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

Annotations already cover safety (readOnly, idempotent, non-destructive), so the bar is lower. The description adds real value by explaining how to interpret cap_period: the stopping limit is the smaller of the plan limit and set_spending_cap, and that paid plans count over the billing period — semantics an agent could not derive from schema or 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?

Three dense sentences, front-loaded with the returned metrics before the cap_period nuance and the key requirement. Compact for the amount of meaning conveyed, with no filler.

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?

No output schema exists, so the description carries the burden of describing return values — and it does: connection time, speech time, session count, plan limits, and cap_period semantics. Units and per-plan breakdown detail are left unstated, a minor gap for a single-object read tool whose safety profile is fully annotated.

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 required param at 100% schema coverage would baseline at 3, but the description adds provenance the schema lacks: the key must be the one issued in the admin panel. That is genuine added meaning beyond 'API キー (sk_live_…)'.

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?

Specific verb+resource: it returns the current month's usage metrics (connection time as billing basis, speech time, session count) plus plan limits. An agent can tell it apart from list_plans or get_account by the metric set, though it never explicitly names 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?

Usage is implied rather than stated — you call it to check consumption against limits. It gives a prerequisite (api_key must be issued in the admin panel) but no explicit when-to-use/when-not-to-use guidance or alternative tools for the same question.

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