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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint, so the safety profile is covered. The description adds useful behavioral context beyond annotations: it specifies that all monetary values are in pounds, documents three calculation modes, and references the source authority. No contradictions with annotations.

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 compact and well-structured: a one-line purpose statement, three mode explanations, and a short currency/source note. Each sentence earns its place, and the most important information is front-loaded.

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?

The tool has moderate complexity with three conditionally-required parameter sets, but the schema covers the conditional requirements and the description covers the mode semantics and currency. With annotations handling side-effect safety and an output schema present, nothing essential is missing for correct invocation.

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 the schema already documents every parameter. The description adds only the currency convention and restates mode meanings that the schema already provides. This meets the baseline for full schema coverage without substantially extending parameter understanding.

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 leads with a concrete action ('Work out a discount and sale price') and then enumerates three distinct calculation modes, each tied to a specific input relationship ('pct', 'fixed', 'find'). This makes the tool's role clear and distinguishes it from sibling percentage/discount-related 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 description gives clear mode-by-mode context for when to use the tool: percentage off, fixed amount off, or deriving a discount percentage from two prices. It does not explicitly name alternatives or state when not to use it, so it stops short of a 5, but the context is sufficient for routine selection.

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