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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and a closed-world scope, so the safety profile is covered. The description adds the meaningful behavioral limit that only well-formedness (structure + Luhn) is evaluated, not existence or lookup, but says nothing about the response shape or error behavior.

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?

One sentence, front-loaded with the verb and resource, with the validation criteria appended as clarifying detail. Nothing is redundant and nothing is padded.

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-only validator the description covers input and scope, but with no output schema it should say what comes back (a boolean? a per-segment failure reason?). An agent can call it but cannot predict how to interpret a failure.

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 'isin' parameter, so the schema already documents it. The description's decomposition (2-char country code + 9-char national code + check digit) usefully explains why 12 characters are expected, but adds no format/whitespace/prefix handling rules beyond that.

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 ('check') and resource ('ISIN'), and pins it to ISO 6166 while enumerating the three validated components (country code, 9-character national code, Luhn digit). This distinguishes it in substance from cusip_checker/sedol_checker/lei_checker, but it never names those siblings explicitly, so the differentiation is implicit.

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 inferred rather than stated: 'is well formed' quietly scopes the tool to syntactic/checksum validation, which implies it should not be used to confirm an ISIN actually exists or to resolve it to an instrument. No explicit when-to-use, prerequisites, or named alternatives are given.

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