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

With no annotations provided, the description carries the full burden and does disclose key behavior: the call is an enumeration of a fixed catalogue and unfiltered calls return the complete set, and it warns that sibling tools hard-fail on unknown cities. It does not describe return shape or whether matches are fuzzy vs exact, which is a modest gap for a read-only lookup.

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?

Two sentences, both load-bearing: the first states what it returns and how it filters, the second states when to reach for it. The usage instruction is front-loaded enough that an agent scanning for guidance finds it immediately.

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 zero-required-parameter, single-parameter read tool with no output schema, the description covers purpose, filters, and ordering against siblings adequately. The only missing piece is what a returned city looks like (name vs. identifier), which matters slightly because sibling tools presumably need that identifier.

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 the single parameter already documents itself with examples and the "omit for all 110" note. The description restates the same filter categories (country, continent, name fragment) without adding syntax or matching semantics, so the 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?

"List which cities this server knows about" gives a specific verb (list) and resource (cities covered by the server), and the filter clause scopes it further. It also implicitly distinguishes itself from the sibling tools (compare_destinations, destination_costs, walking_route), which all operate on known cities rather than enumerating them.

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?

"Call this first when a city might not be covered" gives an explicit trigger condition, and "the other tools fail rather than guess" explains why this tool must precede the siblings instead of them. Both the when-to-use and the reason for the ordering are stated rather than inferred.

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