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

Annotations already cover the safety profile (readOnlyHint=true, destructiveHint=false, openWorldHint=false), yet the description still adds real value: the required auth scope (billing:read or billing:write), the meaning of cap_jpy including the fallback behavior when unset (owner approves every time), and the fact that pending approvals come with URLs. What it lacks is any note on data freshness, failure modes, or cost of the call. The parenthetical about cap_jpy is also internally tangled ("before tax, the monthly contract total"), which blunts the disclosure.

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 purpose and followed by the actionable directive; nearly every clause names a distinct returned field, so the length is earned. The cap_jpy parenthetical is dense and hard to parse in one read, which is the only structural weakness.

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?

With no parameters and no output schema, the description carries the full burden of explaining what comes back, and it does so by enumerating each returned element including approval URLs and the price list. Combined with the auth scope and the follow-up instruction, an agent has everything needed to call it and act on the 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 semantics to explain and the baseline of 4 applies. The description correctly implies the call is scoped entirely by the authenticated account rather than by any argument.

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 opens with a specific verb and resource ("Return this account's billing state") and enumerates exactly what is returned: contract existence, per-site plans and amounts, AI cap, contract total, pending approvals, and the price list. An agent can tell this is a read of billing/plan data. It does not distinguish itself from the closely related siblings plan_status and plan_profile, which is the main clarity gap.

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 explicit sequencing guidance: "Call it before plan_set and tell the user how much it will increase," which tells the agent both when to call it and what to do with the result. However, it offers no guidance on choosing between this and plan_status/plan_profile, which appear to overlap, and plan_set is not even present in the sibling list, so the referenced alternative cannot be cross-checked.

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