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

Annotations already mark readOnlyHint and openWorldHint, and the description adds genuinely useful behavior beyond them: the call itself is free and consumes no renders ('Free, and it renders nothing'), and the three auth modes (header/binding/trial) map to observable failure signatures such as quota_exceeded with an unrecognized limit. No contradiction with annotations.

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 dense paragraphs, each earning its place: the first front-loads the resource and return fields, the second gives when-to-call and interpretation rules. No filler or redundancy.

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 zero-parameter, read-only identity tool with no output schema, the description covers the return fields, quota meaning, authentication variants, a usage rule, and a troubleshooting scenario. An agent has what it needs to decide when to call whoami and how to interpret its result.

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?

The tool takes zero parameters, so there is no parameter burden for the description to carry; the empty schema and 100% coverage fully document invocation. Baseline 4 applies, and the description appropriately says nothing about arguments.

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?

The description identifies a concrete resource — the account behind the current connection — and enumerates exactly what it returns: id, plan, renders_remaining, and authentication origin. It also carves out a distinct niche from siblings by stating the auth-method field is 'the part no other tool answers,' so an agent can distinguish whoami from usage, billing_portal, and the rest.

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 direct call triggers: 'Call it before a batch' and 'the moment anything about quota surprises you,' plus a concrete diagnostic workflow for quota_exceeded errors that point to a keyless trial. It does not spell out when-not-to-call or compare against sibling tools like usage, so it falls just short of a full 5.

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