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

Annotations already declare readOnly, idempotent, open-world, and non-destructive behavior. The description adds valuable contextual behavior: lodging rates vary by month/season and M&IE is a flat daily rate. No contradiction with the annotations exists.

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?

Three focused sentences cover the resource, the return semantics, and a concrete call example. Every sentence earns its place and the main purpose is front-loaded.

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?

With no output schema, the description compensates by specifying the returned values precisely. It could additionally mention behavior for invalid or unsupported zip codes, but for a simple read-only lookup the information provided is largely sufficient.

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 input schema has 100% description coverage for all three parameters, including the zip format, year default, and optional API key. The description adds only a concrete example and doesn't need to repeat parameter details, so the baseline score of 3 is appropriate.

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 states a specific verb and resource: "Get US federal per diem rates ... by zip code." It also clarifies what is returned: the monthly lodging ceiling and the flat M&IE daily rate. Though it doesn't explicitly contrast itself with the sibling rates_by_city tool, the zip-code qualifier makes the core distinction clear.

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 intended use case is clear from "by zip code" and the example call, but there is no explicit guidance about when to choose rates_by_zip over rates_by_city or what to do when only a city name is available. This is implied rather than explicit routing among alternatives.

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