A2A Coordination Semantics: HCT for Google's Agent Protocol

We recently announced the Model Context Protocol (MCP) adaptation of Harmonic Coordination Theory (HCT). Today, we are expanding the ecosystem to support Google's Agent-to-Agent (A2A) Protocol.

While MCP focuses on the standardized client-host connection, A2A is designed for decentralized, multi-agent communication. HCT provides the shared language—the music theory—that enables these distributed agents to play together effectively.

The Extension: hct-a2a

We have released the hct-a2a repository, a formal extension that injects HCT signals into A2A messages.

Key Features

1. Protobuf-First Design: Unlike the JSON-centric MCP implementation, the A2A extension is defined in Protocol Buffers (.proto). This ensures strict typing and compatibility with high-performance gRPC backends.
2. Auto-Generated Specification: The hct.proto file is not hand-edited. It is auto-generated via CI/CD from the central hct-spec repository, guaranteeing that our A2A and MCP implementations never drift apart. They sing from the same sheet music.
3. Polyglot Support: We released reference implementations in four languages:
   • Python: A FastAPI-based Performer and Conductor.
   • TypeScript: Node.js actors for web-native agents.
   • Rust: High-performance, type-safe actors using Tokio.
   • Go: Struct-based orchestration for microservices.

Why A2A needs HCT

In a distributed agent mesh, "intent" is often lost. An agent sends a task ("Analyze this file") but fails to convey the context:

• Is this a Rush job (tempo: presto)?
• Should I use Cheap models or Deep Reasoning (dynamics: pp vs ff)?
• Should I Wait for approval (fermata)?

HCT answers these questions elegantly within the metadata of the A2A envelope, allowing the receiver to adjust its "performance" without changing the core task definition.

Get Started

Check out the repository or browse the Live Spec.

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This work is part of the Harmonic Coordination Theory research initiative.