HIPs/docs/protocols.md

Hanzo Protocol Boundaries

ZAP, MCP, A2A, ACP — four protocols, four boundaries. None absorbs the others.

The decision rule

• ZAP is how Hanzo talks to Hanzo (internal RPC + schema + codegen).
• MCP is how models/clients call Hanzo tools (Model Context Protocol).
• A2A is how agents collaborate with Hanzo agents (Agent2Agent federation).
• ACP is compatibility for agent client/connect networks (NOT canonical core — adapter only).

Protocol matrix

Protocol	Canonical?	Scope	Used by Hanzo for
MCP	Yes (external)	model/client → tools/resources	Tool exposure, IDE/assistant integration, Flow/Auto/ML tool publication
A2A	Yes (external)	agent → agent	Cross-vendor agent federation, task delegation, long-running handoff
ACP	Adapter only	varies — see below	Optional IDE/coding-agent client compatibility
ZAP	Yes (internal)	service → service	Hanzo internal RPC, generated Go/TS/Py/Rust clients, in-process or over-wire

Stack diagram

External agent/client ecosystem:
  MCP / A2A / ACP adapters
       │
Hanzo product/control plane:
  cloud / ai / mcp / ml / flow / auto / o11y / insights
       │
Hanzo internal service protocol:
  ZAP

MCP — Model Context Protocol

Use for:

• Tool exposure to external models and IDE clients (Claude Code, Cursor, Zed, JetBrains, ChatGPT desktop, etc.).
• Resource exposure (files, search indices, KB documents) read by external models.
• Publication of Flow workflows, Auto jobs, and ML primitives as named tools callable by any MCP client.
• Prompt and template publication to model clients.

Do NOT use for:

• Internal service RPC between Hanzo subsystems — that is ZAP.
• ML job orchestration on the cluster — that is the operator + ZAP control plane.
• Agent-to-agent negotiation, delegation, or long-running task handoff — that is A2A.
• Persistent product object model (Tool, Agent, Job, Trace records) — that lives in the ZAP schemas and is exposed through MCP as views, not as the system of record.

MCP is request/response and stateless at the protocol layer. Hanzo MCP servers (Go path at ~/work/hanzo/mcp/go/, Rust path at ~/work/hanzo/mcp/rust/) translate MCP calls into ZAP-typed calls against the appropriate Hanzo subsystem.

A2A — Agent2Agent

Use for:

• Cross-vendor agent collaboration where the counterparty is not a Hanzo agent.
• Capability discovery: agents publish their AgentCard, others discover and route.
• Task delegation with long-running, resumable execution.
• White-label federation: customer-deployed Hanzo agent fleets that interoperate with non-Hanzo agents.
• Marketplace agent routing where the broker is external.

Do NOT use for:

• Low-level tool calls — those are MCP.
• Hanzo internal RPC between subsystems — that is ZAP.
• Kubernetes-side ML job reconciliation — that is the operator's job, expressed as CRDs internally and as ZAP/HTTP externally.
• IDE tool integration — that is MCP, or optionally ACP as an Agent Client Protocol adapter.

A2A sits ABOVE MCP. A Hanzo A2A agent may internally use MCP tools, but the external contract is agent capability + task collaboration, not raw tool invocation. The MCP boundary handles "give me this tool"; the A2A boundary handles "delegate this goal."

ACP — Three meanings, Hanzo picks one

ACP is ambiguous in the ecosystem:

• Agent Connect Protocol — registry/runs/threads/descriptor style federation aimed at agent marketplaces.
• Agent Client Protocol — IDE-to-agent integration popularized by editor and coding-agent vendors.
• Agent Communication Protocol — broader generic agent-to-agent communication framing.

Hanzo treats ACP as adapter compatibility, not canonical core. The canonical agent federation protocol for Hanzo is A2A; the canonical tool protocol is MCP. ACP exists in Hanzo only as an adapter surface.

Until customers explicitly demand Agent-Connect-style federation, the default ACP-in-Hanzo means Agent Client Protocol — for exposing Hanzo coding agents to IDE clients that speak that protocol. The other two senses ship as separate adapter targets only when a real customer asks.

ZAP — internal

ZAP is Hanzo's typed wire protocol. Schema-first. .zap files compile via zapc to Go/TS/Py/Rust clients.

Use for:

• Typed service contracts (cloud ↔ ai, cloud ↔ ml, cloud ↔ o11y, ml-controller ↔ ml-types, etc.).
• Generated clients in every supported language from one schema.
• Split deployments: the same subsystem can mount in-process (func Mount(r, deps)) or run remotely (ZAP RPC over the wire) without changing call sites.
• Durable internal API surface that outlives any one wire format.
• Schema-first product objects: TrainingJob, Experiment, Model, Trace, Tool, Agent, Run, Thread, etc. live in .zap schemas and are exposed externally through MCP/A2A/HTTP — never the reverse.

ZAP is NOT for external clients. External clients use MCP/A2A/ACP/HTTP. The boundary is enforced by the gateway: external traffic terminates at HTTP or one of the external protocols; internal traffic between Hanzo subsystems travels as ZAP.

Where each Hanzo subsystem fits

Subsystem	Internal	External
cloud	ZAP (Deps clients)	HTTP via gateway
ai (LLM control plane)	ZAP	HTTP
mcp	ZAP internal	MCP external
ml (Rust primitives)	n/a (library)	n/a
operator (k8s reconciler)	ZAP for cloud.Mount	K8s CRDs
flow	ZAP	HTTP + MCP publication
auto	ZAP	HTTP + MCP publication
o11y	ZAP internal	OTLP external
insights	ZAP internal	HTTP + UI
agents	ZAP internal	A2A external
brain	ZAP	n/a

Final decision rule

ZAP is how Hanzo talks to Hanzo.
MCP is how models/clients call Hanzo tools.
A2A is how agents collaborate with Hanzo agents.
ACP is compatibility for agent clients/connect networks.
ML is the Hanzo-native operator/control plane for ML workloads (hanzoai/operator).

See also

• HIP-0105 In-Process Extension Runtime (substrate)
• HIP-0106 Unified Hanzo Cloud Binary (where subsystems mount)
• HIP-0108 On-Demand Subsystem Supervisor (warm pool / idle eviction)
• HIP-0109 Hanzo ML Cloud Toolkit (operator absorption of Kubeflow estate)