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

Annotations already declare read-only, idempotent, non-destructive, non-open-world behavior. The description adds genuinely new behavioral context: figures are read directly from flytel.fr, and the result deliberately excludes estimation and proration of partial months. That 'no proration' clause is important for interpreting the returned amounts and is not expressed anywhere in the schema or annotations.

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 front-loaded sentence with the verb 'Calcule' first and the computed outputs as a parallel list; every clause carries information. It is dense but not padded, with only minor cost in readability from packing four items into one sentence.

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 and annotations cover the safety profile, so the description need only ground the computation. It does that plus the no-estimation/no-proration disclaimer and lists the computed components, leaving only the source/pagination-style edge cases unaddressed.

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 defaults (esim, today's date), a plan_id pattern example, and enumerated allowance values documented inline. The description only restates these at a high level (eSIM ou carte SIM, date de souscription, recharges), adding 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 (calcule) and resource (ce que coûte un forfait Flytel) and enumerates the four outputs computed: order-time cost, monthly TTC price by subscription date, billing start, and a month with top-ups. This is far more specific than a sibling like get_plan, but it never names an alternative tool, so it stops short of full sibling 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?

Usage is implied by the enumeration of computed outputs and by the closing constraint 'sans estimation ni prorata', which hints that estimation-style tools exist elsewhere. However, there is no explicit statement of when to call this versus compare_plans, flytel_estimer_donnees, or flytel_tarifs_pays.

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