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

Annotations already declare readOnlyHint, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely useful context beyond them: dates are processed in memory and not stored, and results are for planning rather than legal advice. That is meaningful disclosure the structured fields do not carry.

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?

Three tight sentences, zero filler. The core computation is front-loaded, followed by the data-handling note and the disclaimer, each earning its place.

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?

For a two-parameter calculation tool with no output schema, the description conveys what is computed (used and remaining days), the governing rule, the input source, and the planning-only caveat. Return structure is only loosely implied, but nothing essential for correct invocation 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 description coverage is 100%, with the trips array and referenceDate fully documented (YYYY-MM-DD formats, exit inclusive, omit exit for single day, defaults to today). The description adds no syntax or format detail beyond the schema, so the baseline 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 gives a specific verb (calculate) and resource (Schengen short-stay days under the rolling 90/180-day rule), which is concrete enough to distinguish it from the sibling schengen_earliest_reentry by what it computes (used/remaining vs. a reentry date). It never names a sibling, so it falls short of full explicit differentiation.

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?

"From trips explicitly provided in this request" implies the tool works only on caller-supplied data rather than stored history, and the planning-only disclaimer signals intent. But there is no explicit statement of when to use this versus etias_check or schengen_earliest_reentry, leaving routing to inference.

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