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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is covered. The description adds behavioral context the annotations don't: this is a current-state snapshot, not a historical record, and data is scoped to 'Shiptrail's page'. It does not mention freshness, rate limits, or behavior for an unknown MMSI.

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?

A single sentence, front-loaded with the core action and scope, ending with a useful boundary. The long field enumeration partly duplicates what the output schema already lists, which costs some efficiency, but nothing is filler.

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?

Given a one-parameter read tool with annotations and an output schema, the description covers scope, snapshot nature, and the absence of history. The main remaining gap is not stating what happens when the MMSI is unknown or how current the data is.

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?

Only one parameter, and the schema already documents it fully as 'Her MMSI, 9 digits.' The description restates that lookup is keyed by MMSI but adds no format, validation, or fallback detail beyond the schema. Baseline 3 applies when schema coverage is 100%.

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 and resource ('Everything ... about one ship, by MMSI') and enumerates the payload: identity, position, destination/ETA, port calls, particulars, photo, Wikipedia, nearby ships. The phrase 'one ship, by MMSI' cleanly separates it from search_ships, which would return many.

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?

Makes the trigger condition clear: you have an MMSI and want the full profile of that single vessel. The closing 'No history over time' is an explicit exclusion that prevents misuse for time-series questions. It stops short of naming an alternative (e.g., search_ships for name-based lookup), so not a 5.

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