Tools & MCP — Quick Reference
Tool Design & MCP Integration · 18% of the exam. Density over narrative — this is for review, not first-pass learning.
What makes a good tool
A precise, example-rich description matters more than a clever name. The model chooses tools based on the description text.
tool_use vs tool_result
tool_use is emitted by the model (request to call). tool_result is sent by the client (the outcome), matched by id.
MCP vocabulary
Server = exposes capabilities. Client = 1:1 connection to a server. Host = the application (e.g. Claude Code) managing multiple clients. Primitives = Tools (model-invoked), Resources (contextual data), Prompts (templates).
Transport choice
Local/trusted process → stdio. Remote or shared → Streamable HTTP.
Error handling
Always return structured, actionable errors (is_error:true + clear message) — never a raw stack trace — so the model can self-correct on retry.
Built-in tool vs MCP vs skill
One-off local capability → built-in tool if one exists. Shared/external system integration → MCP server. Team workflow, no external system → skill or slash command.