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

Annotations already declare a safe, idempotent, closed-world read, so the safety profile is covered. The description adds genuinely new behavioral context: it performs no estimation and no currency conversion, and only consumes supplied numbers. It does not state failures (e.g., what happens when amount_per_package is zero or the peptide factor is unknown).

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?

Two tight sentences, front-loaded with what is computed, followed immediately by the constraints. No filler and nothing that could be cut without losing meaning.

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, return values need not be explained, and the description correctly covers what is calculated, which unit variants are supported, and the non-conversion/non-estimation constraint. It is complete for invocation; only edge-case failure behavior is left unstated.

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%, so parameters such as price, shipping, currency, amount_unit and the peptide IU factor are already documented in the schema. The description's mention of 'a peptide's documented factor' for per-IU pricing simply restates what the peptide parameter description says, adding no syntax or format detail beyond it. 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?

States a precise verb+resource: purchase price arithmetic producing total cost and cost per mg/mcg/IU. The scope (total cost with optional shipping, unit costs, IU via a peptide's documented factor) is distinctive enough that an agent can separate it from convert_units or solution_concentration without opening a 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?

The description bounds usage clearly by negation: it uses only the numbers given, never estimates prices or shipping, and converts no currencies – which implicitly steers currency-conversion needs to convert_units. However, no sibling is named explicitly as the alternative, so it stops short of a 5.

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