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

The description goes beyond the readOnlyHint and idempotentHint annotations by disclosing that the tool is keyless/offline, works without network or auth, and explicitly states the limitation of not resolving the issuer. It also details the validation logic (structure and mod-10 check digit) and what it returns (ISO country prefix and NSIN), providing rich behavioral context.

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?

The description is highly concise, consisting of two sentences with no wasted words. It front-loads the main verb ('Validate an ISIN') and immediately provides essential details (format, example, validation method, returns, and limitations). Every sentence earns its place.

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 simple single-parameter tool, the description covers purpose, validation logic, return value components, authentication needs, and limitations. It does not specify the exact output structure (e.g., whether it returns a boolean or an object with fields), but the absence of an output schema makes this a minor gap. Overall, it is quite complete for the tool's complexity.

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?

The schema already fully describes the 'isin' parameter as 'A 12-character ISIN, e.g. "US0378331005"' with 100% coverage. The description reinforces this with the ISO 6166 reference and example, but adds no additional parameter-level semantics beyond what the schema provides. This aligns with the baseline of 3.

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 clearly states the tool's purpose: validating an ISIN by checking structure and the mod-10 check digit, and returning the ISO country prefix and NSIN. It specifies the exact resource (ISIN) and action (validate), and distinguishes itself from siblings like validate_cusip by focusing on ISIN and explicitly noting it does not resolve the issuer.

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?

The description provides clear context for when to use the tool (validating ISINs) and notes that it is keyless/offline, suggesting no authentication needed. It also states a key exclusion ('does NOT resolve the issuer'), which implicitly tells the agent not to use this for issuer resolution. However, it does not explicitly name alternative tools for issuer resolution, so it falls short of the highest score.

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