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

Annotations already declare readOnly, idempotent, non-destructive, open-world behavior, so the description's added value is the commercial and payment model: $0.01 USDC per successful call via x402 on Base. It also lists the return fields (registration status, corporate number, registered name, dates), useful since there is no output schema. It does not restate the schema's nuance that shape-invalid input is rejected before payment.

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?

Content is well front-loaded and chunked into purpose, use-when, returns, and price. The full Japanese duplication doubles the length without adding new information, though it is defensible for a Japanese-market tool.

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 two-parameter read-only lookup with no output schema, the description covers purpose, trigger condition, returned fields, and cost/payment mechanism—everything an agent needs before invoking it.

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 description coverage is 100%, with the registration_number pattern, normalization rules, and the paid-vs-rejected distinction documented in the schema itself. The description adds only the 'T + 13 digits' restatement, so baseline 3 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 a specific verb and resource—verify a Japanese qualified-invoice registration number (T + 13 digits) against the NTA registry. The clause 'You already have a ... number' distinguishes it from extraction-oriented siblings such as jp_invoice_extract_and_verify, so an agent can pick correctly without inspecting schemas.

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 an explicit triggering condition ('Use when: You already have a Japanese T+13-digit qualified-invoice registration number'), which implicitly excludes cases needing extraction first. It does not name a specific alternative tool or state when-not-to-use, which keeps it short of a 5.

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