convergence down, fill the gaps, and extend it.
| Doc | Purpose |
|---|---|
docs/design.md |
The design: problem, positioning, the two-profile architecture, taxonomy, data model, manifest and negotiation, trust, event registry, open questions, and roadmap. |
docs/prior-art.md |
The survey of shipping harness plugin and hook systems (CLI, IDE, frameworks) and protocol prior art (LSP, MCP, WIT/WASM, VS Code, A2A, OpenTelemetry GenAI), with sources. |
Design draft, pre-specification. The next planned step is to draft Hook Profile v0,
the formalization of the already-converged CLI hook layer. See the roadmap in
docs/design.md.
The name AHXP is provisional and open for discussion.