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

Beyond the annotations, which already declare this a read-only, idempotent, open-world lookup, the description discloses cost ('Credits: 1 per vessel returned'), free-tier degradation (answers 24 hours behind), a spatial restriction (at most 1 square degree), a monthly quota (1,000 credits), and coverage limits ('among the vessels our sources have heard'). That is substantial operational context an agent could not infer from the 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?

The purpose comes first, followed by the integration use case and then cost/plan constraints, so the most important information is front-loaded. Four sentences with no filler, though the free-tier clause is dense enough that it could be tightened.

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 does the work of telling the agent what comes back ('her particulars and her last position'), plus cost, quota, latency, and coverage caveats. Ordering and pagination behavior for results are not described, which is a minor remaining gap for a search endpoint.

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 query, limit, near_lat, and near_lon are already documented in the schema. The description reinforces the prefix behavior of the query string but says nothing about how the near_lat/near_lon ranking works or how limit interacts with credit cost, so it adds only marginal meaning beyond the schema.

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 definition gives a concrete verb and resource and states exactly what can be matched: 'Find vessels by name, MMSI or IMO number (a prefix of at least 2 characters works)'. It also frames the tool's role against siblings by saying 'Use it to turn a name into an MMSI for the other tools', which separates it from lookup-by-area tools like vessels_near or vessels_in_area.

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 supplies a clear primary use case (name-to-MMSI resolution ahead of other calls) and states the matching semantics for prefix queries. It does not explicitly name an alternative for geographic browsing, so the when-not-to-use side is only implied rather than spelled out.

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