HCP: Harness Communication Protocol
An open protocol for secure communication between AI agent harnesses — enabling standardized task delegation, safety validation, session lifecycle management, and event-driven result exchange across autonomous agent systems.
一个开放的 AI 智能体 Harness 间安全通信协议 —— 提供标准化的任务委派、安全验证、会话生命周期管理，以及跨自主智能体系统的事件驱动结果交换。
What is HCP? / 什么是 HCP？
HCP defines how one AI agent harness delegates work to another. Unlike tool-invocation protocols (e.g., MCP) that call passive functions, HCP communicates with autonomous agents — the callee harness receives an intent, independently decides how to execute it, and streams progress and results back to the caller.
HCP 定义了一个 AI 智能体 harness 如何将工作委派给另一个 harness。与调用被动函数的工具调用协议（如 MCP）不同，HCP 与自主智能体通信——被调用方 harness 接收一个意图，独立决定如何执行，并将进度和结果以流的方式返回给调用方。
Protocol Stack / 协议栈
| Layer | Name | Responsibility |
|---|---|---|
| L4 | Task Layer / 任务层 | Intent description, constraints, result exchange |
| L3 | Safety & Contract / 安全契约层 | Risk assessment, permission audit, session token issuance |
| L2 | Session & Lifecycle / 会话与生命周期层 | Session state machine, event streaming, checkpoint/recovery |
| L1 | Transport & Encoding / 传输与编码层 | Message envelope, AMQP topology, channel specification |
English
| Document | Description |
|---|---|
| Overview | Protocol overview and design principles |
| Architecture | Layered architecture and interaction model |
| L1: Transport & Encoding | AMQP topology, message envelope, stream continuity |
| L2: Session & Lifecycle | Session state machine, event stream, checkpoint |
| L3: Safety & Contract | Risk assessment, capability declaration, session token |
| L4: Task | Task submission, results, error handling |
中文
| 文档 | 说明 |
|---|---|
| 协议总览 | 协议概述与设计原则 |
| 协议架构 | 分层架构与交互模型 |
| L1: 传输与编码层 | AMQP 拓扑、消息信封、流式续流 |
| L2: 会话与生命周期层 | 会话状态机、事件流、检查点 |
| L3: 安全契约层 | 风险评估、能力声明、Session Token |
| L4: 任务层 | 任务提交、结果返回、错误处理 |
Key Design Principles / 核心设计原则
- Unidirectional / 单向调用: Strict Caller → Callee model
- Safety as Core / 安全为核心: Mandatory L3 safety gate for every task
- Harness Autonomy / Harness 自治: Caller describes what; callee decides how
- Standardized Transport / 标准化传输: AMQP 0-9-1 as the single transport protocol
- Protocol Simplicity / 协议简洁: Core spec covers common patterns; advanced features via extensions
HCP vs MCP
| MCP | HCP | |
|---|---|---|
| Callee / 被调方 | Passive tool | Autonomous agent |
| Interaction / 交互 | Call function → get result | Submit intent → agent iterates → stream results |
| Lifecycle / 生命周期 | Stateless | Stateful sessions with events |
| Safety / 安全性 | None | Mandatory pre-execution risk assessment (R1–R5) |
| Transport / 传输 | Stdio / HTTP+SSE | AMQP 0-9-1 with durable delivery |
| Scope / 适用范围 | Inside a harness | Between harnesses |
MCP and HCP are complementary. A harness uses MCP internally to invoke tools, and HCP externally to delegate work to other harnesses.
MCP 和 HCP 互为补充。Harness 内部使用 MCP 调用工具，对外使用 HCP 委派工作给其他 harness。