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

Annotations already declare readOnlyHint=true, destructiveHint=false, idempotentHint=true, and openWorldHint=false, so 'READ ONLY' and 'Stores and charges nothing' largely restate structured data. The description adds some genuine context (that prices are current all-in figures including the first-letter promotion conditions and a list of what Piloxa does not offer), but no further behavioral traits such as caching, freshness, or rate limits are disclosed.

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?

Front-loads the safety status, then the comparison scope, then example queries in a single dense passage with no filler. It is slightly long and the example triggers could arguably be one instead of three, but each sentence contributes.

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, so return formatting need not be described, and the description covers scope, pricing basis, and eligible questions well. A brief note on how the comparison is returned or whether it is cached would make it fully complete.

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 optional page_count parameter is already documented with its default of 1. The description ties page count to pricing ('the exact current all-in price of each for a page count'), adding minor interpretive value, but the schema already carries the semantics.

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?

Names a specific verb (compares) and resource (Piloxa's mailing services) and enumerates the comparison dimensions: what each includes, when to use it, which is the default, exact all-in price for a page count, promotion conditions, and what is not offered. This clearly separates it from sibling tools like quote_certified_letter (prices one letter) and check_certified_letter (single-item status).

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?

Provides three concrete trigger phrasings ('should I send this with return receipt?', 'what's the difference between your options?', 'how much more is the Evidence Pack?') that make the intended usage unambiguous. It stops short of explicitly naming an alternative tool or stating when not to use it, which keeps it from a 5.

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