Designing MCP tools that agents can use safely

Goal

Expose capabilities to language-model agents so that the model can choose the right tool from its description, call it correctly from its schema and interpret the result without guessing.

Prerequisites

An MCP server implementation (the official SDKs) and a clear list of the operations agents legitimately need.

Steps

1. One purpose per tool with a verb-noun name (search, read_section); avoid catch-all tools that take a mode argument.
2. Declare an input schema with bounded types (limits on lengths and page sizes) and an output schema; the specification's tool definition carries both inputSchema and outputSchema, and structured results let clients validate what they receive.
3. Set annotations truthfully: readOnlyHint, destructiveHint, idempotentHint, openWorldHint. A read-only server exposes no tool that writes.
4. Bound every result: page sizes, text lengths, timeouts; return cursors for more.
5. Return anticipated failures as tool errors with a stable code and message (not found, quota exceeded with a retry hint) so the model can react; reserve crashes for real bugs.
6. Keep tool descriptions in application code and review them like API documentation; never derive them from content that users or agents can edit.
7. Enforce quotas per tool call and validate Host/Origin as the transport documentation requires.

Expected result

An agent reads tools/list, picks the tool by description, sends valid arguments on the first try and receives structured content or a clear error.

Limits and test basis

Good schemas do not prevent misuse by a poorly instructed model; keep destructive operations out of reach rather than relying on descriptions. The design mirrors this wiki's own read-only server and the cited specification.