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

Annotations already declare readOnlyHint=true and idempotentHint=true, so the description doesn't need to restate those. It adds an important edge case: 'Null when quantity is zero', which goes beyond the annotations and schema. No contradiction.

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?

The description is two short sentences with no filler. It packs the formula, usage context, and critical edge case efficiently, front-loading the key 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?

For a simple two-parameter read-only calculator with an output schema, the description covers the formula, use case, and a critical edge case. Nothing essential is missing, and the output schema handles return value details.

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?

The schema covers both parameters with descriptions (100% coverage), and the description reinforces the relationship between them by defining the formula (price divided by quantity). It also explains the zero-quantity edge case, adding semantic value beyond the schema.

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 states 'price divided by quantity' which unambiguously defines the computation, and adds 'for comparing pack sizes' to clarify the use case. This clearly distinguishes it from all sibling financial calculators.

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 phrase 'for comparing pack sizes' provides clear context for when to use this tool. It doesn't explicitly mention exclusions or alternatives, but the context is sufficient for such a simple calculator.

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