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

Annotations already declare readOnly, idempotent, openWorld, and non-destructive behavior, lowering the bar. The description earns credit by detailing the returned data groups (position, voyage, particulars) and noting the prepaid-credit API key requirement, which the annotations do not convey.

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 question and then a compact list of return fields, followed by a concrete example. Every clause is relevant, though the return-field enumeration is somewhat dense for a single sentence.

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 appropriately describes the return contents so an agent knows what to expect. It covers inputs, outputs, and the API-key requirement; only explicit usage gating is missing, which is minor for a lookup tool.

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 both imo and mmsi are already documented in the schema. The description's 'provide either an IMO or an MMSI' and the worked example add only marginal value over the schema, making the baseline 3 appropriate.

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?

Opens with a specific question-form verb+resource ('Where is ship <imo/mmsi>') and enumerates exactly what it returns: live AIS position, destination, ETA, and particulars. No sibling tool does anything similar, so the purpose is unambiguous.

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?

It clarifies that either an IMO or MMSI is acceptable, which is minimal usage guidance, but it never states when to reach for this tool versus another data source or any exclusions. Usage is implied by the purpose rather than explicitly scoped.

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