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

Annotations already provide readOnlyHint and idempotentHint, and the description adds meaningful behavioral detail: it returns the last heard report and indicates which MMSIs were not heard recently. This goes beyond the structured annotations without contradicting them.

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 tight sentences: the first front-loads the purpose and output scope, the second gives routing guidance. No filler or redundant restatement of the tool title.

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 only one parameter, full schema coverage, an output schema, and safety annotations, the description covers the key usage context: what to retrieve and when to use the sibling instead. Nothing essential 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 description coverage is 100% and already documents the mmsi parameter, including anonymous/personal token limits. The description adds little beyond restating that lookups are by MMSI, so the baseline score is 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?

Description states a specific operation — retrieving current position and details by MMSI — and explicitly distinguishes it from search_vessels_by_name. It also names the resource (vessels by MMSI) and adds the unique 'not heard in 30 minutes' detail.

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?

Explicitly tells the agent to use search_vessels_by_name first when only a name is available, routing between alternatives. This is clear when-to-use guidance for the main decision point.

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