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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world). The description adds genuinely new context beyond them: it is free, requires no key, stores nothing, and performs purely formal validation rather than a registry lookup.

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?

Two sentences, front-loaded with the most decision-relevant facts (free, no key, nothing stored) before the technical detail. Every clause carries 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 no output schema, the description covers what is checked and what is deliberately out of scope. It could briefly state the return shape (e.g. valid/invalid plus reason), but the scope disclosure largely fills the gap.

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?

With a single parameter and 100% schema description coverage, the schema already documents the accepted formats (CHE-123.456.788, CHE123456788, optional MWST/TVA/IVA). The description adds no format detail beyond that, so the baseline of 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?

States a specific verb and resource ('Check a Swiss UID') and pins down exactly what is validated: prefix, nine digits, modulo 11 check digit. This clearly separates it from sibling validation tools like iban_check or verify.

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?

Gives a clear scope statement and an explicit exclusion: the UID and VAT registers are not queried, so it never reports existence or VAT registration. That tells the agent when NOT to reach for this tool, though it doesn't name a specific alternative for those needs.

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