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

With no annotations supplied, the description carries the full burden and does well: it declares the operation read-only, discloses the authorization failure mode (scoped keys refused), and clarifies the data granularity (today, this month, account-level not graph-level). It does not describe the literal response shape, but it enumerates the data the caller gets, so the remaining gap is small for a zero-argument read.

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 sentences, front-loaded with the resource and its contents, followed by the read-only/auth caveat. No filler, and the important exclusion (scoped keys refused) is not buried.

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 tool with no output schema, the description supplies everything an agent needs: what data comes back, that it is account-scoped, and that scoped credentials will be rejected. Nothing material is missing.

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 nothing for the description to disambiguate beyond what the empty schema already conveys. Baseline of 4 applies.

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 names the exact resource (the caller's plan and meter) and enumerates its three contents: free-tier calls used today vs the daily cap, hosted-key spend this month, and the upgrade URL. No sibling in the list (all graph/run operations) overlaps with account billing usage, so it is unambiguously distinguishable.

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 states a concrete context for use (checking plan, quota consumption, and where to upgrade) and gives a hard exclusion: scoped keys are refused because billing is account-level. There are no sibling alternatives to route between, so the absence of an explicit 'when not to use' comparison is a minor gap rather than a real one.

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