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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so the safety profile needs no restating. The description does add value by enumerating what a result entry contains (USPS definition words, page, other names, postalref note, postalref page), which is the main behavioral disclosure. It says nothing about matching behavior (exact vs. fuzzy), what happens on a miss, or ordering, which would matter for a lookup tool.

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?

A single sentence, front-loaded with the core question it answers, and no redundant restatement of the title or annotations. The dense enumeration of return contents is earned because it explains what 'define' yields in this registry.

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 rich annotations, an output schema, and full schema coverage, the description only needs to establish what the tool does and what a result broadly contains — both are covered. The remaining gap is behavioral matching rules, but that is a minor omission given the structured data already present.

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 description coverage is 100% and both parameters are documented, including examples (mailer id, MID, CRID) and the default limit of 5, so the baseline is 3. The description's only semantic contribution is the plural 'each registry entry it names', which implies that one name may map to multiple entries and thus justifies the limit parameter — a modest but real addition.

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 precisely what the tool returns for a USPS term: registry entries with USPS definitions, page references, alternate names, and postalref explanations. It is immediately clear this is a terminology-definition lookup rather than a search or document reader. It stops short of naming which sibling (decode, lookup, search) it displaces, so sibling differentiation is left implicit.

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?

There is no explicit when-to-use or when-not-to-use guidance, and none of the six siblings (decode, lookup, search, search_documents, read_source, diff) are referenced. An agent must infer from the wording that this is the definitional lookup versus broader search. The phrase 'each registry entry it names' hints at multi-entry behavior but is not framed as a usage rule.

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