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

Annotations already cover the safety profile (readOnlyHint=true, destructiveHint=false, openWorldHint=true), so the bar is lower; the description still adds the non-obvious auth constraint ('agency key only') and the shape of the payload, including the conditional disclosure that account lists are only returned for account users. That conditional return behavior is genuine context beyond the annotations, though pagination/error behavior is unaddressed.

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 tight clauses plus the endpoint path; the credential gate is front-loaded and nothing is padded or repeated. Every phrase carries information.

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 single-resource GET with no output schema, the description compensates by naming the returned fields and the account-user conditional. Only marginal gaps remain (no error/not-found behavior, no note on what happens with a non-agency key).

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?

One parameter at 100% schema description coverage, so the schema already documents user_id as a positive WhatConverts user id. The description repeats the id concept implicitly ('by id') without adding format, source, or lookup guidance, so 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?

States a specific verb+resource+cardinality ('get one user by id') and enumerates what comes back (role, notification settings, accessible accounts). This cleanly distinguishes it from the sibling whatconverts_list_users without the agent needing to open any schema.

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?

'Agency key only' gives a concrete precondition that determines when this tool is callable, which is real usage context rather than restated purpose. It does not name an alternative or an explicit when-not case, but the singular-vs-list split with whatconverts_list_users is inferable.

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