# Tool design and MCP checklist (cheat sheet)

> Tools a model can use correctly and safely.

Source: https://conversationalaidesign.com/cheatsheets/tool-design-and-mcp.html. Author: Nelson Abacherli Kawakami. Reviewed October 27, 2026.

## Tool design
- Few tools with distinct jobs
- Names that say what happens: cancel_booking
- Descriptions say when not to use it
- Strict schemas: enums, formats, required fields
- Concise structured results; helpful error messages
- Separate reads from writes
- Least privilege and per-user credentials
- Confirmation before consequential writes

## Loop essentials
- Validate arguments
- Timeouts and capped iterations
- Idempotency keys on writes
- Log every call with arguments and results

## MCP 2026-07-28 in short
- Stateless core: no initialize handshake or session ID
- Requests carry version and capabilities inline
- Routing headers; cacheable list results
- Server questions became multi-round-trip requests
- Extensions: Tasks, MCP Apps. Roots, Sampling, Logging and HTTP+SSE deprecated
- State = explicit handles the model passes between tools
- Pin SDK versions and read the migration notes

## Security
- Tool descriptions can be poisoned
- Vet third-party servers like code
- Human confirmation and audit logs
- Stateless is not secure

Related modules: Tools, function calling and MCP (https://conversationalaidesign.com/modules/08-tools-and-mcp.html)