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

The annotations already declare readOnlyHint and idempotentHint, so the description correctly does not need to restate that this is a safe, side-effect-free lookup. It adds useful context by naming the Census ACS 2020-2024 data source and enumerating the returned data, which helps an agent understand freshness and scope.

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 a single sentence that front-loads the lookup action and then provides a compact list of returned data. Every clause adds meaning, and there is no filler or repetition.

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 single-parameter, read-only lookup, the description plus the schema provide the input format and a thorough list of returned fields. It does not specify behavior for nonexistent ZIP codes, but that is a minor omission given how simple and well-scoped the tool is.

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 covers the only parameter, zip_code, at 100% with a pattern and example. The description therefore does not need to add parameter-level guidance; it appropriately focuses on the output fields instead.

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 opens with a specific verb and resource, 'Look up a US ZIP code,' and lists the exact output categories: city, state, county, coordinates, time zone, and Census ACS 2020-2024 demographics. This clearly defines what the tool does and makes it easy to distinguish from unrelated siblings such as cve_kev_lookup or domain_rank.

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 rather than explicitly stated: the description makes clear this is for retrieving US ZIP code information, but it does not explicitly discuss when to prefer it over an alternative or when not to use it. Sibling tools are mostly unrelated, so the lack of explicit routing is a moderate gap rather than a serious one.

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