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

Beyond the annotations (readOnly/idempotent/openWorld/non-destructive), the description discloses material operational traits: an AIS Trail API key must be sent as an 'Authorization: Bearer ap_live_…' header, a 100-port-call cap, a 366-day range cap, and a cost of 1 credit per port call returned. These are exactly the facts an agent needs before invoking.

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?

It is a single dense paragraph that front-loads the resource and scope before moving to event, auth, and limits. Nearly every clause carries information, though the return-field enumeration and limits could be split for faster scanning.

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 correctly enumerates the return payload; with 50% parameter coverage it fills the enum and port_id gaps; and it covers auth, caps, and cost. For a read-only 4-parameter tool, an agent has everything needed to call it correctly.

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 50%, so the description must compensate and it largely does: 'event: any (in port at any time in the range, default), arrival or departure' explains the enum semantics rather than just repeating the labels, and it tells the agent where port_id comes from. from/to formats are left to the schema, leaving a small gap.

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?

The description names a specific verb+resource ('Ships that called at one port between two times') and enumerates the returned fields (ship with flag, arrival, departure, draught, cargo estimate, previous/next port) plus the data source ('AIS Trail archive'). It also routes the agent to a sibling ('Get the port_id from find_port') and relates itself to vessel_port_calls, so it is distinguishable without opening schemas.

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 gives clear context: obtain port_id from find_port, choose event with its default, and notes the credit rules 'apply as for vessel_port_calls'. It does not explicitly say when to prefer port_calls over vessel_port_calls or vessels_in_area, so there is no full exclusion guidance, but the intended usage is unambiguous.

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