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 so well: it discloses coverage bounds, a 30-vessel cap per call, a 3-minute cache, third-party provenance (Atlas Data / Fintraffic Digitraffic, CC BY 4.0), a cost of 0.02 USD per call via x402, and a 'not for navigation' fitness caveat. These are exactly the operational facts an agent needs before calling.

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 geographic limitation, which is the single most important constraint, then the query modes, then provenance/cost. It is a long run-on sentence rather than a structured block, but essentially every clause (coverage, modes, limits, cache, license, price, disclaimer) earns its place.

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 10-parameter tool with no annotations and no output schema, the description covers returned fields (name, IMO, type, speed, course, nav status, destination, ETA, size), result limits, freshness, licensing, pricing, and the safety caveat. Nothing an agent needs to call it correctly is 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 coverage is 100%, so the baseline is 3. The description still adds value beyond the schema by stating the maximum radius (1000 km) and that results are sorted by distance, plus the up-to-30 clamp on maxItems — constraints the schema does not state.

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+resource — live AIS vessel positions — and immediately bounds it geographically (Finnish AIS coverage, ~57–66°N, 17–35°E, not global). The two query modes (proximity vs. identifier/category) make it self-differentiating from maritime_port_calls, which clearly covers port calls rather than live positions.

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 how to use it: proximity search near a point with radius and distance sorting, or lookup by MMSI/IMO/name/category. It does not explicitly name maritime_port_calls as the alternative when historical port-call data is wanted, so there is no stated when-not.

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