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

Annotations already declare readOnlyHint=true and destructiveHint=false, covering the safety profile. The description adds useful behavioral context: it is 'heavier on the backend', it returns a specific enriched set of fields, and it notes staleness handling with a pointer to an estimation tool. This goes beyond the 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?

The description is two sentences, but the second sentence is dense and packs a lot of information. It is still efficient and front-loaded: purpose first, then usage guidance, then the staleness alternative. No fluff, but slightly long due to the amount of necessary detail.

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?

An output schema exists, so return-value structure is already covered. The description fully addresses when to use the tool, what it returns, why it is heavier, and even suggests a fallback tool for stale positions. For a tool with this level of complexity and a well-defined niche, 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?

The input schema has 100% description coverage, with each parameter (mmsi, imo, uuid) fully documented and the 'exactly one' constraint stated in both the schema and the description. The description reinforces the exclusivity requirement but adds no new syntactic or semantic details beyond the schema, so the baseline of 3 applies.

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 opens with a clear, specific statement: it is the 'Richer, heavier variant of get_vessel' and lists exactly what additional data it returns (destination/origin port, ATD, ETA, draught). It explicitly distinguishes itself from the sibling get_vessel, so an agent can immediately tell them apart.

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 gives explicit when-to-use guidance: 'Prefer get_vessel for ordinary position or identity lookups; use get_vessel_pro only when the user asks about the destination or origin port, departure time, arrival time or ETA, or draught.' It even names the alternative for stale positions (estimated_vessel_position), making the decision path clear.

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