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

Goes well beyond the readOnly/idempotent annotations: defines port-call semantics (starts when stopped inside a port area, ends on leaving), notes departure is null while in port, flags the sanctions column as informational only, and discloses credit cost, rate limits and the 24-hour delay on non-live plans.

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 purpose and return fields, then constraints and auth in descending priority. It is a dense single paragraph; splitting the access/limit caveats into their own sentence would scan better, but no clause 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?

With no output schema, the description fully compensates by listing every returned field (port, arrival, departure, draughts, cargo estimate, adjacent ports, sanctions) and covering auth, limits and known-unknown behaviors like the null departure.

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, but the description adds value by stating 'give mmsi or imo' — the schema marks neither as required, so this clarifies a real constraint the structured data leaves ambiguous, and confirms UTC/ISO 8601 for the time bounds.

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 ('Port calls of a ship between two times ... from the AIS Trail archive') and enumerates the returned fields, so the agent knows exactly what it gets. The archive/API-key framing plus the title's '(API key)' distinguishes it from the sibling port_calls variant.

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 operating context: supply mmsi or imo, a UTC ISO 8601 window, max 100 calls and 366 days, and that an AIS Trail API key is required via the Authorization header. It does not explicitly contrast with siblings like vessel_events or vessel_track, so an agent must infer which to prefer.

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