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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the bar is lower, and the description adds genuinely useful context beyond them: the exclusion of Waply plans/taxes/international authentication, and the caution that this is a snapshot-based estimate rather than a live rate lookup. The only missing behavioral detail is pagination/rounding, which is minor here.

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?

Five tight sentences, front-loaded with purpose, then rate-card choice, then return shape, then scope limits, then the response-formatting instruction. Every sentence carries distinct information and nothing is repeated from the annotations or schema.

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 usefully enumerates the return shape ('total, category breakdown and free service allowance') and points at the rate-card date, which the formatting instruction reinforces. Given 8 parameters at 88% schema coverage, the only gaps are edge-case behavior (all-zero inputs, rounding) that an agent can safely omit.

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 88%, so the schema already carries nearly all parameter meaning, including the mode enum and the market/allowance rules. The description adds only the 'current or October 2026 rates' framing for mode, which the schema already states more precisely. Baseline 3 is appropriate when the schema does the heavy lifting.

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?

Clear specific verb+resource: 'Estimate monthly WhatsApp message costs in USD', with the mechanism named ('same rate snapshot and calculation as Waply's web calculator'). It distinguishes itself from most siblings (list_meta_pricing_markets, get_plan_details) but never explicitly contrasts with calculate_call_cost or the very similar compare_message_costs, which an agent must infer from the singular 'Choose current or October 2026 rates'.

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?

Scope exclusions are stated ('Meta costs only; excludes Waply plans, taxes and international authentication'), which tells the agent when this tool is not the right answer. However, no alternative is ever named — not compare_message_costs for multi-card comparison, nor calculate_call_cost for the call-side estimate — so routing is left to inference.

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