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

No annotations are provided, so the description carries the burden, but the tool takes no parameters and is a safe informational lookup, which the description makes evident. It adds concrete behavioral content by naming what the output contains (the EC 610/2013 reference plus links to an interactive calculator and guide), though it never explicitly states it is a read-only, non-mutating operation.

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?

A single tightly written sentence that front-loads the verb and the rule being explained, then appends the reference and resource links. There is no filler and every clause carries information.

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 input parameters and no output schema, the description needs to signal what the caller gets back, and it does by naming the legal reference and the linked calculator/guide. For a trivial explainer tool this is nearly complete, with only an explicit read-only statement and sibling routing 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 tool has zero parameters, so per the baseline there is nothing for the description to disambiguate. No parameter-level meaning is needed and none is missing.

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 verb ("Explain") and a precise resource ("Schengen 90/180 rolling-window rule"), so the agent knows this returns conceptual/legal explanation content. It implicitly distinguishes itself from the computational siblings calculate_stay and check_trip, but never names them, so the differentiation is inferable rather than explicit.

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?

Usage is implied: an "explain" tool is the natural choice for understanding the rule rather than computing a stay, while calculate_stay and check_trip handle calculations. However, the description never states when to use this versus those siblings nor any prerequisites, leaving the routing to inference.

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