Volume VII

Protocols & Tools

MCP, A2A, ACI, humans

Tools are structured outputs with side effects. Protocols (MCP, A2A) standardize discovery and transport. The agent-computer interface — names, schemas, errors — is where most reliability is won or lost.

VII.1Tools Are Structured OutputsFactor 4: a tool call is JSON the model emitted. The 'tool' is your function. The ACI — names, descriptions, parameter schemas, error shapes — is the real prompt.7 min
VII.2Model Context ProtocolMCP is USB-C for tools, resources, and prompts. Servers expose capabilities; clients (harnesses) load them. Do not confuse a protocol with a product strategy.6 min
VII.3Agent-to-Agent (A2A)Google-originated, now Linux Foundation: a protocol for opaque agents to discover, authenticate, and assign tasks across vendors. MCP connects tools. A2A connects firms.5 min
VII.4Computer UseThe environment is a GUI or a browser, not an API. browser-use, OpenHands, and Anthropic's computer-use all shrink the web or desktop into an action space.6 min
VII.5Humans as ToolsFactor 7: contact humans with tool calls. Approval, missing information, and exceptions are tools named ask_human, not a special control-plane myth.5 min