Landscape

HCP sits beside five other open agent standards, each of which answers one question well, and trouble starts when one is stretched to answer the others. HCP answers the install question (why that needs a standard) and composes with the rest, which the sections below lay out standard by standard.

The map#

Standard	Question it answers	Unit	Governance
Agent Skills	How does an agent learn to use a capability?	`SKILL.md` directory	Open spec, multi-vendor clients
MCP	How does a model call a tool over a wire?	MCP server (stdio / Streamable HTTP)	Open spec, steering committee
Agent Plugins	How are Skills and MCP servers packaged so clients can load them consistently?	Plugin directory (`plugin.json`, `skills/`, `mcp.json`)	Open spec, TSC
A2UI	How does an agent render generative UI?	UI messages / catalogs	Open spec
A2A	How do agents talk to each other?	Agent cards, tasks	Open spec
HCP	How does a harness install, authorise, bind, sandbox, and execute a capability, and hear from it?	Harness Pack	Open spec, RFC process

How they compose#

Agent Skill (SKILL.md)              teach
        │
        ▼
Harness Pack (HCP)                  install
  commands · Policy · Binding · Sense · Skill · Adapter
        │                    │
        ▼                    ▼
   CLI / JSON face      optional MCP export        transport
        │
        ▼
   host agent loop · UI (A2UI) · other agents (A2A)

A pack's Skill (HARNESS.md) follows the Agent Skills principles (short lead, examples, anti-patterns, pointers) and is narrower on purpose, because it teaches how to invoke this pack's commands correctly. hcpx activate copies it to $HCP_HOME/skills/<id>/SKILL.md, so hosts that already speak Agent Skills get progressive disclosure with no extra work. A Skill without a pack still teaches, but a pack without a Skill fails lint.

A runtime can expose packs to MCP-only hosts through the reference export, a single MCP tool hcp that forwards to the runtime, so every call still passes Policy and Binding and the pack stays the source of record. A UI host speaking A2UI invokes pack commands the same way an agent does and may subscribe to Sense for chrome, but it never owns mint or Policy. Projections holds the normative rules for these generated surfaces and exports.

HCP and MCP#

	MCP	HCP
Category	Transport between model and tool server	Install unit for a harness
You ship	A server process (stdio or HTTP)	Three files describing an existing CLI or API
Identity of a capability	Tool name on a server	`pack_id` + `verb_key`, projected to CLI, JSON, Skill, and optionally MCP
Authorisation	OAuth 2.1 between client and server	Runtime capability grants per Principal; pack declares only effect class and land
Credentials	Server holds them	Runtime hub mints them; pack names a mode and never holds a secret
Writes	Tool annotations (hints)	Policy `operation: write` + `confirm` enforced by the runtime before any adapter runs
Outbound events	Notifications within a session	Sense: declared triggers, separate grant, cannot mint or act
Human at a terminal	Not a first-class path	The CLI face is the primary face
Sandbox	Client's concern	Default; bin allow-list and cwd jail are part of the profile

The reverse does not hold: an MCP server cannot be the unit you install, because it carries no Policy, Binding mode, Sense declaration or CLI face.

HCP and Agent Plugins#

Agent Plugins defines a portable directory for Skills and MCP servers, and HCP defines one for an executable capability with rules. Both are vendor neutral, and both say "put these files here and clients will find them".

	Agent Plugins	HCP
Manifest	`plugin.json` (name, version, metadata)	`surface.json` (id, adapter, Binding mode, commands)
Teaching	`skills/*/SKILL.md`	`HARNESS.md` (copied to a `SKILL.md` on activate)
Execution	`mcp.json` pointing at servers	Named commands run through an Adapter under Policy
Permissions	Left to each client	Policy rows + five capability grants, enforced by the runtime
Outbound	None	Sense

A plugin's directory could carry an HCP pack next to its Skills and MCP servers, with the pack as what the harness installs and the plugin as how a client discovers it. That nesting is a proposal on the Roadmap and has not shipped.

Name disambiguation#

These names mean something narrower here than elsewhere, and the glossary defines the rest.

Term	Meaning here
Harness	The thing driving an agent; in HCP, the runtime that installs packs
Skill	`HARNESS.md`, the pack's teaching file; or an Agent Skill in general
Surface	Legacy filename `surface.json` = the pack manifest. Not a UI surface
Agent teaching pack	An agent's identity and instructions. Invokes HCP; is not an HCP pack
Connector	Not an HCP type. Vendor SDK glue inside a product runtime