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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint, so the safety profile is covered. The description adds the validation method (Mod-9) and notes a "signed XDR-1 receipt" is returned, which is useful behavioral color, but it never says what happens on an invalid number (error vs. false result) — the key behavioral question for a validator.

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 short sentences, front-loaded with the core verb and resource. The middle sentence is mildly promotional but does convey real usage context, so nothing is wasted beyond a little framing.

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 an output schema present, the description needn't explain the receipt structure, and the annotations cover safety and idempotency. The remaining gap is that it doesn't clarify granularity versus japan_supplier_check — whether this is checksum-only or registry-backed — which materially affects tool selection in this crowded sibling set.

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 a single required parameter already documented as a 13-digit number with an example. The description restates the same "13-digit" constraint and adds no syntax, normalization, or edge-case guidance (e.g. handling of hyphens or the leading check digit), so it earns the baseline, not more.

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 (validate), a precise resource (13-digit Japanese Corporate Number issued by the National Tax Agency) and even the algorithm (Mod-9 check digit), which is far more than a restatement of the name. It does not, however, distinguish itself from near-siblings like japan_supplier_check or company_number_format, leaving the agent to infer whether this is a format check, a registry lookup, or a full supplier screen.

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?

"Required for Japanese B2B SaaS and enterprise billing" implies the usage context (onboarding/billing validation) but gives no explicit when-not guidance and names no alternative. An agent facing japan_supplier_check and company_number_format gets only a hint, not a routing rule.

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