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

Annotations already declare readOnly, idempotent, and closed-world behavior, so the bar is lower. The description still adds non-obvious operational context: the call is free, ids are the key other tools consume, and postcode coverage is partial by country (Spain full, France partial, Portugal only parts of Lisbon, others unsupported). It does not discuss result ordering beyond what the schema states for postcode.

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 tight sentences with no filler: purpose and return shape first, then the key id-consumption rationale, then the coverage caveat that most affects tool selection. The postcode-coverage caveat is front-loaded at the top level rather than buried in the parameter, which is the right call for a decision-affecting constraint.

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 carries the return-value burden and does so ('their ids and parent chain'). Combined with the coverage limits, the free-cost note, and full schema coverage of all six parameters, an agent has everything needed to invoke it correctly.

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 every parameter including lang, size, country, postcode_match, and the query/postcode mutual requirement is already documented in the schema. The description only restates the query-vs-postcode duality and postcode coverage, adding little syntax or format detail beyond the structured fields. 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?

Names a specific verb (find) and resource (locations), enumerates the supported place levels, and states the two lookup modes (name/alias, postcode) plus what is returned (ids and parent chain). It also positions itself against siblings with 'The ids are what every other tool takes', so an agent knows this is the prerequisite lookup tool rather than one of the area-insights tools.

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?

Gives clear context for when to reach for it (you need location ids, or you have a postcode) and discloses the 'Free' cost, which helps the agent choose it over costlier alternatives. It stops short of explicit when-not-to-use guidance or naming a sibling alternative, so it is clear but not fully routing.

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