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

Annotations already declare read-only, idempotent, closed-world, non-destructive. The description adds genuine behavior beyond them: it renders a screen (AFFICHE UN ÉCRAN) rather than returning a plain data payload, and it discloses a guardrail that the window never reaches back before the organization's signup. It does not, however, describe rate limits or the exact delta format.

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 compact sentences, front-loaded with the most important behavioral fact (that it displays a screen) followed by what is compared and the signup-boundary constraint. No filler.

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 one-parameter, read-only, idempotent tool with full schema coverage and no output schema, the description adequately conveys the comparison semantics (before/after per indicator, with deltas) and the signup-boundary rule. Minor gap: it leaves the exact returned structure somewhat implicit in the 'écran' phrasing.

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% and the single parameter period_days is already documented in the schema with its default and bounds. The description restates its meaning as 'period_days derniers jours' but adds no syntax or edge-case detail 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?

State a specific verb (compare) and resource (current period vs. immediately preceding equivalent period, with deltas), plus a concrete detail that each indicator is shown before/after. It distinguishes itself reasonably from point-in-time siblings like get_stats, but never names an alternative explicitly.

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 only implied: an agent infers it should call this when a period-over-period comparison is wanted. There is no explicit when-to-use statement, no exclusion, and no reference to alternatives such as ficiel_get_stats or ficiel_synthese that also expose metrics.

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