Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

The annotations already declare readOnlyHint and idempotentHint, so the side-effect profile is covered. The description adds context about the intended result ('address you can write to'), but it does not describe return shape, limits, or what fields may be included, so it remains modest rather than rich.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Two single-sentence pieces, proceeding from action to use case, with no filler or repetition. Every sentence earns its place and the most important behavior is front-loaded.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a one-parameter, read-only, idempotent lookup tool, the description is nearly complete: it specifies what to query, why, and what the result can enable. It could be stronger by noting whether the result is a minimal match list or a full user record, but the low complexity keeps this gap small.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage for the only parameter is 100%, and the description merely repeats 'username or display name' without adding matching, normalization, or query-format details. With full schema coverage, baseline 3 is appropriate.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

Description uses a specific verb and resource — 'Look somebody up by username or display name' — and names the output purpose ('an address you can write to'). It is clear and stands apart conceptually from users_list or users_get, though it never explicitly distinguishes itself from those siblings.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

It gives a concrete trigger: 'Use this to turn a name somebody mentioned into an address you can write to.' This clearly answers when to use the tool. It does not name alternatives or state when not to use it, but the guidance is specific enough for selection.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.