Author
Jon Postel
1 reading card from 1 book · 1981.
1 card
RFC 793 — Transmission Control Protocol · 1981
The tool description is the prompt you did not write, yet the model reads it on every turn.
A tool is a name, a one-paragraph description and a typed schema. The model's call is the "liberal" side: validate, coerce, return an error the model can repair. The tool's output is the "conservative" side: compact, structured, free of surprises, truncated with a note. MCP (Model Context Protocol) is the open standard for tools, resources and prompts between a client (the agent) and servers; donated by Anthropic to the Agentic AI Foundation under the Linux Foundation on 9 December 2025, with over 10,000 public servers and, in a 2026 survey (Stacklok), 41% of organisations running MCP in production. But Postel's law has a known dark side: liberal acceptance is exactly the door injection walks through — a tool's output is treated as data, not as instructions (lesson 17).
“TCP implementations will follow a general principle of robustness: be conservative in what you do, be liberal in what you accept from others.”