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

Annotations already declare readOnly/idempotent/non-destructive, and the description adds genuinely new behavioral context: incoming calls are free, the rate is an unverified heuristic rather than an official Meta tariff, the result must be presented as a reference estimate needing rate-card confirmation, and Waply plans, AI/provider fees and taxes are excluded. This is exactly the kind of caveat an agent cannot get from structured fields.

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?

Purpose is front-loaded in the first clause, and each following sentence carries non-redundant information (free incoming, heuristic caveat, exclusions). No filler or restatement of the tool name.

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 still implies the returned value is a total estimate and flags the required disclaimer, plus lists what is out of scope. A minor gap remains around the shape/currency of the returned estimate, but for a 3-parameter read-only calculator this is close to sufficient.

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 67% with both market and outgoingMinutes documented in-schema (locale is an obvious enum). The description nonetheless reinforces parameter meaning by framing minutes as outgoing-only monthly minutes and by tying market pricing to a reference rate, though it adds little about the locale enum that the schema does not already convey.

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 uses a specific verb and resource ('Estimate WhatsApp outgoing call costs from monthly minutes') and clarifies scope by stating 'Incoming calls are free', which naturally separates it from calculate_message_cost. It never names a sibling tool explicitly, so differentiation rests on the resource noun rather than direct routing language.

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: estimate call costs from a monthly minute figure, tied to the same reference rate as Waply's web calculator. There is no 'use this instead of X' routing and no stated precondition beyond the inputs, so an agent must infer when this beats calculate_message_cost or compare_message_costs.

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