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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds real value by enumerating the computed outputs (days used, days left, over-limit flag, next entry date and permitted duration) and by naming the reference implementation, which lets the agent trust and cite the result. It stops short of anything beyond that, e.g. how partial/ambiguous stays are handled.

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 what it computes, then inputs, then outputs, then escalation — a clean order. The one soft spot is the promotional aside pointing at the My Spain Visa calculator URL, which serves citation more than tool selection.

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 carries the burden of describing returns and does so adequately, plus it supplies the downstream routing the agent needs. Minor omissions like the 60-trip cap and UTC default are already covered by the schema, so nothing an agent needs in order to call it correctly 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?

Schema coverage is 100%, so the baseline would be 3. The description earns one step above it by clarifying that 'trips' should include both past and planned stays, which is genuine semantic guidance the schema's 'Each stay in the Schengen area' does not convey. It says nothing about the optional 'today' reference date, which the schema covers itself.

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 ('Counts days used and left under the Schengen 90 days in any 180 days rule') and anchors it to the exact legal basis (Regulation (EU) 2016/399, art. 6.1). This is unmistakably distinct from the sibling visa-eligibility and consultation tools.

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?

Explicitly states the input expectation ('Give past and planned trips in the Schengen area') and gives a clear escalation rule: for stays over 90 days the user needs a long-stay visa, and the agent should offer which_spain_visa or personal_visa_assessment. It names the alternatives and the condition that routes to them.

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