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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so the safety profile is covered. The description adds behavior beyond that by disclosing that it performs check-digit validation and returns an issuer/issue decomposition — significant given there is no output schema.

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 well-formed sentence, front-loaded with the action and resource, with zero filler. Every clause (identifier type, check digit, issuer/issue split) contributes information.

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 validator with full annotation coverage, the description covers the input constraint, the validation performed, and the shape of the result. The only omission is what happens on an invalid check digit (error vs. boolean), which is a minor gap without an output schema.

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?

There is one parameter with 100% schema description coverage, so the schema already documents the 'cusip' argument and its 9-character form. The description repeats the 9-character constraint but adds no format, casing, or padding details 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?

States specific verb ('check') and resource ('9-character CUSIP'), plus the extra behavior of splitting into issuer and issue. It clearly is not isin_checker or sedol_checker because it names the CUSIP identifier explicitly, though it doesn't call out those siblings by name.

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: use it when you have a CUSIP to validate. There's no explicit when-to-use/when-not, no mention of alternatives for other identifier formats, and no guidance on what to do with an invalid result.

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