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

Annotations already declare readOnly/idempotent/non-destructive, so the safety profile is covered. The description adds the auth requirement ('api_key が必要: 管理画面で発行したキー') and the post-subscription precondition, which are genuine behavioral details not present in the structured fields.

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?

A single compact sentence with parenthetical detail; the core action is front-loaded and no sentence is wasted. Slightly dense but appropriate for the amount of information conveyed.

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 one-parameter read tool with no output schema, the description covers what is returned (a portal URL), the precondition, and the auth need. Nothing critical is missing, though the return/error behavior is left implicit.

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?

Schema coverage is 100% for the single api_key parameter, so the baseline is 3. The description reinforces that the key must be issued from the dashboard and shows the format, but adds little syntax or validation detail beyond the schema.

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 a specific verb and resource ('Stripe の顧客ポータルの URL を返します') and enumerates what the portal lets the user do (payment method changes, receipts, cancellation). It is clearly distinct from create_checkout/list_plans, though it does not explicitly name the sibling an agent should pick instead.

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?

'有料プランの契約後に使えます' states a concrete precondition for use, which is real context beyond the name. It stops short of naming alternatives or stating when NOT to use it (e.g., for new subscriptions use create_checkout).

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