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

The annotations already declare readOnly, idempotent, open-world and non-destructive behaviour, so the bar is lower; the description still adds real context by naming the two external sources (Team Cymru and RDAP) and the IP-to-AS mapping rule. It omits failure behaviour, e.g. private/reserved IPs or an ASN with no registry record, and mentions no rate limits.

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 tight sentences: the capability and returned fields come first, the less common IP-input behaviour second. No filler, no repetition of the title, and nothing left dangling.

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, annotation-rich lookup with no output schema, the description covers what is queried, which sources are consulted and what data is returned — enough for correct invocation. Only edge-case behaviour (unknown ASN, non-public IP) is missing.

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% and the single parameter already documents both accepted forms (AS13335/13335 or a public IP address). The description restates those forms and adds only the mapping semantics for IP input, so it neither compensates for nor significantly extends 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?

The description names a specific verb and resource ('looks up an AS number') and enumerates the returned fields (name, organisation, country, registry, registration dates), plus the two sources. The final sentence clarifies the IP-address path. It is not explicitly contrasted with nearby tools such as ip_lookup or whois_lookup, so it falls short of a 5.

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?

It gives usable context by stating that a public IP address is accepted and is first mapped to the announcing AS, which tells the agent this tool also serves IP-to-AS questions. It never says when to prefer asn_lookup over ip_lookup, whois_lookup or reverse_dns, and it gives no exclusions or prerequisites.

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