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

Annotations already declare readOnlyHint, idempotentHint, non-destructive and openWorld, so safety is covered. The description nevertheless adds real value beyond them: per-item failures do not fail the whole call, the registered name is returned only in the no-`date` mode, and it costs 1 API request per number. Slightly short of 5 only because the exact response shape is not sketched.

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?

Roughly four sentences, front-loaded with the core purpose and format, then the use case, then the mode split, then failure and cost semantics. Every sentence carries information an agent needs to call it correctly; nothing is padding.

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 output schema, the description still conveys the return behavior that matters (per-item results, registered name present only without `date`). Combined with the 100%-covered input schema and the safety annotations, an agent has everything needed to invoke and interpret the call.

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%, so baseline is 3, but the description goes beyond the schema by explaining that a bare 13-digit number is accepted as a T-number and that omitting `date` changes both the evaluation date (today, JST) and the returned fields. That is genuine semantic enrichment rather than repetition.

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?

States a specific verb and resource ('check whether invoice registration numbers ... are valid') plus the exact format (T + 13 digits) and batch scope (up to 20). An agent can immediately distinguish this validation tool from the sibling company-lookup tools (bango_get_company, bango_match_companies).

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?

Gives an explicit typical use case (verifying the T-number before claiming input tax credit) and, crucially, tells the agent how to choose between the two modes: omit `date` for today's validity plus the registered name, or supply `date` for point-in-time validity with no name. There is no meaningful alternative tool, so no exclusion clause is needed.

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