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

Annotations already declare readOnly/idempotent/non-destructive, but the description adds substantive context beyond them: nothing is prepared, stored, mailed or charged, there is no account, subscription or minimum, and the quoted price is the all-in total with nothing added at checkout. Concrete price examples anchor the behavior.

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-loaded with the purpose and the alternative-tool routing; pricing, safety profile and scope follow in a logical order. Slightly long, and the 'no account / no subscription / no minimum' clause is mildly promotional, but each sentence carries usable information.

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 an output schema present, the description needn't document return structure, and it still explains that the returned figure is the all-in total. Defaults, scope, safety and the sibling relationship are all covered, leaving nothing an agent needs missing.

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%, so baseline is 3, but the description adds real meaning by tying service options to concrete totals ($16.17 with Electronic Return Receipt, $13.18 without), which helps an agent interpret the enum values. It does not add much beyond what the schema already states for page_count.

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?

Opens with a specific verb+resource: it 'says what a USPS Certified Mail letter costs'. It also explicitly distinguishes itself from the sibling prepare_certified_letter, so an agent can route without opening either schema.

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?

Gives an explicit when-to-use ('safe to call just to answer what would that cost') and names the alternative plus its triggering condition ('Use prepare_certified_letter once the person wants the letter sent'). Scope is also bounded ('U.S. destinations').

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