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

Annotations already provide readOnlyHint, openWorldHint, idempotentHint, and destructiveHint, so the burden on the description is lower. The description adds valuable behavioral context beyond annotations: it discloses the data source (GeoNames), licensing (CC BY 4.0), and that there is no upstream cost. It also describes the kind of output fields (lat/lng, admin area names, ISO-3166-2 codes), which informs the agent of what to expect. No contradictions with annotations.

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 several sentences but well-structured and front-loaded with the main action. It includes examples, output qualities, country coverage, and licensing without excessive fluff. Each sentence contributes useful information, though it could be tightened (e.g., merging the country list and licensing sentence). It remains concise considering the information needed for a global lookup tool.

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 four optional parameters, an output schema, and rich annotations, the description covers the tool's purpose, search modes, country filtering, coverage, and data source. The output schema handles return-value details, so the description need not list them exhaustively. The only missing element is guidance on when not to use this tool or how it compares to other postal lookups, but the overall definition is complete enough for correct invocation.

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 covers all parameters with detailed descriptions, including examples for postal_code and place_name, so schema coverage is 100%. The description adds some redundant examples (e.g., '10001', 'SW1A 1AA') and reinforces the country filter, but it does not provide meaningfully new information beyond what the schema already documents. Baseline of 3 is appropriate since the schema carries the heavy lifting.

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 clearly states a specific verb ('look up') and resource ('postal / ZIP codes ... using the GeoNames database'), and differentiates from siblings by noting the global coverage (100+ countries) and the specific data source. The examples and output fields further solidify what the tool does, making it distinguishable from more localized postal lookup tools like address.ukpost.lookup or postcode-japan.postal.lookup.

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 description provides clear context on how to use the tool: search by postal code or place name, use the country filter to narrow results, and covers a broad set of countries. It does not explicitly mention when to use this tool versus other postal lookup alternatives, but the global scope and GeoNames origin implicitly guide the agent. The 'Use the country filter' instruction is a concrete usage tip, though no 'when not to use' or sibling comparison is given.

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