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

Goes well beyond the readOnly/idempotent annotations: it discloses credit cost (1, 2 for nearby), caching, the 30 km free-tier radius ceiling, and per-country data truncation quirks (Ireland returns only Eircode routing keys, Malta only the letter prefix). These are exactly the operational facts an agent cannot infer from structured fields.

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?

Front-loaded with the core purpose, then modes, then caveats, then cost — a sensible order. It is a dense paragraph with semicolon-chained clauses, but each clause carries real information and little is redundant.

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 9-parameter tool with an output schema, the description covers what the schema cannot: cost model, caching, geographic coverage limits, and per-country data caveats. Nothing an agent needs to call this correctly appears to be 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 description coverage is 100%, so the schema already documents every parameter including the mode enum, radius bounds, and country code formats. The description largely restates the mode semantics rather than adding syntax or edge-case meaning beyond the schema, so baseline 3 applies.

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?

States a specific verb and resource (look up postal codes in the GeoNames postal database) and enumerates the three operating modes with their exact semantics. It also scopes coverage (122 countries) so the agent can distinguish it from sibling geocoders like geonames_reverse_geocode.

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?

Modes map cleanly to intents, and it routes the agent to geonames_list_reference (topic postal_countries) to verify coverage before calling. It does not address the overlap with geonames_reverse_geocode for coordinate-based lookups, which is the closest sibling to mode nearby.

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