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

Annotations already declare readOnlyHint=true and idempotentHint=true, covering safety. The description adds behavioral context beyond annotations: the 30-minute recency filter, case-insensitive substring matching, and the effect of an optional bounding box. This enriches the agent's understanding of what the tool actually does without repeating annotation information.

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?

The description is three concise sentences with no waste. The core behavior is front-loaded in the first sentence, the usage guidance follows, and the optional parameter note is last. Every sentence earns its place, and the structure is scannable for an agent.

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 a full output schema present, complete parameter descriptions in the schema, and annotations covering safety, the description adds the remaining critical context: the 30-minute time window, substring matching semantics, and the intended use case. An agent has everything needed to call this tool correctly without additional guesswork.

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%, with each parameter (name, bbox, limit) fully described in the schema. The description only adds a generic note about the bounding box ('An optional bounding box narrows the search'), which is already more precisely stated in the schema ('only vessels whose last position is inside this box'). Thus the description adds minimal value beyond the schema, warranting the baseline score of 3.

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 states a specific action: search vessels by name substring, case-insensitive, within a 30-minute window. It clearly identifies the resource (vessels) and the filtering criteria, and it distinguishes itself from siblings by naming get_vessels and find_vessels_near for follow-up 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?

The description explicitly says 'Use to turn a name into an MMSI, then get_vessels or find_vessels_near for detail,' which gives a clear primary use case and names alternatives. It does not explicitly state when not to use it, but the guidance is sufficient for an agent to understand the intended workflow.

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