Harness Runtime API complements existing agent protocols:
User interface AG-UI or application-specific events
Agent peers A2A
Control plane Harness Runtime API
Coding client ACP
Tools and context MCP
An adapter may internally use ACP or a framework SDK, but the portable runtime contract additionally defines execution idempotency, capability preflight, cursor replay, terminal-state semantics, and action correlation.
The rows are responsibilities, not a network stack or required request order. One product may use several of these protocols at the same time.